[MS] How can I tell C++ that I want to discard a nodiscard value? - devamazonaws.blogspot.com

C++ lets you add the [[nodiscard]] attribute to a function return value to indicate that the caller must use the result.

Given the declaration

[[nodiscard]] int important();

simply calling the function and allow the value to be discarded produces diagnostics.

void test()
{
    important();
}

clang: ignoring return value of 'int important()', declared with attribute 'nodiscard' [-Wunused-result]

gcc: ignoring return value of 'int important()', declared with attribute 'nodiscard' [-Wunused-result]

msvc: C4834: discarding return value of function with [[nodiscard]] attribute

Explicitly casting to (void) works:

void test()
{
    (void)important();
}

Note that this requires a C-style cast. You cannot static_cast or reinterpret_cast to void.

Another option is to store the result into a variable which is attributed as unused, and then allowing the variable to go out of scope immediately.

void test()
{
    { [[maybe_unused]] auto&& unused = important(); }
}

There is a proposal for C++26 to express the discard with std::ignore:

void test()
{
    std::ignore = important();
}

Although the ability to assign to std::ignore is not formally required, in practice, you have always been able to do it, and the C++ Core Guidelines even recommends it!

The first is tersest, though it suffers from pedagogical issues discussed in the std::ignore proposal. The third is fairly brief and has the benefit of clarity, but suffers from technically not being allowed (though everybody allows it in practice, so much so that even the C++ Core Guidelines were fooled). The second is most verbose, and the only things it has going for it are the pedagogical avoidance of the (void) cast and the language-lawyer avoidance of undocumented use of std::ignore. (In other words, the third option is "technically" the most correct, the best kind of correct.)

There's an alternate C++26 proposal for expressing the discard with a new [[discard]] attribute.

void test()
{
    [[discard("reason")]] important();
}

Post Updated on March 29, 2024 at 02:00PM
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