私有继承和隐式转换 [英] Private inheritance and implicit conversion

问题描述

我有一个从 std :: string 私下继承的类，并添加了一些函数。我希望能够像 std :: string 一样使用这个类，所以我试图定义一个隐式转换运算符（ operator string（） ）。但是，我不断收到无法访问的基础错误。

I have a class that inherits privately from std::string, and adds some functions. I want to be able to use this class just like std::string, so I am trying to define an implicit conversion operator (operator string()). However, I keep getting inaccessible base error.

#include <iostream>
#include <string>

using namespace std;
class Test:private string {
    int _a;
    public:
    operator string() {
        return "hello";
    }
};

int main() {
    Test t;
    if(t == "hello") {
        cout<<"world\n";
    }
}

错误：

trial.cpp: In function ‘int main()’:
trial.cpp:15:13: error: ‘std::basic_string<char>’ is an inaccessible base of ‘Test’
 if(t == "hello") {
         ^

问题：

1. 定义这样的转换是不是一个坏主意？这会破坏任何推荐的编程习惯吗？


2. 我该如何使用？

编辑：Clang更有帮助

Clang was more helpful

trial.cpp:8:5: warning: conversion function converting 'Test' to its base class 'std::basic_string<char>' will never be used
    operator string() {
    ^
trial.cpp:15:8: error: cannot cast 'Test' to its private base class 'basic_string<char, std::char_traits<char>, std::allocator<char> >'
    if(t == "hello") {
       ^
trial.cpp:5:12: note: declared private here
class Test:private string {
       ^~~~~~~~~~~~~~

推荐答案

私有基类的转换函数违背了私有继承的目的。这就是为什么标准禁止这种行为的部分原因：

A conversion function to a private base class defeats the purpose of private inheritance. That's part of the reason why the standard forbids this kind of behavior:

转换函数从不用于转换（可能是cv-qualified） ）将
对象（可能是cv限定的）相同的对象类型（或对它的引用），到该类型的（可能是cv限定的）基类（或对它的引用），或者（可能是cv-qualified） void 。

A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void.

如果如果要访问基类，则应将继承公开。这使代码可以保持可读性和全面性，以便其他人可能需要维护它。

If you want access to the base class you should make the inheritance public. This keeps the code readable and comprehensive for others who may have to maintain it.

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