Unused parameter in c++11

Question

In c++03 and earlier to disable compiler warning about unused parameter I usually use such code:

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

For example

int main(int argc, char *argv[])
{
    UNUSED(argc);
    UNUSED(argv);

    return 0;
}

But macros are not best practice for c++, so. Does any better solution appear with c++11 standard? I mean can I get rid of macros?

Thanks for all!

Answer 1

I have used a function with an empty body for that purpose:

template <typename T>
void ignore(T &&)
{ }

void f(int a, int b)
{
  ignore(a);
  ignore(b);
  return;
}

I expect any serious compiler to optimize the function call away and it silences warnings for me.

Answer 2

You can just omit the parameter names:

int main(int, char *[])
{

    return 0;
}

And in the case of main, you can even omit the parameters altogether:

int main()
{
    // no return implies return 0;
}

See "§ 3.6 Start and Termination" in the C++11 Standard.

Answer 3

There is the <tuple> in C++11, which includes the ready to use std::ignore object, that's allow us to write (very likely without imposing runtime overheads):

void f(int x)
{
    std::ignore = x;
}

Answer 4

Nothing equivalent, no.

So you're stuck with the same old options. Are you happy to omit the names in the parameter list entirely?

int main(int, char**)

In the specific case of main, of course, you could simply omit the parameters themselves:

int main()

There are also the typical implementation-specific tricks, such as GCC's __attribute__((unused)).

Answer 5

To "disable" this warning, the best is to avoid writing the argument, just write the type.

void function( int, int )
{
}

or if you prefer, comment it out:

void function( int /*a*/, int /*b*/ )
{
}

You can mix named and unnamed arguments:

void function( int a, int /*b*/ )
{
}

With C++17 you have [[maybe_unused]] attribute specifier, like:

void function( [[maybe_unused]] int a, [[maybe_unused]] int b )
{
}

Answer 6

Macros may not be ideal, but they do a good job for this particular purpose. I'd say stick to using the macro.

Answer 7

What do you have against the old and standard way?

void f(int a, int b)
{
  (void)a;
  (void)b;
  return;
}

Answer 8

There's nothing new available.

What works best for me is to comment out the parameter name in the implementation. That way, you get rid of the warning, but still retain some notion of what the parameter is (since the name is available).

Your macro (and every other cast-to-void approach) has the downside that you can actually use the parameter after using the macro. This can make code harder to maintain.

Answer 9

The Boost header <boost/core/ignore_unused.hpp> (Boost >= 1.56) defines, for this purpose, the function template boost::ignore_unused().

int fun(int foo, int bar)
{
  boost::ignore_unused(bar);
#ifdef ENABLE_DEBUG_OUTPUT
  if (foo < bar)
    std::cerr << "warning! foo < bar";
#endif

  return foo + 2;
}

---

PS C++17 has the [[maybe_unused]] attribute to suppresses warnings on unused entities.

Answer 10

I really like using macros for this, because it allows you better control when you have different debug builds (e.g. if you want to build with asserts enabled):

#if defined(ENABLE_ASSERTS)
  #define MY_ASSERT(x) assert(x)
#else
  #define MY_ASSERT(x)
#end

#define MY_UNUSED(x)

#if defined(ENABLE_ASSERTS)
  #define MY_USED_FOR_ASSERTS(x) x
#else
  #define MY_USED_FOR_ASSERTS(x) MY_UNUSED(x)
#end

and then use it like:

int myFunc(int myInt, float MY_USED_FOR_ASSERTS(myFloat), char MY_UNUSED(myChar))
{
  MY_ASSERT(myChar < 12.0f);
  return myInt;
}

Answer 11

I have my own implementation for time critical segments of code. I've been researching a while a time critical code for slow down and have found this implementation consumes about 2% from the time critical code i have being optimized:

#define UTILITY_UNUSED(exp) (void)(exp)
#define UTILITY_UNUSED2(e0, e1) UTILITY_UNUSED(e0); UTILITY_UNUSED(e1)
#define ASSERT_EQ(v1, v2) { UTILITY_UNUSED2(v1, v2); } (void)0

The time critical code has used the ASSERT* definitions for debug purposes, but in release it clearly has cutted out, but... Seems this one produces a bit faster code in Visual Studio 2015 Update 3:

#define UTILITY_UNUSED(exp) (void)(false ? (false ? ((void)(exp)) : (void)0) : (void)0)
#define UTILITY_UNUSED2(e0, e1) (void)(false ? (false ? ((void)(e0), (void)(e1)) : (void)0) : (void)0)

The reason is in double false ? expression. It somehow produces a bit faster code in release with maximal optimization.

I don't know why this is faster (seems a bug in compiler optimization), but it at least a better solution for that case of code.

Note: Most important thing here is that a time critical code slow downs without above assertions or unused macroses in release. In another words the double false ? expression surprisingly helps to optimize a code.

Answer 12

windows.h defines UNREFERENCED_PARAMETER:

#define UNREFERENCED_PARAMETER(P) {(P) = (P);}

So you could do it like this:

#include <windows.h>
#include <stdio.h>
int main(int argc, char **argv) {
  UNREFERENCED_PARAMETER(argc);
  puts(argv[1]);
  return 0;
}

Or outside of Windows:

#include <stdio.h>
#define UNREFERENCED_PARAMETER(P) {(P) = (P);}
int main(int argc, char **argv) {
  UNREFERENCED_PARAMETER(argc);
  puts(argv[1]);
  return 0;
}