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 shall be 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.
The allocation function attempts to allocate the requested amount of storage. If it is successful, it shall return the address of the start of a block of storage whose length in bytes shall be at least as large as the requested size. There are no constraints on the contents of the allocated storage on return from the allocation function. The order, contiguity, and initial value of storage allocated by successive calls to an allocation function are unspecified. The pointer returned shall be suitably aligned so that it can be converted to a pointer to any suitable complete object type ([new.delete.single]) and then used to access the object or array in the storage allocated (until the storage is explicitly deallocated by a call to a corresponding deallocation function). Even if the size of the space requested is zero, the request can fail. If the request succeeds, the value returned shall be a non-null pointer value ([conv.ptr]) p0 different from any previously returned value p1, unless that value p1 was subsequently passed to an operator delete. Furthermore, for the library allocation functions in [new.delete.single] and [new.delete.array], p0 shall represent 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 as a request for zero size is undefined.38
An allocation function that fails to allocate storage can invoke the currently installed new-handler function ([new.handler]), if any. [ Note: A program-supplied allocation function can obtain the address of the currently installed new_handler using the std::get_new_handler function ([set.new.handler]). — end note ] If an allocation function that has a non-throwing exception specification ([except.spec]) fails to allocate storage, it shall return a null pointer. Any other allocation function that fails to allocate storage shall indicate 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 ([expr.new]), or called directly using the function call syntax ([expr.call]), or called indirectly through calls to the functions in the C++ standard library. [ Note: In particular, a global allocation function is not called to allocate storage for objects with static storage duration ([basic.stc.static]), for objects or references with thread storage duration ([basic.stc.thread]), for objects of type std::type_info ([expr.typeid]), or for an exception object ([except.throw]). — 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.