CODEWITHSUNDEEP
c++lambdamove-semantics
glades
glades

Reputation: 4779

Should I precautionary move callables (e.g. lambdas?)

I have this striped-down example of a timer that I'd like to be instantiable with any kind of callable. Is it advisable to precautionary move the callable into a data member for efficiency?

#include <concepts>
#include <cstdio>
#include <string>
#include <utility>

template <std::invocable Cb>
class timer {
public:
    timer(Cb cb)
        : cb_ { std::move(cb) }
    {
    }

    auto call()
    {
        cb_();
    }

private:
    Cb cb_;
};

int main()
{
    std::string something_to_print = "Hello World!\n";
    timer some_timer([&]() { printf(something_to_print.c_str()); });
    some_timer.call();

    return 0;
}

I can't see any difference in the assembly if I move or copy the lambda. Does it ever make a difference?

Upvotes: 2

Views: 62

Answers (1)

bitmask
bitmask

Reputation: 34636

Your lambda has only reference captures. Moving an lvalue-reference does exactly the same as copying it. If you had [=] captures, the move would actually do something.

The answer to whether or not to do this in general is: "it depends on the situation." W.r.t. performance: measure.

Upvotes: 5

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