c++ - 隐式转换中的隐式参数

Question

#include <iostream>
using namespace std;

struct From{
    int i;
    From(){}
};

struct By{
    By(const From& f){}
};

struct To{
    To(const By& b){}
};

int main() {
    From f;
    To to = f;  // error: conversion from 'From' to non-scalar type 'To' requested
    // By by = f;
    // To to = by;
    return 0;
}

有没有什么方法可以在不修改 By 和 To 的情况下完成这项工作？而且也没有这样做:

 By by = f;
 To to = by;

Answer 1

只需在 From 中提供一个用户定义的转换运算符:

struct From
{
    int i;
    From(){}
    operator To() const
    {
         To to;
         //fill it
         return to;
    }
};