How do I best silence a warning about unused variables?

Phil_Hannent_24459 0
Viewed 271622

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?

gcc-warning gcc warnings c++
21 Answers

I have seen this instead of the (void)param2 way of silencing the warning:

void foo(int param1, int param2)
{
    std::ignore = param2;
    bar(param1);
}

Looks like this was added in C++11

krupan_277290

Lol! I dont think there is another question on SO that reveal all the heretics corrupted by Chaos better that this one!

With all due respect to C++17 there is a clear guideline in C++ Core Guidelines. AFAIR, back in 2009 this option was available as well as today. And if somebody says it is considered as a bug in Doxygen then there is a bug in Doxygen

user_15300180

In C++11, this is the solution I'm using:

template<typename... Ts> inline void Unreferenced(Ts&&...) {}

int Foo(int bar) 
{
    Unreferenced(bar);
    return 0;
}

int Foo2(int bar1, int bar2) 
{
    Unreferenced(bar1, bar2);
    return 0;
}

Verified to be portable (at least on modern msvc, clang and gcc) and not producing extra code when optimizations are enabled. With no optimization, the extra function call is performed and references to the parameters are copied to the stack, but there are no macros involved.

If the extra code is an issue, you can use this declaration instead:

(decltype(Unreferenced(bar1, bar2)))0;

but at that point, a macro provides better readability:

#define UNREFERENCED(...) { (decltype(Unreferenced(__VA_ARGS__)))0; }
user_100227270

I found most of the presented answers work for local unused variable only, and will cause compile error for unused static global variable.

Another macro needed to suppress the warning of unused static global variable.

template <typename T>
const T* UNUSED_VARIABLE(const T& dummy) { 
    return &dummy;
}
#define UNUSED_GLOBAL_VARIABLE(x) namespace {\
    const auto dummy = UNUSED_VARIABLE(x);\
}

static int a = 0;
UNUSED_GLOBAL_VARIABLE(a);

int main ()
{
    int b = 3;
    UNUSED_VARIABLE(b);
    return 0;
}

This works because no warning will be reported for non-static global variable in anonymous namespace.

C++ 11 is required though

 g++  -Wall -O3  -std=c++11 test.cpp
user_17371320
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