operator

 

operator 是 C++ 一个重载操作符的关键字，但是 operator 除了可以重载操作符以外，还可以用做对两个不同类的转换。

具体使用如下：

#include<iostream>
using namespace std;

struct X
{
    X(){
        num = 10;
    }

    // implicit conversion
    operator int() {
        return 7;
    }

    // explicit conversion
    explicit operator int*() {
        return &num;
        // return nullptr;
    }

    // it's ok through typedef
    using arr_t = int[3];
    operator arr_t*() const {
        return nullptr;
    }

    // Error: array operator not allowed in conversion-type-id
    // operator int(*)[3]() const { return nullptr; }

    // Error: conversion to array not allowed in any case
    // operator arr_t() const

    int num;
    arr_t* arr;
};

struct To
{
    To() = default;
    To(const struct From&) {
        cout << "const struct From&" << endl;
    }
};

struct From
{
    operator To() const {
        cout << "operator To() in class From" << endl;
        return To();
    }
};

int main()
{
    X x;
    int n = static_cast<int>(x);
    int m = n; 
    cout << "n : " << n << endl;
    cout << "m : " << m << endl;

    int *p = static_cast<int*> (x);
    cout << "*p : " << *p << endl;


    int (*pa)[3] = x;
    // ? how to reurn a pointer that is a type of int[3]

    From f;
    To t1(f);   // direct-initialization: calls the constructor

    To t3 = static_cast<To> (f);    // direct-initialization: calls the constructor
    const To& r = f;    // reference-initialization
}

 

通过初始化的方式，直接初始化或者拷贝初始化，将一个类转化为另一个类。