6 Basics [basic]

6.7 Memory and objects [basic.memobj]

6.7.5 Storage duration [basic.stc] Dynamic storage duration [basic.stc.dynamic] Allocation functions [basic.stc.dynamic.allocation]

An allocation function shall be a class member function or a global function; a program is ill-formed if an allocation function is declared in a namespace scope other than global scope or declared static in global scope.
The return type shall be void*.
The first parameter shall have type std​::​size_­t ([support.types]).
The first parameter shall not have an associated default argument ([dcl.fct.default]).
The value of the first parameter is interpreted as the requested size of the allocation.
An allocation function can be a function template.
Such a template shall declare its return type and first parameter as specified above (that is, template parameter types shall not be used in the return type and first parameter type).
Template allocation functions shall have two or more parameters.
An allocation function attempts to allocate the requested amount of storage.
If it is successful, it returns the address of the start of a block of storage whose length in bytes is at least as large as the requested size.
The order, contiguity, and initial value of storage allocated by successive calls to an allocation function are unspecified.
Even if the size of the space requested is zero, the request can fail.
If the request succeeds, the value returned by a replaceable allocation function is a non-null pointer value ([basic.compound]) p0 different from any previously returned value p1, unless that value p1 was subsequently passed to a replaceable deallocation function.
Furthermore, for the library allocation functions in [new.delete.single] and [new.delete.array], p0 represents the address of a block of storage disjoint from the storage for any other object accessible to the caller.
The effect of indirecting through a pointer returned from a request for zero size is undefined.36
For an allocation function other than a reserved placement allocation function ([new.delete.placement]), the pointer returned on a successful call shall represent the address of storage that is aligned as follows:
  • If the allocation function takes an argument of type std​::​align_­val_­t, the storage will have the alignment specified by the value of this argument.
  • Otherwise, if the allocation function is named operator new[], the storage is aligned for any object that does not have new-extended alignment ([basic.align]) and is no larger than the requested size.
  • Otherwise, the storage is aligned for any object that does not have new-extended alignment and is of the requested size.
An allocation function that fails to allocate storage can invoke the currently installed new-handler function ([new.handler]), if any.
[Note 1:
A program-supplied allocation function can obtain the address of the currently installed new_­handler using the std​::​get_­new_­handler function ([get.new.handler]).
— end note]
An allocation function that has a non-throwing exception specification ([except.spec]) indicates failure by returning a null pointer value.
Any other allocation function never returns a null pointer value and indicates failure only by throwing an exception ([except.throw]) of a type that would match a handler ([except.handle]) of type std​::​bad_­alloc ([bad.alloc]).
A global allocation function is only called as the result of a new expression, or called directly using the function call syntax, or called indirectly to allocate storage for a coroutine state ([dcl.fct.def.coroutine]), or called indirectly through calls to the functions in the C++ standard library.
[Note 2:
In particular, a global allocation function is not called to allocate storage for objects with static storage duration, for objects or references with thread storage duration, for objects of type std​::​type_­info, or for an exception object.
— end note]
The intent is to have operator new() implementable by calling std​::​malloc() or std​::​calloc(), so the rules are substantially the same.
C++ differs from C in requiring a zero request to return a non-null pointer.