variadic functions in macro and templates

Question

I try to create a macro that call a variadic function that uses a template. I use the following code, but the linker cannot resolve a call to the macro...

This code is part of the Logger class:

template< typename ... Args >
void Logger::logTrace(Args const& ... args)
{
    std::ostringstream stream;
    using List = int[];
    (void)List{ 0, ((void)(stream << args), 0) ... };
    BOOST_LOG_SEV(log_, trace) << stream.str();
}

The logger class:

class Logger {

public:
    static Logger* getInstance(const char *logFile = "LogClient.log");

    template< typename ... Args >
    void logTrace(Args const& ... args);

private:
    Logger(std::string fileName);   
    virtual ~Logger();

    void initialize(std::string fileName);

    static Logger* logger_; // singleton instance
};

and the macro:

#define LOG_TRACE(...) Logger::getInstance()->logTrace(__VA_ARGS__);

A call to the macro:

LOG_TRACE("A log with a number: %d", 5);

Thanks for your help!

EDIT AND SOLVE:

The problem was not related to variadic function or even macro, but with linking. Implement logTrace in the class definition resolve the problem.

code Working:

`The logger class:

class Logger {

public:
    static Logger* getInstance(const char *logFile = "LogClient.log");

    template< typename ... Args >
    void logTrace(Args const& ... args)
    {
        std::ostringstream stream;
        using List = int[];
        (void)List{ 0, ((void)(stream << args), 0) ... };
        BOOST_LOG_SEV(log_, trace) << stream.str();
    }

private:
    Logger(std::string fileName);   
    virtual ~Logger();

    void initialize(std::string fileName);

    static Logger* logger_; // singleton instance
};

and the macro:

#define LOG_TRACE(...) Logger::getInstance()->logTrace(__VA_ARGS__);

A call to the macro:

LOG_TRACE("A log with a number: %d", 5);

Answer 1

You may have called your macro (and thus the logTrace template function) in a source file where Logger::logTrace() is declared but not defined/implemented (e.g.: in a source file where you included Logger.h). The full definition of the logTrace template function is required for your macro to work.

I suggest you define the logTrace member function into the Logger class:

class Logger {

public:
    static Logger* getInstance(const char *logFile = "LogClient.log");

    template< typename ... Args >
    void logTrace(Args const& ... args)
    { /* add implementation here */ }
    ...