C++ chaining of the operator

I'm currently programming a logger class, but the operator<< method causes a compiler error. Here's a minimized version of the class, in file "logger.h":

#include 
class Logger {
public:
    Logger() : m_file(std::cout) {}

    template 
    Logger &operator<<(const T &a) {
        m_file<        return *this;
    }


protected:
    std::ostream& m_file;
};

It is included in my main.cpp and works perfecly when I output a string literal:

log << "hi"; 

However, the following won't compile.

#include "logger.h"
int main() {
    Logger log;

    log << std::endl;
}

The g++ compiler reports:

src/main.cpp:5: error: no match for 'operator<<' in 'log << std::endl'

Answer

Your problem is not about the chain of << , a single log << endl would also cause the problem. It is because std::endl is a template function:

template 

basic_ostream& endl(basic_ostream& os);

One of the overload of operator<< in basic_ostream is:

template  >
class basic_ostream : virtual public basic_ios {
public:
    basic_ostream& operator<<(
    basic_ostream& (*pf)(basic_ostream&));

//...
};

So the template parameters can be deduced when std::cout< is used. However, when the left side is the class Logger, the compile cannot deduce the template parameters of endl. Explicitly give the template parameters can let program compile and work:

#include 
class Logger
{
public:

    std::ostream &m_file;
    Logger(std::ostream &o = std::cout):m_file(o){};

    template 
    Logger &operator<<(const T &a) {
        m_file<        return *this;
    }
};


int main()
{
    Logger log;
    log< >;
    log<<"hi"<<" stackoverflow"< >;
    return 0;
}

Or you can add a new overload of operator<< in class Logger to let compiler deduce the template parameters of std::endl:

#include 
class Logger
{
public:
    std::ostream &m_file;
    Logger(std::ostream &o = std::cout):m_file(o){};

    template 
    Logger &operator<<(const T &a) {

        m_file<        return *this;
    }

    Logger &operator<<(std::ostream& (*pf) (std::ostream&)){
        m_file<        return *this;
    }
};


int main()
{
    Logger log;
    log<    log<<"hi"<<" stackoverflow"<    return 0;
}

Also, if you don't need the output to be flushed immediately, you can use '\n' instead of endl.