“内联运算符T *（）const”是什么意思？

Question

I reviewed some codes on nullptr and found that:

const
class nullptr_t
{
public:
    template<class T>
    inline operator T*() const
        { return 0; }

    template<class C, class T>
    inline operator T C::*() const
        { return 0; }

private:
    void operator&() const;
} nullptr = {};

what do inline operator T*() const and inline operator TC::*() const mean?

Do they work the same way as inline T operator *() const or inline T operator C::*() const ?

Why not specify the return type in the declaration?

Answer 1

They are user defined conversion operator functions.

A simpler function:

struct Foo
{
   inline operator int () const { return 0; }
};

Given that you can use

Foo f;
int i = f; // Invokes f.operator int () and initializes i with
           // return value.

For you class, which is an anonymous class for nullptr , what it means is that it can be converted to any pointer or member function pointer. You can use:

int* ip = nullptr;

It uses the return value of the first user defined conversion operator function to initialize ip .

struct Bar
{
   void bar() {}
};

void (Bar::*ptr)() = nullptr;

It uses the return value of the second user defined conversion operator function to initialize ptr .