how to save any callable in c++ template class

Question

I wanted to define a scopeguard, which runs some clean up code when scope exit. And I don't want to use std::function<void()> since it has some overhead. Then I come up with this implementation that is templated.

template <typename Fn>
class ScopeGuard {
public:
    ScopeGuard(Fn&& fn) : fn_(std::forward<T>(fn)) {}
    ~ScopeGuard() { fn_(); }
private:
    Fn fn_;
};

This works nicely for lambdas. However, it does not accept global functions, which are lvalues. How to solve this problem?

I think this is a more general question, how to make a template that can save any callable object (global functions, lambdas, class instance that has operator() with minimal overhead.

Is the following a better way?

template <typename Fn>
class ScopeGuard {
public:
    template <typename T>
    ScopeGuard(T&& fn) : fn_(std::forward<T>(fn)) {}
    ~ScopeGuard() {
        fn_();
    }
private:
    Fn fn_;
};

Does it have to be as generic as `std::function`? If _yes_, then ok, `std::function` is probably as good as it gets. — Ted Lyngmo, Apr 15 '23 at 05:09
Unrelated: Scopeguarding a functor seems wrong. From a resource it's captured or... what? I don't get it. — Ted Lyngmo, Apr 15 '23 at 05:10
_"This works nicely for lambdas"_ ... unless the lambdas are capturing. Then it does not work for lambdas. — Ted Lyngmo, Apr 15 '23 at 05:13
@TedLyngmo why do you think lambdas that are capturing does not work? — doraemon, Apr 15 '23 at 05:19
They'll _work_ in the sense that they'll do the _work_ but the carrying functor types are incompatible. — Ted Lyngmo, Apr 15 '23 at 05:30

score 0 · Answer 1 · answered May 05 '23 at 23:59

0

Actually the first implementation works for global functions as well - just use

ScopeGuard g(&foo);

answered May 05 '23 at 23:59

muji4ok

16

how to save any callable in c++ template class

1 Answers1