重载运算符<<：无法将"std：：<char>basic_ostream"左值绑定到"std：：basic_ostream<char>&&"

Overloading operator<<: cannot bind ‘std::basic_ostream<char>’ lvalue to ‘std::basic_ostream<char>&&’

本文关键字：lt gt basic ostream std char 绑定 运算符 重载      更新时间：2023-10-16

搜索这个问题的标题会让我有很多人引用相同的错误，但在不同的情况下，不幸的是，那里提供的答案是针对他们的情况的，我看不出他们如何帮助我。

我正在尝试为模板类重载operator<<。下面是一个测试用例：

Vector.h：

#ifndef __INCL_VECTOR_H__
#define __INCL_VECTOR_H__
#include <array>
template < class T, unsigned int N >
class Vector
{
public:
    Vector();
    Vector( std::array< T, N > );
    template < class U, unsigned int M > friend Vector< U, M > operator+ ( const Vector< U, M >&, const Vector< U, M >& );
    template < class U, unsigned int M > friend std::ostream& operator<< ( std::ostream&, Vector< U, M >& );
    T& operator[] ( const unsigned int& );
protected:
    std::array< T, N > _values;
};
#include "Vector.hpp"
#endif

Vector.hpp：

#include "Vector.h"
#include <iostream>
template < class T, unsigned int N >
Vector< T, N >::Vector()
{
}
template < class T, unsigned int N >
Vector< T, N >::Vector( std::array< T, N > otherArray )
{
    _values = *( new std::array< T, N >( otherArray ) );
}
template < class U, unsigned int M >
Vector< U, M > operator+ ( const Vector< U, M > &lhVector, const Vector< U, M > &rhVector )
{
    Vector< U, M > sumVector;
    for( unsigned int i = 0; i < M; i++ )
        sumVector[i] = lhVector[i] + rhVector[i];
    return sumVector;
}
template < class U, unsigned int M >
std::ostream& operator<< ( std::ostream &out, Vector< U, M > &cVector )
{
    out << "< ";
    for( int i = M - 1; i >= 0; i-- )
    {
        out << cVector[i];
        if( i )
            out << ", ";
    }
    out << " >";
    return out;
}
template < class T, unsigned int N >
T& Vector< T, N >::operator[] ( const unsigned int &index )
{
    return _values[ index ];
}

向量测试.cpp：

#include "Vector.h"
#include <iostream>
#include <array>
using namespace std;
int main( int argc, char* argv[] )
{
    Vector< int, 3 > u( array< int, 3 > {  1, 4,  2 } );
    Vector< int, 3 > v( array< int, 3 > { -2, 3, -1 } );
    cout << "u = " << u << endl;
    cout << "v = " << v << endl;
    cout << "u + v = " << u + v << endl;
    return 0;
}

导致错误的行是cout << "u + v = " << u + v << endl;;前两行按预期工作。

错误消息如下(编译为 g++ -std=c++11 Vector.h vectorTest.cpp(：

vectorTest.cpp: In function ‘int main(int, char**)’:
vectorTest.cpp:15:31: error: cannot bind ‘std::basic_ostream<char>’ lvalue to ‘std::basic_ostream<char>&&’
In file included from /usr/include/c++/4.7/iostream:40:0,
                 from Vector.hpp:2,
                 from Vector.h:34:
/usr/include/c++/4.7/ostream:600:5: error:   initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = Vector<int, 3u>]’
In file included from Vector.h:34:0:
Vector.hpp: In instantiation of ‘Vector<U, M> operator+(const Vector<U, M>&, const Vector<U, M>&) [with U = int; unsigned int M = 3u]’:
vectorTest.cpp:15:31:   required from here
Vector.hpp:40:9: error: passing ‘const Vector<int, 3u>’ as ‘this’ argument of ‘T& Vector<T, N>::operator[](const unsigned int&) [with T = int; unsigned int N = 3u]’ discards qualifiers [-fpermissive]
Vector.hpp:40:9: error: passing ‘const Vector<int, 3u>’ as ‘this’ argument of ‘T& Vector<T, N>::operator[](const unsigned int&) [with T = int; unsigned int N = 3u]’ discards qualifiers [-fpermissive]

我无法理解这些错误消息告诉我什么。我将不胜感激任何帮助。

第一个问题：

要使程序编译，只需使用 lvalue 引用 const 作为operator <<的第二个参数(在 friend -声明和该函数的定义中(：

template < class U, unsigned int M >
std::ostream& operator<< ( std::ostream &out, Vector< U, M > const& cVector )
//                                                           ^^^^^

程序无法编译的原因是operator <<重载接受对非const的左值引用作为其第二个参数，而对非const的左值引用不能绑定到右值。

由于两个Vector实例之间的operator +结果是临时的，而临时是右值，因此编译器无法调用您的operator <<，因此无法解析调用。

第二个问题：

解决上述问题后，您必须解决第二个问题：您的Vector类模板不提供operator []的const版本，因此您重写的operator <<现在接受对const向量的引用，将无法访问向量的元素。

template < class T, unsigned int N >
class Vector
{
    // ...
    T& operator[] ( const unsigned int& );
    T const& operator[] ( const unsigned int& ) const; // <== ADD THIS!
    // ...
};

当然还有相应的定义：

template < class T, unsigned int N >
T const& Vector< T, N >::operator[] ( const unsigned int &index ) const
{
    return _values[ index ];
}

更改以下内容：

std::ostream& operator<< ( std::ostream&, Vector< U, M >& );

对此：

std::ostream& operator<< ( std::ostream&, const Vector< U, M >& );
//                                        ^^^^^

编译器告诉您，C++不会允许您将临时Vector(如 u + v(绑定到非常量Vector&。

而且你不修改那个Vector，所以它应该一开始就const。

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