Marc Gregoire’s Blog

C++17 introduces three new code attributes:

• [[fallthrough]]
• [[maybe_unused]]
• [[nodiscard]]

The first one was discussed in detail in my C++17: Fallthrough in switch statements blog post. The others are briefly explained below.

[[maybe_unused]]

This attribute can be used to mark variables or functions that might be unused to avoid the compiler from generating a warning for such variables and functions.

For example, suppose you have code as follows:

bool DoSomething()
{
    // ... do something ...
    return true;
}

int main()
{
    bool result = DoSomething();
    assert(result);
}

When you build a release build of your code, the assert() macro becomes a no-operation. As such, in release builds, the result variable is unused, while in debug you need the result variable for the assert(). To avoid the compiler warning, use the [[maybe_unused]] attribute as follows:

int main()
{
    [[maybe_unused]] bool result = DoSomething();
    assert(result);
}

This attribute can also be used on functions. Maybe you have a function that is only executed in debug. In such a case, you can mark the function with the attribute. For example:

static bool OnlyUsedInDebug() { return true; }

int main()
{
    assert(OnlyUsedInDebug());
}

When compiling with gcc, the compiler gives the following warning:

warning: ‘bool OnlyUsedInDebug()’ defined but not used [-Wunused-function]

The avoid this warning, use [[maybe_unused]]:

[[maybe_unused]] static bool OnlyUsedInDebug() { return true; }

[[nodiscard]]

This attribute can be used to make sure the caller of a function does not just discard the return value of that function. For example:

bool DoSomething() { return true; }

int main()
{
    DoSomething();
}

The user is not doing anything with the return value of the DoSomething() function, but the compiler will not give any warning about this. Now, the following code adds the [[nodiscard]] attribute:

[[nodiscard]] bool DoSomething() { return true; }

When you try to call it as in the previous main() function, the compiler will complain with something as follows:

warning C4834: discarding return value of function with ‘nodiscard’ attribute