Deallocation functions shall be class member functions or global functions; a program is ill-formed if deallocation functions are declared in a namespace scope other than global scope or declared static in global scope.
Each deallocation function shall return void and its first parameter shall be void*. A deallocation function can have more than one parameter. The global operator delete with exactly one parameter is a usual (non-placement) deallocation function. The global operator delete with exactly two parameters, the second of which has type std::size_t, is a usual deallocation function. Similarly, the global operator delete[] with exactly one parameter is a usual deallocation function. The global operator delete[] with exactly two parameters, the second of which has type std::size_t, is a usual deallocation function.37 If a class T has a member deallocation function named operator delete with exactly one parameter, then that function is a usual deallocation function. If class T does not declare such an operator delete but does declare a member deallocation function named operator delete with exactly two parameters, the second of which has type std::size_t, then this function is a usual deallocation function. Similarly, if a class T has a member deallocation function named operator delete[] with exactly one parameter, then that function is a usual (non-placement) deallocation function. If class T does not declare such an operator delete[] but does declare a member deallocation function named operator delete[] with exactly two parameters, the second of which has type std::size_t, then this function is a usual deallocation function. A deallocation function can be an instance of a function template. Neither the first parameter nor the return type shall depend on a template parameter. [ Note: That is, a deallocation function template shall have a first parameter of type void* and a return type of void (as specified above). — end note ] A deallocation function template shall have two or more function parameters. A template instance is never a usual deallocation function, regardless of its signature.
If a deallocation function terminates by throwing an exception, the behavior is undefined. The value of the first argument supplied to a deallocation function may be a null pointer value; if so, and if the deallocation function is one supplied in the standard library, the call has no effect. Otherwise, the behavior is undefined if the value supplied to operator delete(void*) in the standard library is not one of the values returned by a previous invocation of either operator new(std::size_t) or operator new(std::size_t, const std::nothrow_t&) in the standard library, and the behavior is undefined if the value supplied to operator delete[](void*) in the standard library is not one of the values returned by a previous invocation of either operator new[](std::size_t) or operator new[](std::size_t, const std::nothrow_t&) in the standard library.
If the argument given to a deallocation function in the standard library is a pointer that is not the null pointer value ([conv.ptr]), the deallocation function shall deallocate the storage referenced by the pointer, rendering invalid all pointers referring to any part of the deallocated storage. Indirection through an invalid pointer value and passing an invalid pointer value to a deallocation function have undefined behavior. Any other use of an invalid pointer value has implementation-defined behavior.38
This deallocation function precludes use of an allocation function void operator new(std::size_t, std::size_t) as a placement allocation function ([diff.cpp11.basic]).
Some implementations might define that copying an invalid pointer value causes a system-generated runtime fault.