I define a class to wrap raw pointer, so the pointer can be delete automatically. Here is my code

#include <iostream>
class Entity
{
private:
    std::string e_name;
public:
    Entity():e_name("unknown"){}
    void whoami() {
        std::cout << e_name;
    }
};
class ScopePointer
{
private:
    Entity *_e;
public:
    ScopePointer(Entity *e) : _e(e) {}
    ~ScopePointer() { delete _e; }
    Entity *operator->() { return _e; }
};

int main(int argc, char *argv[])
{
    ScopePointer p(new Entity());
    p->whoami();
}

It works well but I quite don't understand how p->whoami(); works. I mean the operator -> overload in ScopePointer should work like below

p.operator->()->whoami();or p->->whoami();

the first -> should return Entity* and the next -> call class method. why only one -> can make it work ?

CodePudding user response：

I hope that this quote from the C 14 Standard will help to understand how the operator works.

The C 14 Standard (13.5.6 Class member access, p #1)

An expression x->m is interpreted as (x.operator->())->m for a class object x of type T if T::operator->() exists and if the operator is selected as the best match function by the overload resolution mechanism.

So the overloaded operator in fact is a "prefix expression" of the built-in operator-> that supplies a pointer used in the built-in operator.

CodePudding user response：

I quite don't understand how p->whoami(); works

There is an exception to the ordinary rules of operators for ->, such that you only need one ->. The language requires the implementation recursively evaluate operator-> until it finds a pointer object, instead of a object of some class type.

From [over.ref]

A class member access operator function is a function named operator-> that is a non-static member function taking no parameters. For an expression of the form

postfix-expression->template_optid-expression

the operator function is selected by overload resolution ([over.match.oper]), and the expression is interpreted as

(postfix-expression.operator->() ) -> template_opt id-expression