Zapisanie do pliku

#include <iostream>
#include <streambuf>
#include <fstream>


class ConsoleLogger
    : public std::streambuf
{
public:
    ConsoleLogger( std::ostream & _stream, const char * _file )
        : m_cout( _stream )
         , m_file( _file )
    {
        m_coutBuf = _stream.rdbuf( this );
    }
   
    ~ConsoleLogger()
    {
        m_cout.rdbuf( m_coutBuf );
    }
   
protected:
    std::ostream & m_cout;
    std::streambuf * m_coutBuf;
    std::ofstream m_file;
   
    int_type overflow( int_type c ) override
    {
        if( c != EOF ) {
            m_coutBuf->sputc( c );
            return m_file.rdbuf()->sputc( c );
        }
       
        return c;
    }
};


int main()
{
    ConsoleLogger logger( std::cout, "log.txt" );
    std::cout << "Test " << 123 << '\n';
}