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二进制表达式的操作数无效('std::ostream'(又名'basic\u ostream<char>')和'sound\u ctl')

stream c++

Omri. B · 技术社区 · 4 年前

我想写一本书 << 接线员。我的代码:

#include <iostream>
#include <ostream>
class sound_ctl {
private:
    int _vol;
    bool _status;
public:
    sound_ctl() : _vol(50), _status(false) {};

    void operator++() {
        if (_vol <= 98) {
            _vol += 2;
        }
    }

    void operator--() {
        if (_vol >= 2) {
            _vol -= 2;
        }
    }

    void operator!() {
        _status = !_status;
    }

    std::ostream &operator<<(std::ostream &os) const {
        os << _status << "Volume: " + _vol << std::endl;
        return os;
    }

};


int main() {
    sound_ctl panel1;
    std::cout << panel1;
}

但是,代码编译时不会出错: Invalid operands to binary expression ('std::ostream' (aka 'basic_ostream<char>') and 'sound_ctl')

我对c++中的流非常缺乏理解,因此,理解底层问题将对我有很大帮助。谢谢

2 回复 | 直到 4 年前

rawrex 4 年前

不应将输入/输出运算符定义为成员函数。

std::ostream &operator<<(std::ostream &os, const sound_ctl& sc) {
    os << sc._status << "Volume: " + sc._vol << std::endl;
    return os;
}

然后将此函数声明为友元:

class sound_ctl {
friend std::ostream &operator<<(std::ostream &os, const sound_ctl& sc);
// ...
};

为什么不能像这样调用output std::cout << panel1 ,对你来说?因为 << operator<<() 功能。如果此函数是成员函数,则左侧将作为隐式传递 this ,而右侧将是参数列表中的参数。意味着您必须这样调用输出:

panel1 << std::cout; // No comments...

我们通过将类的对象作为显式的第二个参数传递给运算符来避免: ostream& operator<<(ostream&, const sound_ctl&)

thread . 这也会有帮助: Overloading The IO Operators .

另外,请注意 模拟内置操作符 在重载运算符中。所以,你的 operator++ 还要定义后缀 递增/递减运算符。

Remy Lebeau 4 年前

所有运算符的实现都不正确。看到了吗 What are the basic rules and idioms for operator overloading? .

但是,关于 operator<< 具体来说,它不能作为类的成员实现,它必须是独立函数。它需要2个参数,流被写入,对象被写入。

试试这个。

#include <iostream>
#include <ostream>

class sound_ctl {
private:
    int _vol;
    bool _status;
public:
    sound_ctl() : _vol(50), _status(false) {};

    sound_ctl& operator++() {
        if (_vol <= 98) {
            _vol += 2;
        }
        return *this;
    }

    sound_ctl operator++(int) {
        sound_ctl tmp(*this);
        ++(*this);
        return tmp;
    }

    sount_ctl& operator--() {
        if (_vol >= 2) {
            _vol -= 2;
        }
        return *this;
    }


    sound_ctl operator--(int) {
        sound_ctl tmp(*this);
        --(*this);
        return tmp;
    }

    bool operator!() const {
        return !_status;
    }

    // even though this is inlined within the class declaration,
    // it is NOT actually a member of the class!
    friend std::ostream& operator<<(std::ostream &os, const sound_ctl& ctl) {
        os << ctl._status << " Volume: " + ctl._vol << std::endl;
        return os;
    }
};

int main() {
    sound_ctl panel1;
    std::cout << panel1;
}

#include <iostream>
#include <ostream>

class sound_ctl {
private:
    int _vol;
    bool _status;
public:
    sound_ctl() : _vol(50), _status(false) {};

    sound_ctl& operator++() {
        if (_vol <= 98) {
            _vol += 2;
        }
        return *this;
    }

    sound_ctl operator++(int) {
        sound_ctl tmp(*this);
        ++(*this);
        return tmp;
    }

    sount_ctl& operator--() {
        if (_vol >= 2) {
            _vol -= 2;
        }
        return *this;
    }


    sound_ctl operator--(int) {
        sound_ctl tmp(*this);
        --(*this);
        return tmp;
    }

    bool operator!() const {
        return !_status;
    }

    void print(std::ostream &os) const {
        os << _status << " Volume: " + _vol << std::endl;
    }
};

std::ostream& operator<<(std::ostream &os, const sound_ctl& ctl) {
    ctl.print(os);
    return os;
}

int main() {
    sound_ctl panel1;
    std::cout << panel1;
}