Overloading output operator for arrays

问题

According to this answer, the correct way to overload output operator << for C-style arrays is this -:

#include <iostream>
using namespace std;

template <size_t arrSize>
std::ostream& operator<<( std::ostream& out, const char( &arr )[arrSize] )
{
    return out << static_cast<const char*>( arr ); // use the original version
}

// Print an array
template<typename T1, size_t arrSize>
std::ostream& operator <<( std::ostream& out, const T1( & arr )[arrSize] )
{
    out << "[";
    if ( arrSize )
    {
        const char* separator = "";
        for ( const auto& element : arr )
        {
            out << separator;
            out << element;
            separator = ", ";
        }
    }
    out << "]";
    return out;
}

int main()
{
    int arr[] = {1, 2, 3};
    cout << arr;
}

But I am still getting the compiler error

error: ambiguous overload for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream<char>}' and 'const char [2]')  

for the out << "["; and out << "]"; statements.

What is the correct way of doing this ?

回答1:

The problem is the standard overload for operator<< that prints a character array is this one:

template< class CharT, class Traits >
basic_ostream<CharT,Traits>& operator<<( basic_ostream<CharT,Traits>& os,
                                         const char* s );

So when you provide yours:

template <size_t arrSize>
std::ostream& operator<<( std::ostream& out, const char( &arr )[arrSize] )

That's going to be ambiguous: we have two different function templates with identical conversion sequences, neither of which is more specialized than the other.

However, since you want your version to JUST call the original, there is really no reason to provide your version at all. Just make your "generic" array printer not accept char using SFINAE:

// Print an array
template<typename T1, size_t arrSize, 
         typename = std::enable_if_t<!std::is_same<T1,char>::value>>
std::ostream& operator <<( std::ostream& out, const T1( & arr )[arrSize] )
{ /* rest as before */ }

来源：https://stackoverflow.com/questions/31135228/overloading-output-operator-for-arrays