How to pass variable argument parameter to another function?

Question

Short version: How can I pass the contents represented by ... in a variable argument function to another function without first parsing it into a va_list?

Long version:

Below are two functions in a class of mine. I would like to draw your attention to the fact that the first four lines of each function are identical. And I have a half dozen other functions in this class with the same first four lines.

void cyclOps::Logger::warn(char* szFile, char* szFunction, int iLine, char* szFormat, ...) {
    va_list vaArguments;
    va_start(vaArguments, szFormat);
    char szOutput[10000];
    _vsnprintf_s(szOutput, CYCLOPSSIZEOF(szOutput), _TRUNCATE, szFormat, vaArguments);
    this->log("WARNING: %s [%s - %s(%d)]", szOutput, szFile, szFunction, iLine);
}

void cyclOps::Logger::info(char* szFormat, ...) {
    va_list vaArguments;
    va_start(vaArguments, szFormat);
    char szOutput[10000];
    _vsnprintf_s(szOutput, CYCLOPSSIZEOF(szOutput), _TRUNCATE, szFormat, vaArguments);
    this->log("INFO: %s", szOutput);
}

I would like to put these four identical lines in a single function called summarizeVariableArguments() and call it something like this...

void cyclOps::Logger::info(char* szFormat, ...) {
      std::string strOutput = this->summarizeVariableArguments(/* TBD */);
    this->log("INFO: %s", strOutput.c_str());
}

...where the contents of strOutput would be the same as the contents of szOutput in the two previous functions. But how do I pass the ... parameter to another function?

Answer 1

You cannot do that portably (or perhaps at compile time, with horrible C++2011 variadic template tricks).

If you want to call at runtime a variadic function, you may want to use the libffi.

Details are operating system, compiler, processor and ABI specific. (but libffi is trying to abstract them).