c++ - 同样的代码为何在重载运算符函数中报错，在普通函数中没有错？

Question

c++新手，使用vs 2015 community。刚写了一个泛型函数用来比较两个容器内的元素是否相同，原来是在重载运算符函数operator==中写的，编译出错： std::begin 未找到匹配的重载函数，我查看了vs中std::begin的定义，其中应该有匹配的函数。于是我把代码放到普通函数中，结果就可以运行通过了，代码如下，希望有人解答 （另外，如果我将两个函数的参数都改成const Vector&，就可以通过编译正确运行）：

#include <iostream>
#include <vector>
#include <string>

//#define NO_OPERATOR

#ifndef NO_OPERATOR

template<class Vector1, class Vector2>
bool operator == (Vector1& con1, Vector2& con2)
{
    auto first1 = std::begin(con1), last1 = std::end(con1);
    auto first2 = std::begin(con2), last2 = std::end(con2);
    for (; first1 != last1 && first2 != last2; ++first1, ++first2)
    {
        if (*first1 != *first2)
            return false;
    }
    return (first1 == last1 && first2 == last2);
}
#endif // !1


template<class Vector1, class Vector2>
bool compare_vector(Vector1& con1, Vector2& con2)
{
    auto first1 = std::begin(con1), last1 = std::end(con1);
    auto first2 = std::begin(con2), last2 = std::end(con2);
    for (; first1 != last1 && first2 != last2; ++first1, ++first2) 
    {
        if (*first1 != *first2)
            return false;
    }
    return (first1 == last1 && first2 == last2);
}



int main()
{
    std::vector<int> s1(10,0), s2(10,0);
#ifndef NO_OPERATOR
    if(s1 == s2)
        std::cout << "equal1" << std::endl;
#endif
    if (compare_vector(s1, s2))
        std::cout << "equal2" << std::endl;
    return 0;
}

Answer 1

Because you overloaded the global operator==, when you determine that s1 == s2, the compiler speculates that you may want to instantiate the template parameter as
<std:: vector<int>, std::vector<int>>, then the compiler sees first1 == last1, then the template parameter is instantiated as
std::vector<int>::iterator, so the type derivation is It failed, and the compiler was dumbfounded.

The container in stl overloads operator==, so just use it directly.

Answer 2

#include <iostream>
#include <vector>
#include <string>

//#define NO_OPERATOR

#ifndef NO_OPERATOR

template<class Vector1, class Vector2>
bool operator == (Vector1& con1, Vector2& con2) 
// 正如楼上 重载的全局 == ， 但这种用模板重载的做法是很危险的，不可取
{
    auto first1 = std::begin(con1), last1 = std::end(con1);
    auto first2 = std::begin(con2), last2 = std::end(con2);
    for (; first1 != last1 && first2 != last2; ++first1, ++first2)
    {
        if (*first1 != *first2)
            return false;
    }
    return (first1 == last1 && first2 == last2); 
    // == 实例化为 operator == (vector<int>::iterator&, vector<int>::iterator&)
    // 然后在 == 内部，std::begin的调用产生了错误
}
#endif // !1


template<class Vector1, class Vector2>
bool compare_vector(Vector1& con1, Vector2& con2)
{
    auto first1 = std::begin(con1), last1 = std::end(con1);
    auto first2 = std::begin(con2), last2 = std::end(con2);
    for (; first1 != last1 && first2 != last2; ++first1, ++first2) 
    {
        if (*first1 != *first2)
            return false;
    }
    return (first1 == last1 && first2 == last2);
}



int main()
{
    std::vector<int> s1(10,0), s2(10,0);
#ifndef NO_OPERATOR
    if(s1 == s2)                                // == 实例化为 operator == (vector<int>&, vector<int>&)
        std::cout << "equal1" << std::endl;
#endif
    if (compare_vector(s1, s2))
        std::cout << "equal2" << std::endl;
    return 0;
}