17 Language support library [support]

17.6 Dynamic memory management [support.dynamic]

17.6.2 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.

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

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[[nodiscard]] void* operator new(std::size_t size);
[[nodiscard]] 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 the first form is called otherwise.

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:

(4.1)
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.
(4.2)
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.
(4.3)
Otherwise, the function calls the current new_handler function. If the called function returns, the loop repeats.
(4.4)
The loop terminates when an attempt to allocate the requested storage is successful or when a called new_handler function does not return.

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[[nodiscard]] void* operator new(std::size_t size, const std::nothrow_t&) noexcept;
[[nodiscard]] 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

]

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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

]

Preconditions: ptr is a null pointer or its value represents 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.

Preconditions: If an implementation has strict pointer safety ([basic.stc.dynamic.safety]) then ptr is a safely-derived pointer.

Preconditions: If the alignment parameter is not present, ptr was returned by an allocation function without an alignment parameter.

If present, the alignment argument is equal to the alignment argument passed to the allocation function that returned ptr.

If present, the size argument is equal to 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> ([cstdlib.syn]).

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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.

Preconditions: If an implementation has strict pointer safety ([basic.stc.dynamic.safety]) then ptr is a safely-derived pointer.

Preconditions: If the alignment parameter is not present, ptr was returned by an allocation function without an alignment parameter.

If present, the alignment argument is equal to the alignment argument passed to the allocation function that returned ptr.

Default behavior: Calls operator delete(ptr), or operator delete(ptr, alignment), respectively.

17.6.2.2 Array forms [new.delete.array]

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[[nodiscard]] void* operator new[](std::size_t size);
[[nodiscard]] 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 the first form is called otherwise.214

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.

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[[nodiscard]] void* operator new[](std::size_t size, const std::nothrow_t&) noexcept;
[[nodiscard]] void* operator new[](std::size_t size, std::align_val_t alignment,
                                   const std::nothrow_t&) noexcept;

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.

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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

]

Preconditions: ptr is a null pointer or its value represents 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[].

Preconditions: If an implementation has strict pointer safety ([basic.stc.dynamic.safety]) then ptr is a safely-derived pointer.

Preconditions: If the alignment parameter is not present, ptr was returned by an allocation function without an alignment parameter.

If present, the alignment argument is equal to the alignment argument passed to the allocation function that returned ptr.

If present, the size argument is equal to 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.

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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.

Preconditions: If an implementation has strict pointer safety ([basic.stc.dynamic.safety]) then ptr is a safely-derived pointer.

Preconditions: If the alignment parameter is not present, ptr was returned by an allocation function without an alignment parameter.

If present, the alignment argument is equal to the alignment argument passed to the allocation function that returned ptr.

Default behavior: Calls operator delete[](ptr), or operator delete[](ptr, alignment), respectively.

214)

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.

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17.6.2.3 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.

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[[nodiscard]] 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

]

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[[nodiscard]] void* operator new[](std::size_t size, void* ptr) noexcept;

Returns: ptr.

Remarks: Intentionally performs no other action.

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void operator delete(void* ptr, void*) noexcept;

Effects: Intentionally performs no action.

Preconditions: If an implementation has strict pointer safety ([basic.stc.dynamic.safety]) then ptr is 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]).

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void operator delete[](void* ptr, void*) noexcept;

Effects: Intentionally performs no action.

Preconditions: If an implementation has strict pointer safety ([basic.stc.dynamic.safety]) then ptr is 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]).

17.6.2.4 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.