Cfrgtkky

Say I have

[[nodiscard]] int foo ()

{

    return 0;

}



int main ()

{

    foo ();

}

then

error: ignoring return value of ‘int foo()’, declared with attribute nodiscard [-Werror=unused-result]

but if

int x = foo ();

then

error: unused variable ‘x’ [-Werror=unused-variable]

Is there a clean way of telling the compiler "I want to discard this [[nodiscard]] value"?

The WG14 nodiscard proposal discusses the rationale for allowing the diagnostic to be silenced by casting to void. It says casting to void is the encouraged (if non-normative) way to silence it which follows what the existing implementation do with __attribute__((warn_unused_result)):

The [[nodiscard]] attribute has extensive real-world use, being implemented by Clang and GCC as __attribute__((warn_unused_result))
, but was standardized under the name [[nodiscard]] by WG21. This proposal chose the identifier nodiscard
because deviation from this name would create a needless incompatibility with C++.

The semantics of this attribute rely heavily on the notion of a use,
the definition of which is left to implementation discretion. However,
the non-normative guidance specified by WG21 is to encourage
implementations to emit a warning diagnostic when a nodiscard function
call is used in a potentially-evalulated discarded-value expression
unless it is an explicit cast to void. This means that an
implementation is not encouraged to perform dataflow analysis (like an
initialized-but- unused local variable diagnostic would require).
...

The C++ way would be static_cast<void>.

See the draft C++ standard [[dcl.attr.nodiscard]p2:

[ Note: A nodiscard call is a function call expression that calls a function previously declared nodiscard, or whose return type is a possibly cv-qualified class or enumeration type marked nodiscard.
Appearance of a nodiscard call as a potentially-evaluated discarded-value expression is discouraged unless explicitly cast to void.
Implementations should issue a warning in such cases.
This is typically because discarding the return value of a nodiscard call has surprising consequences.
— end note]

This is a note, so non-normative but basically this is what existing implementations do with __attribute__((warn_unused_result)). Also, note a diagnostic for nodiscard is also also non-normative, so a diagnostic for violating nodiscard is not ill-formed but a quality of implementation just like suppressing via a cast to void is.

see the clang document on nodiscard, warn_unused_result:

Clang supports the ability to diagnose when the results of a function call expression are discarded under suspicious circumstances. A diagnostic is generated when a function or its return type is marked with [[nodiscard]] (or __attribute__((warn_unused_result))) and the function call appears as a potentially-evaluated discarded-value expression that is not explicitly cast to void.

Cast it to void:

[[nodiscard]] int foo ()

{

    return 0;

}



int main ()

{

    static_cast<void>(foo());

}

This basically tells the compiler "Yes I know I'm discarding this, yes I'm sure of it."

You can also tag the returned int with another tag:

[[nodiscard]] int foo ()

{

    return 0;

}



int main ()

{

    [[maybe_unused]] int i = foo ();

}

Could be useful if you have some debug-only code that requires the value.