ub.tex

%!TEX root = std.tex
\infannex{ub}{Core undefined behavior}

\rSec1[ub.general]{General}

\pnum
This Annex documents
undefined behavior explicitly called out in
\ref{intro} through \ref{\lastcorechapter}
using the following phrases:
\begin{itemize}
\item is undefined
\item the behavior is undefined
\item the behavior of the program is undefined
\item has undefined behavior
\item have undefined behavior
\item result has undefined behavior
\item results in undefined behavior
\end{itemize}
Undefined behavior that is implicit is not covered by this annex.
Each entry contains
a title,
a cross-reference,
a summary of the circumstances,
and code examples.
The code examples are not intended
to exhaustively cover all possible ways of invoking that case.

\rSec1[ub.basic]{\ref*{basic}: Basics}

\ubdescription{intro.object.implicit.create}

\pnum
For each operation that is specified as implicitly creating objects,
that operation implicitly creates and starts the lifetime
of zero or more objects of implicit-lifetime types\iref{basic.types}
in its specified region of storage
if doing so would result in the program having defined behavior.
If no such set of objects would give the program defined behavior,
the behavior of the program is undefined.

\pnum
\begin{example}
\begin{codeblock}
void f()
{
  void *p = malloc(sizeof(int) + sizeof(float));
  *reinterpret_cast<int*>(p) = 0;
  *reinterpret_cast<float*>(p) = 0.0f;  // undefined behavior, cannot create
                                        // both \tcode{int} and \tcode{float} in same place
}
\end{codeblock}
\end{example}

\ubdescription{intro.object.implicit.pointer}

\pnum
After implicitly creating objects within a specified region of storage,
some operations are described as producing a pointer to a
suitable created object\iref{basic.types}.
These operations select one of the implicitly-created objects
whose address is the address of the start of the region of storage,
and produce a pointer value that points to that object,
if that value would result in the program having defined behavior.
If no such pointer value would give the program defined behavior,
the behavior of the program is undefined.

\pnum
\begin{example}
\begin{codeblock}
#include <cstdlib>
struct X {
  int a, b;
  ~X() = delete;        // deleted destructor makes \tcode{X} a non-implicit-lifetime class
};

X* make_x() {
  X* p = (X*)std::malloc(sizeof(struct X));     // Cannot implicitly create an object because \tcode{X}
                                                // is not an implicit-lifetime class.
  p->a = 1;     // undefined behavior, no set of objects give us defined behavior
  return p;
}
\end{codeblock}
\end{example}
\begin{example}
\begin{codeblock}
#include <cstdlib>

struct X {
  int a;
};

struct Y {
  int x;
};

Y* make_y() {
  X* p1 = (X*)std::malloc(sizeof(struct X));    // object \#1
  p1->a = 1;
  Y* p2 = (Y*)p1;
  p2->x = 2;    // undefined behavior, \tcode{p2} points to an in-lifetime object of type \tcode{X},
                // which is not similar to \tcode{Y}\iref{expr.ref}
  return p2;
}
\end{codeblock}
\end{example}

\ubdescription{basic.align.object.alignment}

\pnum
All instances of a type must be created in storage
that meets the alignment requirement of that type.

\pnum
\begin{example}
\begin{codeblock}
struct alignas(4) S {};

void make_misaligned()
{
    alignas(S) char s[sizeof(S) + 1];
    new (&s+1) S();     // undefined behavior, \tcode{\&s+1} will yield a pointer to
                        // a \tcode{char} which is $1$ byte away from an address with
                        // an alignment of $4$ and so cannot have an alignment of $4$
}
\end{codeblock}
\end{example}

\ubdescription{lifetime.outside.pointer.delete}

\pnum
For a pointer pointing to an object outside of its lifetime,
behavior is undefined if the object will be or was of a class type
with a non-trivial destructor
and the pointer is used as the operand of a \grammarterm{delete-expression}.

\pnum
\begin{example}
\begin{codeblock}
struct S {
  float f = 0;
  ~S() {}
};

void f() {
  S* p = new S;
  p->~S();
  delete p;     // undefined behavior, operand of \tcode{delete}, lifetime has ended and \tcode{S}
                // has a non-trivial destructor
}
\end{codeblock}
\end{example}

\ubdescription{lifetime.outside.pointer.member}

\pnum
For a pointer pointing to an object outside of its lifetime, behavior is
undefined if the pointer is used to access a non-static data member or call a
non-static member function of the object.

\pnum
\begin{example}
\begin{codeblock}
struct S {
  float f = 0;
};

float f() {
  S s;
  S* p = &s;
  s.~S();
  return p->f;  // undefined behavior, accessing non-static data member after end of lifetime
}
\end{codeblock}
\end{example}

\ubdescription{lifetime.outside.pointer.virtual}

\pnum
For a pointer pointing to an object outside of its lifetime, behavior is
undefined if the pointer is implicitly converted\iref{conv.ptr} to a pointer
to a virtual base class (or base class of a virtual base class).

\pnum
\begin{example}
\begin{codeblock}
struct B {};
struct D : virtual B {};
void f() {
  D d;
  D* p = &d;
  d.~D();
  B* b = p;     // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{lifetime.outside.pointer.dynamic.cast}

\pnum
For a pointer pointing to an object outside of its lifetime, behavior is
undefined if the pointer is used as the operand of a
\tcode{dynamic_cast}\iref{expr.dynamic.cast}.

\pnum
\begin{example}
\begin{codeblock}
struct B { virtual ~B() = default; };
struct D : B {};
void f()
{
  D d;
  B* bp = &d;
  d.~D();
  D* dp = dynamic_cast<D*>(bp);     // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{lifetime.outside.glvalue.access}

\pnum
Behavior is undefined if a glvalue referring to an object
outside of its lifetime is used to access the object.

\pnum
\begin{example}
\begin{codeblock}
void f() {
  int x = int{10};
  using T = int;
  x.~T();
  int y = x;    // undefined behavior, glvalue used to access the
                // object after the lifetime has ended
}
\end{codeblock}
\end{example}

\ubdescription{lifetime.outside.glvalue.member}

\pnum
Behavior is undefined if a glvalue referring to an object
outside of its lifetime is used to call
a non-static member function of the object.

\pnum
\begin{example}
\begin{codeblock}
struct A {
  void f() {}
};

void f() {
  A a;
  a.~A();
  a.f();    // undefined behavior, glvalue used to access a non-static member
            // function after the lifetime has ended
}
\end{codeblock}
\end{example}

\ubdescription{lifetime.outside.glvalue.virtual}

\pnum
Behavior is undefined if a glvalue referring to an object
outside of its lifetime is bound to a reference to a virtual base class.

\pnum
\begin{example}
\begin{codeblock}
struct B {};
struct D : virtual B {
};

void f() {
  D d;
  d.~D();
  B& b = d;     // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{lifetime.outside.glvalue.dynamic.cast}

\pnum
Behavior is undefined if a glvalue referring to an object outside of its
lifetime is used as the operand of a
\keyword{dynamic_cast} or as the operand of \keyword{typeid}.

\pnum
\begin{example}
\begin{codeblock}
struct B { virtual ~B(); };
struct D : virtual B {};

void f() {
  D d;
  B& br = d;
  d.~D();
  D& dr = dynamic_cast<D&>(br);     // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{original.type.implicit.destructor}

\pnum
The behavior is undefined if
a non-trivial implicit destructor call
occurs when the type of the object inhabiting the associated storage
is not the original type associated with that storage.

\pnum
\begin{example}
\begin{codeblock}
class T {};

struct B {
  ~B();
};

void h() {
  B b;
  new (&b) T;
}               // undefined behavior at block exit
\end{codeblock}
\end{example}

\ubdescription{creating.within.const.complete.obj}

\pnum
Creating a new object within the storage that a const complete object
with static, thread, or automatic storage duration occupies,
or within the storage that such a const object used to occupy
before its lifetime ended, results in undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct B {
  B();
  ~B();
};

const B b;

void h() {
  b.~B();
  new (const_cast<B*>(&b)) const B; // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{basic.indet.value}

\pnum
When the result of an evaluation is an indeterminate value
(but not just an erroneous value) the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
void g() {
  int x [[ indeterminate ]];
  int y = x;    // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{basic.stc.alloc.dealloc.constraint}

\pnum
If the behavior of an allocation or deallocation function
does not satisfy the semantic constraints specified
in~\ref{basic.stc.dynamic.allocation}
and~\ref{basic.stc.dynamic.deallocation},
the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
#include <new>
void* operator new(std::size_t sz) {
  if (sz == 0)
    return nullptr;             // undefined behavior, should return non-null pointer

  return std::malloc(1);        // undefined behavior, if successful should return allocation with
                                // length in bytes is at least as large as the requested size
}
void operator delete(void* ptr) noexcept {
  throw 0;                      // undefined behavior, terminates by throwing an exception
}
\end{codeblock}
\end{example}

\ubdescription{basic.stc.alloc.zero.dereference}

\pnum
The pointer returned when invoking an allocation function with a size of zero
cannot be dereferenced.

\pnum
\begin{example}
\begin{codeblock}
void test()
{
    char* c = static_cast<char*>(operator new(0z));
    c[0] = 'X';     // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{basic.compound.invalid.pointer}

\pnum
Indirection or the invocation of a deallocation function
with a pointer value referencing storage that has been freed
has undefined behavior.
(Most other uses of such a pointer have implementation-defined behavior.)

\pnum
\begin{example}
\begin{codeblock}
void f()
{
    int *x = new int{5};
    delete x;
    int y = *x; // undefined behavior
    delete x;   // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{intro.execution.unsequenced.modification}

\pnum
If a side effect on a memory location\iref{intro.memory}
is unsequenced relative to either another side effect
on the same memory location
or a value computation using the value of any object
in the same memory location,
and they are not potentially concurrent\iref{intro.multithread},
the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
void g(int i) {
  i = 7, i++, i++;  // \tcode{i} becomes 9

  i = i++ + 1;  // the value of \tcode{i} is incremented
  i = i++ + i;  // undefined behavior
  i = i + 1;    // the value of \tcode{i} is incremented
}
\end{codeblock}
\end{example}

\ubdescription{intro.races.data}

\pnum
The execution of a program contains a data race
if it contains two potentially concurrent conflicting actions,
at least one of which is not atomic,
and neither happens before the other,
except for the special case for signal handlers
described in~\ref{intro.races}.
Any such data race results in undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
int count = 0;
auto f = [&] { count++; };
std::thread t1{f}, t2{f}, t3{f};
// undefined behavior, \tcode{t1}, \tcode{t2} and \tcode{t3} have a data race on access of variable \tcode{count}
\end{codeblock}
\end{example}

\ubdescription{intro.progress.stops}

\pnum
The behavior is undefined if a thread of execution
that has not terminated stops making execution steps.

\pnum
\begin{example}
\begin{codeblock}
bool stop() { return false; }

void busy_wait_thread() {
  while (!stop());  // undefined behavior, thread makes no progress but the loop
}                   // is not trivial because \tcode{stop()} is not a constant expression

int main() {
  std::thread t(busy_wait_thread);
  t.join();
}
\end{codeblock}
\end{example}

\ubdescription{basic.start.main.exit.during.destruction}

\pnum
If \tcode{std::exit} is called to
end a program during the destruction of an object with static or thread storage duration, the program has
undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
#include <cstdlib>

struct Exiter {
  ~Exiter() { std::exit(0); }
};

Exiter ex;

int main() {}
// undefined behavior when destructor of static variable \tcode{ex} is called it will call \tcode{std::exit}
\end{codeblock}
\end{example}

\ubdescription{basic.start.term.use.after.destruction}

\pnum
If a function contains a block-scope object of static
or thread storage duration that has been destroyed
and the function is called
during the destruction of an object with static or thread storage duration,
the program has undefined behavior if the flow of control
passes through the definition of the previously destroyed block-scope object.
Likewise, the behavior is undefined if the block-scope object
is used indirectly (i.e., through a pointer) after its destruction.

\pnum
\begin{example}
\begin{codeblock}
struct A {};
void f() {
  static A a;
}

struct B {
  B() { f(); }
};
struct C {
  ~C() { f(); }
};

C c;
B b;    // call to \tcode{f()} in constructor begins lifetime of \tcode{a}

int main() {}
// undefined behavior, static objects are destructed in reverse order, in this case \tcode{a} then \tcode{b} and
// finally \tcode{c}. When the destructor of \tcode{c} is called, it calls \tcode{f()} which passes through the definition of
// previously destroyed block-scope object
\end{codeblock}
\end{example}

\rSec1[ub.expr]{\ref*{expr}: Expressions}

\ubdescription{expr.expr.eval}

\pnum
If during the evaluation of an expression,
the result is not mathematically defined
or not in the range of representable values for its type,
the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
#include <limits>
int main() {
  // Assuming $32$-bit \tcode{int}, the range of values is: $-2,147,483,648$ to $2,147,483,647$.
  int x1 = std::numeric_limits<int>::max() + 1;
        // undefined behavior, $2,147,483,647 + 1$ is not representable as an \tcode{int}
  int x2 = std::numeric_limits<int>::min() / -1;
        // undefined behavior, $-2,147,483,648 / -1$ is not representable as an \tcode{int}
}
\end{codeblock}
\end{example}

\ubdescription{expr.basic.lvalue.strict.aliasing.violation}

\pnum
If a program attempts to access\iref{defns.access} the stored value of an object
whose dynamic type is $T$ through a glvalue
whose type is not similar\iref{conv.qual} to $T$
(or its corresponding signed or unsigned types)
the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
int foo(float* f, int* i) {
  *i = 1;
  *f = 0.f;     // undefined behavior, glvalue is not similar to \tcode{int}

  return *i;
}

int main() {
  int x = 0;

  x = foo(reinterpret_cast<float*>(&x), &x);
}
\end{codeblock}
\end{example}

\ubdescription{expr.basic.lvalue.union.initialization}

\pnum
If a program invokes a defaulted copy/move constructor or
defaulted copy/move assignment operator of a union
with an argument that is not an object of a similar type
within its lifetime, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
union U { int x; };
void f()
{
   char u[sizeof(U)];
   U o = reinterpret_cast<U&>(u);   // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{expr.type.reference.lifetime}

\pnum
Evaluating a reference when an equivalent use of a pointer denoting the same object
would be invalid has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
void g()
{
  int* ip = new int(5);
  int& i = *ip;
  delete ip;
  i;    // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{conv.lval.valid.representation}

\pnum
Performing an
lvalue-to-rvalue conversion
on an object whose
value representation
is not valid for its type
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
bool f() {
  bool b = true;
  char c = 42;
  memcpy(&b, &c, 1);
  return b;         // undefined behavior if \tcode{42} is not a valid value representation for \keyword{bool}
}
\end{codeblock}
\end{example}

\ubdescription{conv.double.out.of.range}

\pnum
Converting a floating-point value to a type
that cannot represent the value has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
#include <limits>

int main() {
  // Assuming 32-bit int, 32-bit float and 64-bit double.
  double d2 = std::numeric_limits<double>::max();
  float f = d2; // undefined behavior on systems where the range of representable values
                // of \tcode{float} is \tcode{[-max,+max]}; on systems where the range of representable
                // values is \tcode{[-inf,+inf]} this would not be undefined behavior
  int i = d2;   // undefined behavior, the max value of \tcode{double} is not representable as \tcode{int}
}
\end{codeblock}
\end{example}

\ubdescription{conv.fpint.float.not.represented}

\pnum
When converting a floating-point value to an integer type, if
the value is not representable in the destination type, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
#include <limits>

int main() {
  // Assuming $32$-bit \tcode{int}, the range of values is: $-2,147,483,648$ to
  // $2,147,483,647$. Assuming $32$-bit \tcode{float} and $64$-bit \tcode{double}.
  double d = (double)std::numeric_limits<int>::max() + 1;
  int x1 = d;   // undefined behavior, $2,147,483,647 + 1$ is not representable as \tcode{int}
}
\end{codeblock}
\end{example}

\ubdescription{conv.fpint.int.not.represented}

\pnum
When converting a value of integer or unscoped enumeration type
to a floating-point type,
if the value is not representable in the destination type
the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
int main() {
  unsigned long long x2 = -1;
  float f = x2; // undefined behavior on systems where \tcode{f} does not include
                // a representation for infinity and the maximum value for \tcode{float}
                // is smaller than the maximum value for \tcode{unsigned long long}
}
\end{codeblock}
\end{example}

\ubdescription{conv.ptr.virtual.base}

\pnum
Converting
a pointer to a derived class \tcode{D}
to
a pointer to a virtual base class \tcode{B}
that does not point to
a valid object
within its lifetime
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct B {};
struct D : virtual B {};
void f()
{
  D ds[1];
  B* b = &ds[1];    // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{conv.member.missing.member}

\pnum
The conversion of
a pointer to a member of a base class
to a pointer to member of a derived class
that could not contain that member
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct B {};
struct D1 : B { int d1; };
struct D2 : B {};
void f()
{
  int (D1::*pd1) = &D1::d1;
  int (B::*pb) = static_cast<int (B::*)>(pd1);
  int (D2::*pd2) = pb;  // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{expr.call.different.type}

\pnum
Calling a function through an expression whose
function type is not call-compatible with
the function type of the called
function's definition results in undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
using f_float = int (*)(float);
using f_int = int (*)(int);

int f1(float) { return 10; }

int f2(int) { return 20; }

int main() {
  return reinterpret_cast<f_int>(f1)(20);   // undefined behavior, the function type of the expression
                                            // is different from the called function's definition
}
\end{codeblock}
\end{example}

\ubdescription{expr.ref.member.not.similar}

\pnum
In a class member access \tcode{E1.E2},
where \tcode{E2} is a non-static member,
the behavior is undefined
if \tcode{E1} refers to an object
whose type is not similar
to the type of the expression \tcode{E1}.

\pnum
\begin{example}
\begin{codeblock}
struct A { int i; };
struct B { int j; };
A a;
int x = reinterpret_cast<B&>(a).j;  // undefined behavior
\end{codeblock}
\end{example}

\ubdescription{expr.dynamic.cast.pointer.lifetime}

\pnum
Evaluating a \keyword{dynamic_cast} on a non-null pointer
that points to an object (of polymorphic type) of the wrong type
or to an object not within its lifetime
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct B { virtual ~B(); };
void f() {
  B bs[1];
  B* dp = dynamic_cast<B*>(bs+1);   // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{expr.dynamic.cast.glvalue.lifetime}

\pnum
Evaluating a \keyword{dynamic_cast} on a reference
that denotes an object (of polymorphic type)
of a type that is not similar to the referenced type
or an object not within its lifetime
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct B { virtual ~B(); };
void f() {
  B bs[1];
  B& dr = dynamic_cast<B&>(bs[1]);  // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{expr.static.cast.base.class}

\pnum
A glvalue of type \tcode{B}
can be cast to the type ``reference to \tcode{D}''
if \tcode{B} is a base class of \tcode{D},
otherwise the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct B {};
struct D1 : B {};
struct D2 : B {};

void f() {
  D1 d;
  B &b = d;
  static_cast<D2 &>(b); // undefined behavior, base class object of type \tcode{D1} not \tcode{D2}
}
\end{codeblock}
\end{example}

\ubdescription{expr.static.cast.enum.outside.range}

\pnum
If the enumeration type does not have a fixed underlying type,
the value is unchanged if the original value
is within the range of the enumeration values\iref{dcl.enum},
and otherwise, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
enum A { e1 = 1, e2 };

void f() {
  enum A a = static_cast<A>(4); // undefined behavior, $4$ is not within the range of enumeration values
}
\end{codeblock}
\end{example}

\ubdescription{expr.static.cast.fp.outside.range}

\pnum
An explicit conversion of a floating-point value
that is outside the range of the target type
has undefined behavior.

\pnum
\begin{example}
If \tcode{float} does not adhere to \IsoFloatUndated{}
and cannot represent positive infinity,
a sufficiently large \tcode{double} value will be
outside the (finite) range of \tcode{float}.
\begin{codeblock}
void f() {
  double d = FLT_MAX;
  d *= 16;
  float f = static_cast<float>(d);  // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{expr.static.cast.downcast.wrong.derived.type}

\pnum
Casting from a pointer to a base class to a pointer to a derived class
when there is no enclosing object of that derived class at the
specified location has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct B {};
struct D1 : B {};
struct D2 : B {};

void f() {
  B *bp = new D1;
  static_cast<D2 *>(bp);        // undefined behavior, base class object of type \tcode{D1} not \tcode{D2}
}
\end{codeblock}
\end{example}

\ubdescription{expr.static.cast.does.not.contain.original.member}

\pnum
A
pointer to member of derived class \tcode{D}
can be cast to
a pointer to member of base class \tcode{B}
(with certain restrictions on \cv{} qualifiers)
as long as \tcode{B} contains the original member,
or is a base or derived class of the class
containing the original member;
otherwise the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct A {
  char c;
};

struct B {
  int i;
};

struct D : A, B {};

int main() {
  char D::*p1 = &D::c;
  char B::*p2 = static_cast<char B::*>(p1); // undefined behavior, \tcode{B} does not contain the
                                            // original member \tcode{c}
}
\end{codeblock}
\end{example}

\ubdescription{expr.unary.dereference}

\pnum
Dereferencing a pointer that does not point to an object or function
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
int f()
{
  int *p = nullptr;
  return *p;    // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{expr.new.non.allocating.null}

\pnum
If the allocation function is a non-allocating form\iref{new.delete.placement}
that returns null, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
#include <new>
[[nodiscard]] void* operator new(std::size_t size, void* ptr) noexcept {
  return nullptr;       // undefined behavior, should return non-null pointer
}
\end{codeblock}
\end{example}
\begin{example}
\begin{codeblock}
#include <new>

struct A {
  int x;
};

int main() {
  char *p = nullptr;
  A *a = new (p) A;     // undefined behavior, non-allocating new returning nullptr
}
\end{codeblock}
\end{example}

\ubdescription{expr.delete.mismatch}

\pnum
Using array delete on the result of a single object new expression
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
int* x = new int;
delete[] x;         // undefined behavior, allocated using single object new expression
\end{codeblock}
\end{example}

\ubdescription{expr.delete.array.mismatch}

\pnum
Using single object delete on the result of an array new expression has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
int* x = new int[10];
delete x;           // undefined behavior, allocated using array new expression
\end{codeblock}
\end{example}

\ubdescription{expr.delete.dynamic.type.differ}

\pnum
If the static type of the object to be deleted
is different from its dynamic type
and the selected deallocation function
is not a destroying operator delete,
the static type shall be a base class
of the dynamic type of the object to be deleted
and the static type shall have a virtual destructor
or the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct B {
  int a;
};

struct D : public B {
  int b;
};

void f() {
  B* b = new D;
  delete b;             // undefined behavior, no virtual destructor
}
\end{codeblock}
\end{example}

\ubdescription{expr.delete.dynamic.array.dynamic.type.differ}

\pnum
In an array delete expression,
if the dynamic type of the object to be deleted differs
from its static type, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct B {
  virtual ~B();
  void operator delete[](void*, std::size_t);
};

struct D : B {
  void operator delete[](void*, std::size_t);
};

void f(int i) {
  D* dp = new D[i];
  delete[] dp;          // uses \tcode{D::operator delete[](void*, std::size_t)}
  B* bp = new D[i];
  delete[] bp;          // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{expr.mptr.oper.not.contain.member}

\pnum
Abbreviating \grammarterm{pm-expression}.*\grammarterm{cast-expression}
as \tcode{E1.*E2}, \tcode{E1} is called the object expression.
If the dynamic type of \tcode{E1} does not contain the member
to which \tcode{E2} refers, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct B {};
struct D : B {
  int x;
};

void f() {
  B *b = new B;
  D *d = static_cast<D *>(b);
  int D::*p = &D::x;
  (*d).*p = 1;          // undefined behavior, dynamic type \tcode{B} does not contain \tcode{x}
}
\end{codeblock}
\end{example}

\ubdescription{expr.mptr.oper.member.func.null}

\pnum
If the second operand in a \tcode{.*} expression is the null
member pointer value\iref{conv.mem}, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct S {
  int f();
};

void f() {
  S cs;
  int (S::*pm)() = nullptr;
  (cs.*pm)();           // undefined behavior, the second operand is null
}
\end{codeblock}
\end{example}

\ubdescription{expr.mul.div.by.zero}

\pnum
Division by zero has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
int main() {
  int x = 1 / 0;        // undefined behavior, division by zero
  double d = 1.0 / 0.0; // undefined behavior on systems where the range of
                        // representable values of \tcode{double} is \tcode{[-max,+max]}, on systems where
                        // representable values is \tcode{[-inf,+inf]} this would not be undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{expr.mul.representable.type.result}

\pnum
If the
quotient \tcode{a/b}
is representable in the type of the result,
\tcode{(a/b)*b + a\%b}
is equal to \tcode{a};
otherwise, the behavior of both \tcode{a/b} and \tcode{a\%b} is undefined.

\pnum
\begin{example}
\begin{codeblock}
#include <limits>

int main() {
  int x = std::numeric_limits<int>::min() / -1;
        // Assuming \tcode{LP64} $-2,147,483,648$ which when divided by $-1$
        // gives us $2,147,483,648$ which is not representable by \tcode{int}.
}
\end{codeblock}
\end{example}

\ubdescription{expr.add.out.of.bounds}

\pnum
Creating an out of bounds pointer has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
static const int arrs[10]{};

int main() {
  const int *y = arrs + 11;             // undefined behavior, creating an out of bounds pointer
}
\end{codeblock}
\end{example}

\begin{example}
\begin{codeblock}
static const int arrs[10][10]{};

int main() {
  const int(*y)[10] = arrs + 11;        // undefined behavior, creating an out of bounds pointer
}
\end{codeblock}
\end{example}

\ubdescription{expr.add.sub.diff.pointers}

\pnum
Subtracting pointers that are not part of the same array
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
#include <cstddef>
void f() {
  int x[2];
  int y[2];
  int* p1 = x;
  int* p2 = y;
  std::ptrdiff_t off = p1 - p2; // undefined behavior, \tcode{p1} and \tcode{p2} point to different arrays
}
\end{codeblock}
\end{example}

\ubdescription{expr.add.not.similar}

\pnum
For addition or subtraction of two expressions $P$ and $Q$,
if $P$ or $Q$ have type ``pointer to \cv{}~\tcode{T}'', where \tcode{T} and the array
element type are not similar\iref{conv.rval}, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct S     {  int i;    };
struct T : S {  double d; };

void f(const S* s, std::size_t count) {
  for (const S* end = s + count; s != end; ++s) {
        // undefined behavior, \tcode{T} is not similar to \tcode{S} but \tcode{s} points to a \tcode{T[]}
    @\commentellip@
  }
}

int main() {
  T test[5];
  f(test, 5);
}
\end{codeblock}
\end{example}

\ubdescription{expr.shift.neg.and.width}

\pnum
Shifting by a negative amount or equal or greater than
the bit-width of a type has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
int y = 1 << -1;        // undefined behavior, shift is negative

static_assert(sizeof(int) == 4 && CHAR_BIT == 8);
int y1 = 1 << 32;       // undefined behavior, shift is equal to the bit width of \tcode{int}
int y2 = 1 >> 32;       // undefined behavior, shift is equal to the bit width of \tcode{int}
\end{codeblock}
\end{example}

\ubdescription{expr.assign.overlap}

\pnum
Overlap in the storage between the source and destination
can result in undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
int x = 1;
char* c = reinterpret_cast<char*>(&x);
x = *c;         // undefined behavior, source overlaps storage of destination
\end{codeblock}
\end{example}

\rSec1[ub.stmt]{\ref*{stmt}: Statements}

\ubdescription{stmt.return.flow.off}

\pnum
Flowing off the end of a function other
than \tcode{main} or a coroutine results in undefined behavior if the return type
is not \cv{}~\keyword{void}.

\pnum
\begin{example}
\begin{codeblock}
int f(int x) {
  if (x)
    return 1;
  // undefined behavior if \tcode{x} is \tcode{0}
}

void b() {
  int x = f(0); // undefined behavior, using \tcode{0} as an argument will cause \tcode{f(...)} to flow
                // off the end without a \tcode{return} statement
}
\end{codeblock}
\end{example}

\ubdescription{stmt.return.coroutine.flow.off}

\pnum
Flowing off the end of a coroutine function body
that does not return \tcode{void} has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
#include <cassert>
#include <coroutine>
#include <iostream>
#include <vector>

class resumable {
 public:
  struct promise_type;
  using coro_handle = std::coroutine_handle<promise_type>;
  resumable(coro_handle handle) : handle_(handle) { assert(handle); }
  resumable(resumable&) = delete;
  resumable(resumable&&) = delete;
  bool resume() {
    if (not handle_.done())
      handle_.resume();
    return not handle_.done();
  }
  ~resumable() { handle_.destroy(); }
  const char* return_val();

 private:
  coro_handle handle_;
};

struct resumable::promise_type {
  using coro_handle = std::coroutine_handle<promise_type>;
  const char* string_;
  auto get_return_object() { return coro_handle::from_promise(*this); }
  auto initial_suspend() { return std::suspend_always(); }
  auto final_suspend() noexcept { return std::suspend_always(); }
  void unhandled_exception() { std::terminate(); }
  void return_value(const char* string) { string_ = string; }
};

const char* resumable::return_val() {
  return handle_.promise().string_;
}

resumable foo() {
  std::cout << "Hello" << std::endl;
  co_await std::suspend_always();
  // undefined behavior, falling off the end of coroutine that does not return \tcode{void}
}

int main() {
  resumable res = foo();
  while (res.resume())
    ;
  std::cout << res.return_val() << std::endl;
}
\end{codeblock}
\end{example}

\ubdescription{stmt.dcl.local.static.init.recursive}

\pnum
If control re-enters the declaration recursively while the
variable is being initialized, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
int foo(int i) {
  static int s = foo(2 * i);    // undefined behavior, recursive call
  return i + 1;
}
\end{codeblock}
\end{example}

\rSec1[ub.dcl]{\ref*{dcl}: Declarations}

\ubdescription{dcl.type.cv.modify.const.obj}

\pnum
Any attempt to modify a const object during its lifetime results in
undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
const int* ciq = new const int(3);  // initialized as required
int* iq = const_cast<int*>(ciq);    // cast required
*iq = 4;                            // undefined behavior, modifies a const object
\end{codeblock}
\end{example}

\ubdescription{dcl.type.cv.access.volatile}

\pnum
If an attempt is made to access an object
defined with a volatile-qualified type
through the use of a non-volatile glvalue,
the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
volatile int x = 0;
int& y = const_cast<int&>(x);
std::cout << y;         // undefined behavior, accessing volatile through non-volatile glvalue
\end{codeblock}
\end{example}

\ubdescription{dcl.ref.incompatible.function}

\pnum
Initializing a reference to a function
with a value that is a function
that is not call-compatible\iref{expr.call}
with the type of the reference
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
void f(float x);
void (&g)(int) = reinterpret_cast<void (&)(int)>(f);    // undefined behavior
\end{codeblock}
\end{example}

\ubdescription{dcl.ref.incompatible.type}

\pnum
Initializing a reference to an object
with a value that is not
type-accessible\iref{basic.lval} through
the type of the reference
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
float g;
int& i = reinterpret_cast<int&>(g);     // undefined behavior
\end{codeblock}
\end{example}

\ubdescription{dcl.ref.uninitialized.reference}

\pnum
Evaluating a reference
prior to initializing that
reference has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
extern int &ir1;
int i2 = ir1;   // undefined behavior, \tcode{ir1} not yet initialized
int i3 = 17;
int &ir1 = i3;
\end{codeblock}
\end{example}

\ubdescription{dcl.fct.def.coroutine.resume.not.suspended}

\pnum
Invoking a resumption member function for a coroutine
that is not suspended
results in undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
#include <coroutine>

struct minig {
  struct promise_type {
    int val;
    minig get_return_object() { return {*this}; }
    constexpr suspend_always initial_suspend() noexcept { return {}; }
    constexpr suspend_always final_suspend() noexcept { return {}; }
    constexpr void return_void() noexcept {}
    [[noreturn]] void unhandled_exception() noexcept { throw; }
    suspend_always yield_value(int v) noexcept {
      val = v;
      return {};
    }
  };
  using HDL = coroutine_handle<promise_type>;
  HDL coro;
  minig(promise_type& p) : coro(HDL::from_promise(p)) {}
  ~minig() { coro.destroy(); }
  bool move_next() {
    coro.resume();
    return !coro.done();
  }
  int current_value() { return coro.promise().val; }
};

static minig f(int n) {
  for (int i = 0; i < n; ++i)
    co_yield i;
}

int main() {
  auto g = f(10);
  int sum = 0;
  while (g.move_next())
    sum += g.current_value();

  g.move_next();    // undefined behavior, will call \tcode{coro.resume()} but \tcode{final_suspend}
                    // has already returned, even though it returned \tcode{suspend_always}

  return sum;
}
\end{codeblock}
\end{example}

\ubdescription{dcl.fct.def.coroutine.destroy.not.suspended}

\pnum
Invoking \tcode{destroy()} on a coroutine that is not suspended has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
#include <coroutine>

struct minig {
  struct promise_type {
    int val;
    minig get_return_object() { return {*this}; }
    constexpr suspend_always initial_suspend() noexcept { return {}; }
    constexpr suspend_always final_suspend() noexcept { return {}; }
    constexpr void return_void() noexcept {}
    [[noreturn]] void unhandled_exception() { throw; }
    suspend_always yield_value(int v) noexcept {
      val = v;
      return {};
    }
  };
  using HDL = coroutine_handle<promise_type>;
  HDL coro;
  minig(promise_type& p) : coro(HDL::from_promise(p)) {}
  ~minig() { coro.destroy(); }
  bool move_next() {
    coro.resume();
    return !coro.done();
  }
  int current_value() {
    // this coroutine is not suspended therefore a call to destroy has undefined behavior
    coro.destroy();
    return coro.promise().val;
  }
};

static minig f(int n) {
  for (int i = 0; i < n; ++i)
    co_yield i;
}

int main() {
  auto g = f(10);
  int sum = 0;
  while (g.move_next())
    sum += g.current_value();

  return sum;
}
\end{codeblock}
\end{example}

\ubdescription{dcl.attr.assume.false}

\pnum
If an assumption expression would not evaluate to true at the point where it
appears the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
int g(int x) {
  [[assume(x >= 0)]];
  return x/32;
}

int f() {
  return g(-10);    // undefined behavior, assumption in \tcode{g} is \tcode{false}
}
\end{codeblock}
\end{example}

\ubdescription{dcl.attr.noreturn.eventually.returns}

\pnum
If a function \tcode{f} is called
where \tcode{f} was previously declared
with the \tcode{noreturn} attribute and
\tcode{f} eventually returns,
the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
[[noreturn]] void f(int i) {
  if (i > 0)
    throw "positive";
}

int main() {
  f(0);         // undefined behavior, returns from a \tcode{[[noreturn]]} function
}
\end{codeblock}
\end{example}

\rSec1[ub.class]{\ref*{class}: Classes}

\ubdescription{class.dtor.no.longer.exists}

\pnum
Once a destructor is invoked for an object,
the object's lifetime has ended;
the behavior is undefined if the
destructor is invoked for an object whose lifetime has ended.

\pnum
\begin{example}
\begin{codeblock}
struct A {
  ~A() {}
};

int main() {
  A a;
  a.~A();
}   // undefined behavior, lifetime of \tcode{a} already ended before implicit destructor
\end{codeblock}
\end{example}

\ubdescription{class.abstract.pure.virtual}

\pnum
Calling a pure virtual function from a constructor or destructor
in an abstract class has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct B {
  virtual void f() = 0;
  B() {
    f();        // undefined behavior, \tcode{f} is pure virtual and is being called from the constructor
  }
};

struct D : B {
  void f() override;
};
\end{codeblock}
\end{example}

\ubdescription{class.base.init.mem.fun}

\pnum
Calling a member function
before all the \grammarterm{mem-initializer}s
for base classes have completed
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
class A {
public:
  A(int);
};

class B : public A {
  int j;

public:
  int f();
  B()
      : A(f()),         // undefined behavior, calls member function but base \tcode{A} not yet initialized
        j(f()) {}       // defined, bases are all initialized
};

class C {
public:
  C(int);
};

class D : public B, C {
  int i;

public:
  D()
      : C(f()),         // undefined behavior, calls member function but base \tcode{C} not yet initialized
        i(f()) {}       // defined, bases are all initialized
};
\end{codeblock}
\end{example}

\ubdescription{class.cdtor.before.ctor}

\pnum
For an object with a non-trivial constructor,
referring to any non-static member or base class of the object
before the constructor begins execution results in undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct W {
  int j;
};
struct X : public virtual W {};
struct Y {
  int *p;
  X x;
  Y() : p(&x.j) {       // undefined behavior, \tcode{x} is not yet constructed
  }
};
\end{codeblock}
\end{example}

\ubdescription{class.cdtor.after.dtor}

\pnum
For an object with a non-trivial destructor,
referring to any non-static member or base class of the object
after the destructor finishes execution
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct X {
  int i;
  ~X();     // non-trivial
};
X& g()
{
  static X x;
  return x;
}
void f()
{
  X* px = &g();
  px->~X();
  int j = px->i;    // undefined behavior
}
\end{codeblock}
\end{example}

\ubdescription{class.cdtor.convert.pointer}

\pnum
Converting a pointer to a base class of an object,
when construction of that object has not started
or destruction of that object has finished,
has undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct A { };
struct B : virtual A { };
struct C : B { };
struct D : virtual A { D(A*); };
struct X { X(A*); };

struct E : C, D, X {
  E() : D(this),    // undefined behavior, upcast from \tcode{E*} to \tcode{A*} might use path \tcode{E*} $\rightarrow$ \tcode{D*} $\rightarrow$ \tcode{A*}
                    // but \tcode{D} is not constructed

                    // ``\tcode{D((C*)this)}\!'' would be defined, \tcode{E*} $\rightarrow$ \tcode{C*} is defined because \tcode{E()} has started,
                    // and \tcode{C*} $\rightarrow$ \tcode{A*} is defined because \tcode{C} is fully constructed

  X(this) {}        // defined, upon construction of \tcode{X}, \tcode{C/B/D/A} sublattice is fully constructed
};
\end{codeblock}
\end{example}

\ubdescription{class.cdtor.form.pointer}

\pnum
When forming a pointer to
a direct non-static member of a class,
construction must have started
and destruction must not have finished
otherwise the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct A {
  int i = 0;
};
struct B {
  int *p;
  A a;
  B() : p(&a.i) {}  // undefined behavior
};
\end{codeblock}
\end{example}

\ubdescription{class.cdtor.virtual.not.x}

\pnum
When a virtual function is called directly or indirectly
from a constructor or from a destructor,
including during the construction or destruction
of the class's non-static data members,
and the object to which the call applies
is the object (call it \tcode{x}) under construction or destruction,
the function called is the final overrider in the constructor's or destructor's class
and not one overriding it in a more-derived class.
If the virtual function call uses an explicit class member access\iref{expr.ref}
and the object expression refers to the complete object of \tcode{x}
or one of that object's base class subobjects
but not \tcode{x} or one of its base class subobjects,
the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct V {
  virtual void f();
  virtual void g();
};

struct A : virtual V {
  virtual void f();
};

struct B : virtual V {
  virtual void g();
  B(V *, A *);
};

struct D : A, B {
  virtual void f();
  virtual void g();
  D() : B((A *)this, this) {}
};

B::B(V *v, A *a) {
  f();          // calls \tcode{V::f}, not \tcode{A::f}
  g();          // calls \tcode{B::g}, not \tcode{D::g}
  v->g();       // \tcode{v} is base of \tcode{B}, the call is defined, calls \tcode{B::g}
  a->f();       // undefined behavior, \tcode{a}'s type not a base of \tcode{B}
}
\end{codeblock}
\end{example}

\ubdescription{class.cdtor.typeid}

\pnum
If the operand of \tcode{typeid} refers to
the object under construction or destruction and the static type of the operand is neither the constructor or
destructor's class nor one of its bases, the behavior is undefined.

\pnum
\begin{example}
\begin{codeblock}
struct V {
  virtual void f();
};

struct A : virtual V {};
struct B : virtual V {
  B(V *, A *);
};

struct D : A, B {
  D() : B((A *)this, this) {}
};

B::B(V *v, A *a) {
  typeid(*this);        // \tcode{std::type_info} for \tcode{B}
  typeid(*v);           // defined, \tcode{*v} has type \tcode{V}, a base of \tcode{B} yields \tcode{std::type_info} for \tcode{B}
  typeid(*a);           // undefined behavior, type \tcode{A} not a base of \tcode{B}
}
\end{codeblock}
\end{example}

\ubdescription{class.cdtor.dynamic.cast}

\pnum
If the operand of the \tcode{dynamic_cast}
refers to the object under construction or destruction
and the static type of the operand
is not a pointer to or object of
the constructor or destructor's own class or one of its bases,
the \tcode{dynamic_cast} results in undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
struct V {
  virtual void f();
};

struct A : virtual V {};
struct B : virtual V {
  B(V *, A *);
};

struct D : A, B {
  D() : B((A *)this, this) {}
};

B::B(V *v, A *a) {
  dynamic_cast<B *>(v);         // defined, \tcode{v} of type \tcode{V*}, \tcode{V} base of \tcode{B} results in \tcode{B*}
  dynamic_cast<B *>(a);         // undefined behavior, \tcode{a} has type \tcode{A*}, \tcode{A} not a base of \tcode{B}
}
\end{codeblock}
\end{example}

\rSec1[ub.temp]{\ref*{temp}: Templates}

\ubdescription{temp.inst.inf.recursion}

\pnum
The result of an infinite recursion in template instantiation is undefined.

\pnum
\begin{example}
\begin{codeblock}
template <class T>
class X {
  X<T> *p;      // OK
  X<T *> a;     // implicit instantiation of \tcode{X<T>} requires
                // the implicit instantiation of \tcode{X<T*>} which requires
                // the implicit instantiation of \tcode{X<T**>} which \ldots
};

int main() {
  X<int> x;     // undefined behavior, instantiation will lead to infinite recursion
}
\end{codeblock}
\end{example}

\rSec1[ub.except]{\ref*{except}: Exception handling}

\ubdescription{except.handle.handler.ctor.dtor}

\pnum
Referring to any non-static member or base class of an object
in the handler for a \grammarterm{function-try-block}
of a constructor or destructor for that object
results in undefined behavior.

\pnum
\begin{example}
\begin{codeblock}
#include <iostream>

struct A {
  A() try : x(0 ? 1 : throw 1) {
  } catch (int) {
    std::cout << "y: " << y << std::endl;   // undefined behavior, referring to non-static member \tcode{y} in
                                            // the handler of function-try-block
  }
  int x;
  int y = 42;
};
\end{codeblock}
\end{example}