Tuesday coding tips are super short posts about various tidbits mainly from C++, but also from other programming languages I use. You can also follow the #TuesdayCodingTips hashtag on Mastodon and Linkedin.
Since C++17, one can use [[nodiscard]] annotation to mark API functions whose return values should not be discarded without consideration. This usually applies to methods that make no sense to call without evaluating their return value (like predicate functions such as empty or a factory function). The compiler will issue a warning if the return value is not handled.
class [[nodiscard]] A {};
// OR
class B {
public:
[[nodiscard]] B() = default;
};
// Enums can be nodiscard too
enum class [[nodiscard]] State {
Default,
Foo,
Bar
};
int main()
{
// warning: ignoring temporary created by a constructor declared with 'nodiscard' attribute
A();
B();
}
But beware! Sometimes it might be tempting to return some scope guard that will maintain postconditions automatically at the end of the scope using RAII (such as committing transactions). However, a user of your API can pipe the result of your nodiscard function into a std::ignore helper which causes the temporary to destruct immediately, defeating the idea of end-of-scope cleanup.
#include <tuple> // std::ignore
class [[nodiscard]] A {};
class B {
public:
[[nodiscard]] bool isOk() const { return true; }
};
// no need for nodiscard, A is nodiscard
A foo() {
return A();
}
// B is not nodiscard, mark it explicitly
[[nodiscard]] B bar() {
return B();
}
int main()
{
foo(); // warning: ignoring temporary created by a constructor declared with 'nodiscard' attribute
bar(); // warning
std::ignore = foo(); // ok
auto b = bar(); // ok
b.isOk(); // warning
if (b.isOk()) {} // ok
}