重载运算符<<：不能绑定'std :: basic_ostream <char>'左值为'std :: basic_ostream <char> &&'

Question

搜索这个问题的标题给了我许多人引用相同的错误，但在不同的情况下，不幸的是，那里提供的答案是特定于他们的情况，我不明白他们如何能帮助我。

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 ]; 
} 

vectorTest.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; 
} 

它导致错误是线cout << "u + v = " << u + v << endl;;前两行按预期工作。

错误信息如下（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。我会很感激任何帮助。

Answer 1

第一个问题：

使你的程序编译，只需要使用一个左值参考const为您operator <<（无论是在friend -declaration并且在函数的定义）的第二个参数：

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

之所以你的程序将无法编译是你的operator <<超负荷接受左值参考非const作为第二个参数，并且左值引用非const不能绑定到右值。

由于Vector的两个实例之间的operator +的结果是临时值，并且临时值是右值，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：

一旦固定的上述问题，你就必须要解决的第二个：你Vector类模板不提供const版本的operator []，所以你改写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 ]; 
} 

Answer 2

更改此：

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

这样：

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

编译器告诉你，C++不会让您绑定的临时Vector如u + v到非constVector& 。

而且你不修改那个Vector，所以它应该是const开始。