从流中创建具有恒定参数的对象 [英] Creating object with constant parameters from stream

问题描述

让我们假设我有一个课程

let us assume I have a class with

#include <iostream>
using namespace std;

class Test{
    public:
        friend istream& operator >> (istream& input, Test& test){
            input >> test.dummy;
            return input;
        };
        friend ostream& operator << (ostream& output, Test& test){
            output << test.dummy << endl;
            return output;
        };
    private:
        const int dummy;
}

这不起作用，因为哑是恒定的.有没有一种方法可以从文件加载并使用常量参数重新创建对象?

This does not work because dummy is constant. Is there a way to load from file and recreate an object with parameters which are constant?

推荐答案

我真的非常需要这种方式"

使用const_cast.通常，您将它用于外部看起来像是恒定的对象，但是在内部，它们确实需要不时地更新状态.至少可以说，使用它从流中读取内容有点令人困惑.

The "I really, really need this" way

Use const_cast. Usually you use it for objects that outside look like if they are constant, but internally they do need to update state every now and then. Using it for reading from stream is a little confusing, to say the least.

friend istream& operator >> (istream& input, Test& test){
    input >> const_cast<int&>(test.dummy);
    return input;
};

推荐方式

沟流运算符，使用工厂功能.

The recommended way

Ditch stream operator, use a factory function.

static Test fromStream(istream& input) {
    int dummy;
    input >> dummy;
    return Test(dummy);
}

最好的方法

沟常数相反，如果需要，将整个对象作为const传递.

The best way

Ditch const. Instead, pass your entire object as const if needed.

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