I want to prevent people from calling the lambda without handling the return value.
Clang 4.0 refuses everything I've tried, compiling with -std=c++1z:
auto x = [&] [[nodiscard]] () { return 1; };
// error: nodiscard attribute cannot be applied to types
auto x = [[nodiscard]] [&]() { return 1; };
// error: expected variable name or 'this' in lambda capture list
auto x [[nodiscard]] = [&]() { return 1; };
// warning: nodiscard attribute only applies to functions, methods, enums, and classes
[[nodiscard]] auto x = [&]() { return 1; };
// warning: nodiscard attribute only applies to functions, methods, enums, and classes
auto x = [&]() [[nodiscard]] { return 1; };
// error: nodiscard attribute cannot be applied to types
Is this some sort of bug in clang or a hole in the standard?
Update: C++23 introduced attributes on lambdas immediately following the captures, so your example syntax 1 is now correct!
auto lambda = [] [[nodiscard]] (auto... params) {
// ....
};
In C++20 or earlier, you can't apply nodiscard
to lambdas, but you can write a wrapper:
template <typename F>
struct NoDiscard {
F f;
NoDiscard(F const& f) : f(f) {}
template <typename... T>
[[nodiscard]] constexpr auto operator()(T&&... t) const
noexcept(noexcept(f(std::forward<T>(t)...))) {
return f(std::forward<T>(t)...);
}
};
int main() {
NoDiscard([](int i) {return i;})(0);
}
Demo.