Replaceable allocation functions
(1) (2) (3) (since C++17) (4) (since C++17)Replaceable non-throwing allocation functions
(5) (noexcept since C++11) (6) (noexcept since C++11) (7) (since C++17) (8) (since C++17)Non-allocating placement allocation functions
(9) (noexcept since C++11)User-defined placement allocation functions
void* operator new ( std::size_t count, /* args... */ ); (11) void* operator new[]( std::size_t count, /* args... */ ); (12) (13) (since C++17) (14) (since C++17)Class-specific allocation functions
(15) (16) (17) (since C++17) (18) (since C++17)Class-specific placement allocation functions
void* T::operator new ( std::size_t count, /* args... */ ); (19) void* T::operator new[]( std::size_t count, /* args... */ ); (20) (21) (since C++17) (22) (since C++17)Attempts to allocate requested number of bytes, and the allocation request can fail (even if the requested number of bytes is zero). These allocation functions are called by new expressions to allocate memory in which new object would then be initialized. They may also be called using regular function call syntax.
1-8) Replaceableallocation functions. The standard library provides default implementations for these functions, for the effects of the default implementations, see
below.
If this function is called through placement new and ptr is a null pointer, the behavior is undefined.
11-22) User-defined allocation functions called by new expressions.
Overloads (1-4) are implicitly declared in each translation unit even if the <new> header is not included.
See new expression for the criteria of selecting overload.
[edit] Parameters count - number of bytes to allocate ptr - pointer to a memory area to initialize the object at tag - disambiguation tag used to select non-throwing overloads al - alignment to use, invalid value leads to undefined behavior [edit] Return value 1-4)If the allocation succeeds, a non-null pointer
p0which points to suitably aligned memory of size at least
sizeand is different from any previously returned value
p1, unless that value
p1was subsequently passed to a replaceable
deallocation function; if the allocation fails, does not return (an exception is thrown, see below).
5-8)Same as
(1-4), but returns a null pointer if the allocation fails.
9,10) ptr
11-22)Same as
(1-4)if the function does not return on allocation failure, otherwise same as
(5-8).
[edit] Exceptions 1-4)Throws an exception of a type that would match a handler of type
std::bad_allocon failure to allocate memory.
11-22)Same as
(1-4)if the function does not return on allocation failure, otherwise same as
(5-8).
[edit] Global replacementsOverloads (1-8) are replaceable. The effects of the default versions are:
1)Attempts to allocate the requested storage. Whether the attempt involves a call to
std::mallocor
std::aligned_allocis unspecified.
2) Returns operator new(count).
3) Same as (1).
4) Returns operator new(count, al).
5-8)Calls
(1-4)respectively with the same arguments except for
tag.
On freestanding implementations, it is implementation-defined whether the default versions of (1-8) satisfy the behaviors required above. Freestanding implementations are recommended that if any of these default versions meet the requirements of a hosted implementation, they all should.
(since C++26)Global operators new/delete replacement:
#include <cstdio>
#include <cstdlib>
#include <new>
// no inline, required by [replacement.functions]/3
void* operator new(std::size_t sz)
{
std::printf("1) new(size_t), size = %zu\n", sz);
if (sz == 0)
++sz; // avoid std::malloc(0) which may return nullptr on success
if (void *ptr = std::malloc(sz))
return ptr;
throw std::bad_alloc{}; // required by [new.delete.single]/3
}
// no inline, required by [replacement.functions]/3
void* operator new[](std::size_t sz)
{
std::printf("2) new[](size_t), size = %zu\n", sz);
if (sz == 0)
++sz; // avoid std::malloc(0) which may return nullptr on success
if (void *ptr = std::malloc(sz))
return ptr;
throw std::bad_alloc{}; // required by [new.delete.single]/3
}
void operator delete(void* ptr) noexcept
{
std::puts("3) delete(void*)");
std::free(ptr);
}
void operator delete(void* ptr, std::size_t size) noexcept
{
std::printf("4) delete(void*, size_t), size = %zu\n", size);
std::free(ptr);
}
void operator delete[](void* ptr) noexcept
{
std::puts("5) delete[](void* ptr)");
std::free(ptr);
}
void operator delete[](void* ptr, std::size_t size) noexcept
{
std::printf("6) delete[](void*, size_t), size = %zu\n", size);
std::free(ptr);
}
int main()
{
int* p1 = new int;
delete p1;
int* p2 = new int[10]; // guaranteed to call the replacement in C++11
delete[] p2;
}
Possible output:
// Compiled with GCC-5 in C++17 mode to obtain the following: 1) op new(size_t), size = 4 4) op delete(void*, size_t), size = 4 2) op new[](size_t), size = 40 5) op delete[](void* ptr)
Overloads of operator new and operator new[] with additional user-defined parameters ("placement forms", versions (11-14)) may be declared at global scope as usual, and are called by the matching placement forms of new expressions.
The standard library's non-allocating placement forms of operator new (9,10) cannot be replaced and can only be customized if the placement new expression did not use the ::new syntax, by providing a class-specific placement new (19,20) with matching signature: void* T::operator new(std::size_t, void*) or void* T::operator new[](std::size_t, void*).
The placement form void* operator new(std::size_t, std::size_t) is not allowed because the matching signature of the deallocation function, void operator delete(void*, std::size_t), is a usual (not placement) deallocation function.
(since C++14) [edit] Class-specific overloadsBoth single-object and array allocation functions may be defined as public static member functions of a class (versions (15-18)). If defined, these allocation functions are called by new expressions to allocate memory for single objects and arrays of this class, unless the new expression used the form ::new which bypasses class-scope lookup. The keyword static is optional for these functions: whether used or not, the allocation function is a static member function.
The new expression looks for appropriate allocation function's name firstly in the class scope, and after that in the global scope. Note, that as per name lookup rules, any allocation functions declared in class scope hides all global allocation functions for the new expressions that attempt to allocate objects of this class.
When allocating objects and arrays of objects whose alignment exceeds __STDCPP_DEFAULT_NEW_ALIGNMENT__, overload resolution is performed twice: first, for alignment-aware function signatures, then for alignment-unaware function signatures. This means that if a class with extended alignment has an alignment-unaware class-specific allocation function, it is the function that will be called, not the global alignment-aware allocation function. This is intentional: the class member is expected to know best how to handle that class.
(since C++17)When allocating objects and arrays of objects whose alignment does not exceed __STDCPP_DEFAULT_NEW_ALIGNMENT__, overload resolution is performed twice: first, for alignment-unaware function signatures, then for alignment-aware function signatures.
(since C++20)#include <cstddef>
#include <iostream>
// class-specific allocation functions
struct X
{
static void* operator new(std::size_t count)
{
std::cout << "custom new for size " << count << '\n';
return ::operator new(count);
}
static void* operator new[](std::size_t count)
{
std::cout << "custom new[] for size " << count << '\n';
return ::operator new[](count);
}
};
int main()
{
X* p1 = new X;
delete p1;
X* p2 = new X[10];
delete[] p2;
}
Possible output:
custom new for size 1 custom new[] for size 10
Overloads of operator new and operator new[] with additional user-defined parameters ("placement forms"), may also be defined as class members (19-22)). When the placement new expression with the matching signature looks for the corresponding allocation function to call, it begins at class scope before examining the global scope, and if the class-specific placement new is provided, it is called.
When allocating objects and arrays of objects whose alignment exceeds __STDCPP_DEFAULT_NEW_ALIGNMENT__, overload resolution for placement forms is performed twice just as for regular forms: first, for alignment-aware function signatures, then for alignment-unaware function signatures.
(since C++17)When allocating objects and arrays of objects whose alignment does not exceed __STDCPP_DEFAULT_NEW_ALIGNMENT__, overload resolution for placement forms is performed twice just as for regular forms: first, for alignment-unaware function signatures, then for alignment-aware function signatures.
(since C++20)#include <cstddef>
#include <iostream>
#include <stdexcept>
struct X
{
X() { throw std::runtime_error(""); }
// custom placement new
static void* operator new(std::size_t count, bool b)
{
std::cout << "custom placement new called, b = " << b << '\n';
return ::operator new(count);
}
// custom placement delete
static void operator delete(void* ptr, bool b)
{
std::cout << "custom placement delete called, b = " << b << '\n';
::operator delete(ptr);
}
};
int main()
{
try
{
[[maybe_unused]] X* p1 = new (true) X;
}
catch (const std::exception&)
{}
}
Output:
custom placement new called, b = 1 custom placement delete called, b = 1
If class-level operator new is a template function, it must have the return type of void*, the first argument std::size_t, and it must have two or more parameters. In other words, only placement forms can be templates.
Even though the non-allocating placement new (9,10) cannot be replaced, a function with the same signature may be defined at class scope as described above. In addition, global overloads that look like placement new but take a non-void pointer type as the second argument are allowed, so the code that wants to ensure that the true placement new is called (e.g. std::allocator::construct), must use ::new and also cast the pointer to void*.
If the behavior of a deallocation function does not satisfy the default constraints, the behavior is undefined.
It is unspecified whether library versions of operator new make any calls to std::malloc or std::aligned_alloc(since C++17).
For loading a large file, file mapping via OS-specific functions, e.g., mmap on POSIX or CreateFileMapping(A/W) along with MapViewOfFile on Windows, is preferable to allocating a buffer for file reading.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior CWG 521 C++98 any class derived from std::bad_alloc could be thrown,RetroSearch is an open source project built by @garambo | Open a GitHub Issue
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