How do I best silence a warning about unused variables?

I have a cross platform application and in a few of my functions not all the values passed to functions are utilised. Hence I get a warning from GCC telling me that there are unused variables.

What would be the best way of coding around the warning?

An #ifdef around the function?

#ifdef _MSC_VER
void ProcessOps::sendToExternalApp(QString sAppName, QString sImagePath, qreal qrLeft, qreal qrTop, qreal qrWidth, qreal qrHeight)
#else
void ProcessOps::sendToExternalApp(QString sAppName, QString sImagePath, qreal /*qrLeft*/, qreal /*qrTop*/, qreal /*qrWidth*/, qreal /*qrHeight*/)
#endif
{

This is so ugly but seems like the way the compiler would prefer.

Or do I assign zero to the variable at the end of the function? (which I hate because it's altering something in the program flow to silence a compiler warning).

Is there a correct way?

You can put it in "(void)var;" expression (does nothing) so that a compiler sees it is used. This is portable between compilers.

E.g.

void foo(int param1, int param2)
{
    (void)param2;
    bar(param1);
}

Or,

#define UNUSED(expr) do { (void)(expr); } while (0)
...

void foo(int param1, int param2)
{
    UNUSED(param2);
    bar(param1);
}

In GCC and Clang you can use the __attribute__((unused)) preprocessor directive to achieve your goal.
For example:

int foo (__attribute__((unused)) int bar) {
   return 0;
}

C++17 now provides the [[maybe_unused]] attribute.

http://en.cppreference.com/w/cpp/language/attributes

Quite nice and standard.

Your current solution is best - comment out the parameter name if you don't use it. That applies to all compilers, so you don't have to use the pre-processor to do it specially for GCC.

How do I best silence a warning about unused variables?

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