[解決済み】「std::operator」で「operator<<」にマッチするものがない。

質問

私はC++の初心者ですが、ここで最初のプログラムを試しました。私の目には、このプログラムは正しく見えます。

#include <iostream>

using namespace std;

class mystruct
{
    private:
        int m_a;
        float m_b;

    public:
        mystruct(int x, float y)
        {
                m_a = x;
                m_b = y;
        }

};


int main()
{

        mystruct m = mystruct(5,3.14);

        cout << "my structure " << m << endl;

        return 0;
}

しかし、私は非常に多くのエラーが発生する。なぜかわからない？

cout.cpp: In function ‘int main()’:
cout.cpp:26:29: error: no match for ‘operator<<’ in ‘std::operator<< [with _Traits = std::char_traits<char>]((* & std::cout), ((const char*)"my structure ")) << m’
cout.cpp:26:29: note: candidates are:
/usr/include/c++/4.6/ostream:110:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:110:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&) {aka std::basic_ostream<char>& (*)(std::basic_ostream<char>&)}’
/usr/include/c++/4.6/ostream:119:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>, std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]
/usr/include/c++/4.6/ostream:119:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}’
/usr/include/c++/4.6/ostream:129:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:129:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘std::ios_base& (*)(std::ios_base&)’
/usr/include/c++/4.6/ostream:167:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:167:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘long int’
/usr/include/c++/4.6/ostream:171:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:171:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘long unsigned int’
/usr/include/c++/4.6/ostream:175:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:175:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘bool’
/usr/include/c++/4.6/bits/ostream.tcc:93:5: note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.6/bits/ostream.tcc:93:5: note:   no known conversion for argument 1 from ‘mystruct’ to ‘short int’
/usr/include/c++/4.6/ostream:182:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:182:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘short unsigned int’
/usr/include/c++/4.6/bits/ostream.tcc:107:5: note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.6/bits/ostream.tcc:107:5: note:   no known conversion for argument 1 from ‘mystruct’ to ‘int’
/usr/include/c++/4.6/ostream:193:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:193:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘unsigned int’
/usr/include/c++/4.6/ostream:202:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:202:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘long long int’
/usr/include/c++/4.6/ostream:206:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:206:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘long long unsigned int’
/usr/include/c++/4.6/ostream:211:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:211:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘double’
/usr/include/c++/4.6/ostream:215:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:215:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘float’
/usr/include/c++/4.6/ostream:223:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:223:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘long double’
/usr/include/c++/4.6/ostream:227:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.6/ostream:227:7: note:   no known conversion for argument 1 from ‘mystruct’ to ‘const void*’
/usr/include/c++/4.6/bits/ostream.tcc:121:5: note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]
/usr/include/c++/4.6/bits/ostream.tcc:121:5: note:   no known conversion for argument 1 from ‘mystruct’ to ‘std::basic_ostream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}’
/usr/include/c++/4.6/bits/basic_string.h:2693:5: note: template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.6/ostream:451:5: note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT)
/usr/include/c++/4.6/ostream:456:5: note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char)
/usr/include/c++/4.6/ostream:462:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char)
/usr/include/c++/4.6/ostream:468:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char)
/usr/include/c++/4.6/ostream:473:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char)
/usr/include/c++/4.6/ostream:493:5: note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const _CharT*)
/usr/include/c++/4.6/bits/ostream.tcc:323:5: note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*)
/usr/include/c++/4.6/ostream:510:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*)
/usr/include/c++/4.6/ostream:523:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*)
/usr/include/c++/4.6/ostream:528:5: note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*)

解決方法は？

演算子をオーバーロードする必要があります。 << に対して mystruct クラス

のようなものです。

friend ostream& operator << (ostream& os, const mystruct& m)
{
    os << m.m_a <<" " << m.m_b << endl;
    return os ;
}

参照 こちら