What is the best way to suppress “Unused variable x”-warning [duplicate]

Answer 1

Do not give the variable a name (C++)

void foo(int /*bar*/) {
    ...
}

Tell your compiler using a compiler specific nonstandard mechanism

See individual answers for __attribute__((unused)), various #pragmas and so on. Optionally, wrap a preprocesor macro around it for portability.

Switch the warning off

IDEs can signal unused variables visually (different color, or underline). Having that, compiler warning may be rather useless.

In GCC and Clang, add -Wno-unused-parameter option at the end of the command line (after all options that switch unused parameter warning on, like -Wall, -Wextra).

Add a cast to void

void foo(int bar) {
    (void)bar;
}

as per jamesdlin's answer and http://herbsutter.com/2009/10/18/mailbag-shutting-up-compiler-warnings/

Answer 2

Assign it to itself.

void f(int unused) {
    unused = unused;
}

Works in gcc, but clang needs -Wno-self-assign.

---

Edit: I now think casting to void is the most portable solution: Both gcc and clang understand this, even with full warnings -W{all,extra,pedantic}:

(void)unused;

Answer 3

If this is really what you want, you could use the unused attribute (gcc only), something like:

void foo(int __attribute__((__unused__)) bar) {
    ...
}

Not just for function parameters, of course, but that's the most common use case, since it might be a callback function for an API where you don't actually need all the input.

Additionally, GLib has a G_GNUC_UNUSED macro which I believe expands to that attribute.

Answer 4

It's a very hackish solution, but did you try simply assigning the variable to itself? I think that should fool most compilers into thinking that the variable is used. Should be quite portable too.

Answer 5

The cast to a void is the best approach because it shows that you didn't "accidentally" keep the variable in your code - ie: this function might be an instance where you have a table of function pointers that need the same parameter types and return types, but in this particular table entry you are not using the parameter.

That said, if you don't need it, get rid of it. ;)

Answer 6

#pragma unused <variable>