21 Language support library [language.support]

21.6 Dynamic memory management [support.dynamic]

21.6.2 Storage allocation and deallocation [new.delete]

Except where otherwise specified, the provisions of [basic.stc.dynamic] apply to the library versions of operator new and operator delete.
If the value of an alignment argument passed to any of these functions is not a valid alignment value, the behavior is undefined.

21.6.2.1 Single-object forms [new.delete.single]

void* operator new(std::size_t size); void* operator new(std::size_t size, std::align_val_t alignment);
Effects: The allocation functions called by a new-expression to allocate size bytes of storage.
The second form is called for a type with new-extended alignment, and allocates storage with the specified alignment.
The first form is called otherwise, and allocates storage suitably aligned to represent any object of that size provided the object's type does not have new-extended alignment.
Replaceable: A C++ program may define functions with either of these function signatures, and thereby displace the default versions defined by the C++ standard library.
Required behavior: Return a non-null pointer to suitably aligned storage ([basic.stc.dynamic]), or else throw a bad_­alloc exception.
This requirement is binding on any replacement versions of these functions.
Default behavior:
  • Executes a loop: Within the loop, the function first attempts to allocate the requested storage.
    Whether the attempt involves a call to the C standard library functions malloc or aligned_­alloc is unspecified.
  • Returns a pointer to the allocated storage if the attempt is successful.
    Otherwise, if the current new_­handler ([get.new.handler]) is a null pointer value, throws bad_­alloc.
  • Otherwise, the function calls the current new_­handler function.
    If the called function returns, the loop repeats.
  • The loop terminates when an attempt to allocate the requested storage is successful or when a called new_­handler function does not return.
void* operator new(std::size_t size, const std::nothrow_t&) noexcept; void* operator new(std::size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept;
Effects: Same as above, except that these are called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a bad_­alloc exception.
Replaceable: A C++ program may define functions with either of these function signatures, and thereby displace the default versions defined by the C++ standard library.
Required behavior: Return a non-null pointer to suitably aligned storage ([basic.stc.dynamic]), or else return a null pointer.
Each of these nothrow versions of operator new returns a pointer obtained as if acquired from the (possibly replaced) corresponding non-placement function.
This requirement is binding on any replacement versions of these functions.
Default behavior: Calls operator new(size), or operator new(size, alignment), respectively.
If the call returns normally, returns the result of that call.
Otherwise, returns a null pointer.
[Example
:
T* p1 = new T;                  // throws bad_­alloc if it fails
T* p2 = new(nothrow) T;         // returns nullptr if it fails
end example
]
void operator delete(void* ptr) noexcept; void operator delete(void* ptr, std::size_t size) noexcept; void operator delete(void* ptr, std::align_val_t alignment) noexcept; void operator delete(void* ptr, std::size_t size, std::align_val_t alignment) noexcept;
Effects: The deallocation functions called by a delete-expression to render the value of ptr invalid.
Replaceable: A C++ program may define functions with any of these function signatures, and thereby displace the default versions defined by the C++ standard library.
If a function without a size parameter is defined, the program should also define the corresponding function with a size parameter.
If a function with a size parameter is defined, the program shall also define the corresponding version without the size parameter.
[Note
:
The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function.
end note
]
Requires: ptr shall be a null pointer or its value shall represent the address of a block of memory allocated by an earlier call to a (possibly replaced) operator new(std​::​size_­t) or operator new(std​::​size_­t, std​::​align_­val_­t) which has not been invalidated by an intervening call to operator delete.
Requires: If an implementation has strict pointer safety then ptr shall be a safely-derived pointer.
Requires: If the alignment parameter is not present, ptr shall have been returned by an allocation function without an alignment parameter.
If present, the alignment argument shall equal the alignment argument passed to the allocation function that returned ptr.
If present, the size argument shall equal the size argument passed to the allocation function that returned ptr.
Required behavior: A call to an operator delete with a size parameter may be changed to a call to the corresponding operator delete without a size parameter, without affecting memory allocation.
[Note
:
A conforming implementation is for operator delete(void* ptr, std​::​size_­t size) to simply call operator delete(ptr).
end note
]
Default behavior: The functions that have a size parameter forward their other parameters to the corresponding function without a size parameter.
[Note
:
See the note in the above Replaceable: paragraph.
end note
]
Default behavior: If ptr is null, does nothing.
Otherwise, reclaims the storage allocated by the earlier call to operator new.
Remarks: It is unspecified under what conditions part or all of such reclaimed storage will be allocated by subsequent calls to operator new or any of aligned_­alloc, calloc, malloc, or realloc, declared in <cstdlib>.
void operator delete(void* ptr, const std::nothrow_t&) noexcept; void operator delete(void* ptr, std::align_val_t alignment, const std::nothrow_t&) noexcept;
Effects: The deallocation functions called by the implementation to render the value of ptr invalid when the constructor invoked from a nothrow placement version of the new-expression throws an exception.
Replaceable: A C++ program may define functions with either of these function signatures, and thereby displace the default versions defined by the C++ standard library.
Requires: ptr shall be a null pointer or its value shall represent the address of a block of memory allocated by an earlier call to a (possibly replaced) operator new(std​::​size_­t) or operator new(std​::​size_­t, std​::​align_­val_­t) which has not been invalidated by an intervening call to operator delete.
Requires: If an implementation has strict pointer safety then ptr shall be a safely-derived pointer.
Requires: If the alignment parameter is not present, ptr shall have been returned by an allocation function without an alignment parameter.
If present, the alignment argument shall equal the alignment argument passed to the allocation function that returned ptr.
Default behavior: Calls operator delete(ptr), or operator delete(ptr, alignment), respectively.

21.6.2.2 Array forms [new.delete.array]

void* operator new[](std::size_t size); void* operator new[](std::size_t size, std::align_val_t alignment);
Effects: The allocation functions called by the array form of a new-expression to allocate size bytes of storage.
The second form is called for a type with new-extended alignment, and allocates storage with the specified alignment.
The first form is called otherwise, and allocates storage suitably aligned to represent any array object of that size or smaller, provided the object's type does not have new-extended alignment.219
Replaceable: A C++ program may define functions with either of these function signatures, and thereby displace the default versions defined by the C++ standard library.
Required behavior: Same as for the corresponding single-object forms.
This requirement is binding on any replacement versions of these functions.
Default behavior: Returns operator new(size), or operator new(size, alignment), respectively.
void* operator new[](std::size_t size, const std::nothrow_t&) noexcept; void* operator new[](std::size_t size, std::align_val_t alignment, const std::nothrow_t&) noexcept;
Effects: Same as above, except that these are called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a bad_­alloc exception.
Replaceable: A C++ program may define functions with either of these function signatures, and thereby displace the default versions defined by the C++ standard library.
Required behavior: Return a non-null pointer to suitably aligned storage ([basic.stc.dynamic]), or else return a null pointer.
Each of these nothrow versions of operator new[] returns a pointer obtained as if acquired from the (possibly replaced) corresponding non-placement function.
This requirement is binding on any replacement versions of these functions.
Default behavior: Calls operator new[](size), or operator new[](size, alignment), respectively.
If the call returns normally, returns the result of that call.
Otherwise, returns a null pointer.
void operator delete[](void* ptr) noexcept; void operator delete[](void* ptr, std::size_t size) noexcept; void operator delete[](void* ptr, std::align_val_t alignment) noexcept; void operator delete[](void* ptr, std::size_t size, std::align_val_t alignment) noexcept;
Effects: The deallocation functions called by the array form of a delete-expression to render the value of ptr invalid.
Replaceable: A C++ program may define functions with any of these function signatures, and thereby displace the default versions defined by the C++ standard library.
If a function without a size parameter is defined, the program should also define the corresponding function with a size parameter.
If a function with a size parameter is defined, the program shall also define the corresponding version without the size parameter.
[Note
:
The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function.
end note
]
Requires: ptr shall be a null pointer or its value shall represent the address of a block of memory allocated by an earlier call to a (possibly replaced) operator new[](std​::​size_­t) or operator new[](std​::​size_­t, std​::​align_­val_­t) which has not been invalidated by an intervening call to operator delete[].
Requires: If an implementation has strict pointer safety then ptr shall be a safely-derived pointer.
Requires: If the alignment parameter is not present, ptr shall have been returned by an allocation function without an alignment parameter.
If present, the alignment argument shall equal the alignment argument passed to the allocation function that returned ptr.
If present, the size argument shall equal the size argument passed to the allocation function that returned ptr.
Required behavior: A call to an operator delete[] with a size parameter may be changed to a call to the corresponding operator delete[] without a size parameter, without affecting memory allocation.
[Note
:
A conforming implementation is for operator delete[](void* ptr, std​::​size_­t size) to simply call operator delete[](ptr).
end note
]
Default behavior: The functions that have a size parameter forward their other parameters to the corresponding function without a size parameter.
The functions that do not have a size parameter forward their parameters to the corresponding operator delete (single-object) function.
void operator delete[](void* ptr, const std::nothrow_t&) noexcept; void operator delete[](void* ptr, std::align_val_t alignment, const std::nothrow_t&) noexcept;
Effects: The deallocation functions called by the implementation to render the value of ptr invalid when the constructor invoked from a nothrow placement version of the array new-expression throws an exception.
Replaceable: A C++ program may define functions with either of these function signatures, and thereby displace the default versions defined by the C++ standard library.
Requires: ptr shall be a null pointer or its value shall represent the address of a block of memory allocated by an earlier call to a (possibly replaced) operator new[](std​::​size_­t) or operator new[](std​::​size_­t, std​::​align_­val_­t) which has not been invalidated by an intervening call to operator delete[].
Requires: If an implementation has strict pointer safety then ptr shall be a safely-derived pointer.
Requires: If the alignment parameter is not present, ptr shall have been returned by an allocation function without an alignment parameter.
If present, the alignment argument shall equal the alignment argument passed to the allocation function that returned ptr.
Default behavior: Calls operator delete[](ptr), or operator delete[](ptr, alignment), respectively.
It is not the direct responsibility of operator new[] or operator delete[] to note the repetition count or element size of the array.
Those operations are performed elsewhere in the array new and delete expressions.
The array new expression, may, however, increase the size argument to operator new[] to obtain space to store supplemental information.

21.6.2.3 Non-allocating forms [new.delete.placement]

These functions are reserved; a C++ program may not define functions that displace the versions in the C++ standard library ([constraints]).
The provisions of [basic.stc.dynamic] do not apply to these reserved placement forms of operator new and operator delete.
void* operator new(std::size_t size, void* ptr) noexcept;
Returns: ptr.
Remarks: Intentionally performs no other action.
[Example
:
This can be useful for constructing an object at a known address:
void* place = operator new(sizeof(Something));
Something* p = new (place) Something();
end example
]
void* operator new[](std::size_t size, void* ptr) noexcept;
Returns: ptr.
Remarks: Intentionally performs no other action.
void operator delete(void* ptr, void*) noexcept;
Effects: Intentionally performs no action.
Requires: If an implementation has strict pointer safety then ptr shall be a safely-derived pointer.
Remarks: Default function called when any part of the initialization in a placement new-expression that invokes the library's non-array placement operator new terminates by throwing an exception ([expr.new]).
void operator delete[](void* ptr, void*) noexcept;
Effects: Intentionally performs no action.
Requires: If an implementation has strict pointer safety then ptr shall be a safely-derived pointer.
Remarks: Default function called when any part of the initialization in a placement new-expression that invokes the library's array placement operator new terminates by throwing an exception ([expr.new]).

21.6.2.4 Data races [new.delete.dataraces]

For purposes of determining the existence of data races, the library versions of operator new, user replacement versions of global operator new, the C standard library functions aligned_­alloc, calloc, and malloc, the library versions of operator delete, user replacement versions of operator delete, the C standard library function free, and the C standard library function realloc shall not introduce a data race ([res.on.data.races]).
Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.