C++ could not deduce template argument

Question

I'm writing a generic class for logging which

1. can be called as a functor with the string to log
2. enriches the string with some information (system time, log level, ...)
3. passes the log message to an output class which implements the << operator. This "output channel" can be defined upon construction.

The code:

template<class Writer>
class Logger
{
public:
Logger(Writer* writer);
~Logger(void);

void operator() (char level, std::string message);

private:
Writer* writer;
};

template<class Writer>
Logger<Writer>::Logger(Writer* writer)
    : writer(writer)
{
}

template<class Writer>
Logger<Writer>::~Logger(void)
{
}

template<class Writer>
void Logger<Writer>::operator ()(char level, std::string message) {

    /* do something fancy with the message */
    /* ... */
    /* then write to output channel */

    this->writer << message;
}

However I get the error "Could not deduce template argument" on compilation. The line the error occurs is

this->writer << message;

I'm pretty new to C++ templates, I'm rather coming from the C#-side of the force... Any suggestions?

Thank you in advance...

Answer 1

You are using a pointer as the left operand of operator << :

this->writer << message;
//    ^^^^^^

If you want to use a pointer, you should then do:

*(this->writer) << message; 

Or even better (provided that a Logger class must always be associated to a Writer , so that the writer pointer should never be null), replace the pointer with a reference:

template<class Writer>
class Logger
{
public:
    Logger(Writer& writer);
//         ^^^^^^^
    // ...
private:
    Writer& writer;
//  ^^^^^^^
};

This will allow you to use your original version of the call operator, and write:

this->writer << message;

Now all of this of course is correct under the assumption that an appropriate overload of operator << exists.