对每个可重载的运算符来讲，它既可以重载为友元函数，又可以重载

Applications can define operator new and operator delete functions in the global scope and/or as member functions. When the compiler sees a new or delete expression, it looks for the corresponding operator function to call. If the object being allocated (deallocated) has class type, the compiler first looks in the scope of the class, including any base classes. If the class has a member operator new or operator delete, that function is used by the new or delete expression. Otherwise, the compiler looks for a matching function in the global scope. If the compiler finds a user-defined version, it uses that function to execute the new or delete expression. Otherwise, the standard library version is used. 

When we define the global operator new and operator delete functions, we take over responsibility for all dynamic memory allocation. These functions must be correct: They form a vital part of all processing in the program.