'stringstream'参数命令麻烦

Question

我遇到了一个奇怪的问题stringstream.

#include "stdafx.h"
#include "iostream"
#include "sstream"

using namespace std;

struct Test
{
    float f;
};

wstringstream& operator <<( wstringstream& sstream , const Test& test )
{
    sstream << test.f;
    return sstream;
}

int _tmain(int argc, _TCHAR* argv[])
{
    Test a;
    a.f = 1.2f;

    wstringstream ss;
    ss << L"text" << a << endl; // error C2679!
    ss << a << L"text" << endl; // it works well..

    getchar();
    return 0;
}

问题出在这里:

ss << L"text" << a << endl; // error C2679!
ss << a << L"text" << endl; // it works well..

这两个语句之间的唯一区别是参数顺序.为什么第一个语句失败而第二个语句有效？

Answer 1

不要限制你的operator<<工作wstringstream,写它以便它适用于任何广泛的流:

std::wostream& operator <<(std::wostream& sstream, Test const& test)
{
    return sstream << test.f;
}

或任何流(宽或窄):

template<typename CharT, typename TraitsT>
std::basic_ostream<CharT, TraitsT>&
operator <<(std::basic_ostream<CharT, TraitsT>& sstream, Test const& test)
{
    return sstream << test.f;
}