C++编译错误C2679：binary '<<'无匹配T类型操作数求助

错误信息 #

Severity Code    Description Project File    Line    Suppression State
Error   C2679   binary '<<': no operator found which takes a right-hand operand of type 'T' (or there is no acceptable conversion)

错误触发行：

std::cout << "Popped Item: " << UniqueQueueDictionary->pop() << std::endl; // 错误行

问题原因 #

Queue::pop()返回的是Dictionary临时对象，但当前Dictionary类的operator<<重载声明为非const左值引用参数：

std::ostream& operator<<(std::ostream& ostr, Dictionary& D);

C++规则中临时对象无法绑定到非const左值引用，因此编译器找不到匹配的<<运算符，抛出C2679错误。同时Dictionary::display方法未声明为const成员函数，无法在const对象上调用，会进一步阻碍运算符重载的实现。

修复步骤 #

1. 更新Dictionary.h的运算符与方法声明 #

#ifndef DICTIONARY_H_
#define DICTIONARY_H_

#include <string>
namespace sdds
{
    class Dictionary
    {
        std::string m_term{};
        std::string m_definition{};
    public:
        const std::string& getTerm() const
        {
            return m_term;
        }
        const std::string& getDefinition() const
        {
            return m_definition;
        }
        Dictionary(const std::string& term, const std::string& definition) : m_term{ term }, m_definition{ definition }{}

        const std::string& getTermConst()const
        {
            return m_term;
        }

        Dictionary() : m_term{ "" }, m_definition{ "" }{};
        // 修改为const成员函数
        std::ostream& display(std::ostream& ostr = std::cout) const;
    };
    bool operator==(const Dictionary& lhs, const Dictionary& rhs);
    // 修改为const引用参数
    std::ostream& operator<<(std::ostream& ostr, const Dictionary& D);
}
#endif

2. 在Dictionary.cpp中实现正确的函数逻辑 #

#include "Dictionary.h"
#include <iostream>

namespace sdds {
    std::ostream& Dictionary::display(std::ostream& ostr) const {
        ostr << m_term << ": " << m_definition;
        return ostr;
    }

    std::ostream& operator<<(std::ostream& ostr, const Dictionary& D) {
        return D.display(ostr);
    }

    bool operator==(const Dictionary& lhs, const Dictionary& rhs) {
        return lhs.getTerm() == rhs.getTerm();
    }
}

验证效果 #

修改完成后，pop()返回的临时Dictionary对象可以正常通过<<运算符输出，同时Queue::display方法中遍历输出队列元素的逻辑也能正常编译运行。

内容的提问来源于stack exchange，提问作者sewrew swerew