Why cast unused return values to void?

int fn();

void whatever()
{
    (void) fn();
}

Is there any reason for casting an unused return value to void, or am I right in thinking it's a complete waste of time?

David's answer pretty much covers the motivation for this, to explicitly show other "developers" that you know this function returns but you're explicitly ignoring it.

This is a way to ensure that where necessary error codes are always handled.

I think for C++ this is probably the only place that I prefer to use C-style casts too, since using the full static cast notation just feels like overkill here. Finally, if you're reviewing a coding standard or writing one, then it's also a good idea to explicitly state that calls to overloaded operators (not using function call notation) should be exempt from this too:

class A {};
A operator+(A const &, A const &);

int main () {
  A a;
  a + a;                 // Not a problem
  (void)operator+(a,a);  // Using function call notation - so add the cast.

At work we use that to acknowledge that the function has a return value but the developer has asserted that it is safe to ignore it. Since you tagged the question as C++ you should be using static_cast:

static_cast<void>(fn());

As far as the compiler goes casting the return value to void has little meaning.

The true reason for doing this dates back to a tool used on C code, called lint.

It analyzes code looking for possible problems and issuing warnings and suggestions. If a function returned a value which was then not checked, lint would warn in case this was accidental. To silence lint on this warning, you cast the call to (void).

Casting to void is used to suppress compiler warnings for unused variables and unsaved return values or expressions.

The Standard(2003) says in §5.2.9/4 says,

>Any expression can be explicitly converted to type “cv void.” The expression value is discarded.

So you can write :

//suppressing unused variable warnings
static_cast<void>(unusedVar);
static_cast<const void>(unusedVar);
static_cast<volatile void>(unusedVar);

//suppressing return value warnings
static_cast<void>(fn());
static_cast<const void>(fn());
static_cast<volatile void>(fn());

//suppressing unsaved expressions
static_cast<void>(a + b * 10);
static_cast<const void>( x &&y || z);
static_cast<volatile void>( m | n + fn());

All forms are valid. I usually make it shorter as:

//suppressing  expressions
(void)(unusedVar);
(void)(fn());
(void)(x &&y || z);

Its also okay.

Since c++17 we have the [[maybe_unused]] attribute which can be used instead of the void cast.

Cast to void is costless. It is only information for compiler how to treat it.

For the functionality of you program casting to void is meaningless. I would also argue that you should not use it to signal something to the person that is reading the code, as suggested in the answer by David. If you want to communicate something about your intentions, it is better to use a comment. Adding a cast like this will only look strange and raise questions about the possible reason. Just my opinion...

As of C++11 you can also do:

std::ignore = fn();

This should achieve the same result on functions marked with [[nodiscard]]

C++17 [[nodiscard]]

C++17 standardized the "return value ignored business" with an attribute.

Therefore, I hope that compliant implementations will always warn only when nodiscard is given, and never warn otherwise.

Example:

main.cpp

[[nodiscard]] int f() {
    return 1;
}

int main() {
    f();
}

compile:

g++ -std=c++17 -ggdb3 -O0 -Wall -Wextra -pedantic -o main.out main.cpp

outcome:

main.cpp: In function ‘int main()’:
main.cpp:6:6: warning: ignoring return value of ‘int f()’, declared with attribute nodiscard [-Wunused-result]
    6 |     f();
      |     ~^~
main.cpp:1:19: note: declared here
    1 | [[nodiscard]] int f() {
      | 

The following all avoid the warning:

(void)f();
[[maybe_unused]] int i = f();

I wasn't able to use maybe_unused directly on the f() call:

[[maybe_unused]] f();

gives:

main.cpp: In function ‘int main()’:
main.cpp:6:5: warning: attributes at the beginning of statement are ignored [-Wattributes]
    6 |     [[maybe_unused]] f();
      |     ^~~~~~~~~~~~~~~~

The (void) cast working does not appear to be mandatory but is "encouraged" in the standard: https://stackoverflow.com/questions/53581744/how-can-i-intentionally-discard-a-nodiscard-return-value

Also as seen from the warning message, one "solution" to the warning is to add -Wno-unused-result:

g++ -std=c++17 -ggdb3 -O0 -Wall -Wextra -pedantic -Wno-unused-result -o main.out main.cpp

although I wouldn't of course recommend ignoring warnings globally like this.

C++20 also allows you to add a reason to the nodiscard as in [[nodiscard("reason")]] as mentioned at: https://en.cppreference.com/w/cpp/language/attributes/nodiscard

GCC warn_unused_result attribute

Before the standardization of [[nodiscard]], and for C before they finally decide to standardize attributes, GCC implemented the exact same functionality with the warn_unused_result:

int f() __attribute__ ((warn_unused_result));

int f() {
    return 1;
}

int main() {
    f();
}

which gives:

main.cpp: In function ‘int main()’:
main.cpp:8:6: warning: ignoring return value of ‘int f()’, declared with attribute warn_unused_result [-Wunused-result]
    8 |     f();
      |     ~^~

It should be noted then that since ANSI C does not have a standard for this, ANSI C does not specify which C standard library functions have the attribute or not and therefore implementations have made their own decisions on what should or not be marked with warn_unuesd_result, which is why in general you would have to use the (void) cast to ignore returns of any calls to standard library functions to fully avoid warnings in any implementation.

Tested in GCC 9.2.1, Ubuntu 19.10.

Also when verifying your code complies to MISTA (or other) standards, automatic tools such as LDRA will not allow you to call a function that has a return type without having it return a value unless you explicitly cast the returned value to (void)