Initialize a lambda function in the body of a class constructor

Question

I can compile the following code without any problem (using gcc 11.1.0):

#include <iostream> template <typename Func> class K { Func f; public: K(Func _f): f{_f} {}; void do_thing(int x) {f(x);}; }; int main() { auto f = [](int x) {std::cout << x << std::endl;}; K kl{f}; kl.do_thing(5); return 0; }

however I would like to perform some check in the constructor of the class K (for instance some std::is_convertible_v inside some bool function), so I tried to modify the code to

#include <iostream> template <typename Func> class K { Func f; public: K(Func _f) { ... f = _f;}; void do_thing(int x) {f(x);}; }; int main() { auto f = [](int x) {std::cout << x << std::endl;}; K kl{f}; kl.do_thing(5); return 0; }

which however gives me some error message

error: use of deleted function ‘main()::<lambda(int)>::<lambda>()’

and then

note: a lambda closure type has a deleted default constructor

This confuses me a lot since I cannot understand how it is possible that the former piece of code could compile since the lambda function has not default constructor.

Question
How can I set my f inside the body of the constructor? (This is just a MWE and in my case the class is a bit more complex and the checks I mentioned before make sense.)

you cannot initialize the member in the body of the constructor. It is initialized before the body of the constructor is executed. — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 8, 2021 at 10:49
I see, so how can I set the f to be the same as _f inside the body, after the checks I have to do? — MaPo
– MaPo, Commented Sep 8, 2021 at 10:51
I dont quite understand your motivation. What kind of checks can you do with the second version of the code that you cannot do with the first? — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 8, 2021 at 10:51
sounds like a xy problem. Your complete code will have more issues because each lamda expression has a unique type. Maybe ask for the actual problem you are trying to solve, but consider that this question already has an answer, so perhaps open a new one — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 8, 2021 at 10:54
you could use SFINAE to check the right signature, then initializing the lambda in the initializer list is fine. — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 8, 2021 at 10:56

songyuanyao · Accepted Answer · 2021-09-08 10:58:07Z

How can I initialize my f inside the body of the constructor?

You can't. f = _f; inside the constructor body is assignment but not initialization. So f will be default-initialized firstly, then enter the constructor body (to perform assignment).

You might use std::function instead; which could be default-initialized, then you can assign it in constructor body.

BTW: Since C++20 your code will compile fine even it might not work as you expected (depending on the ... part). For lambdas,

If no captures are specified, the closure type has a defaulted default constructor. Otherwise, it has no default constructor (this includes the case when there is a capture-default, even if it does not actually capture anything).

it will compile, but I am not sure if it achieves what OP wants to achieve by that modification of their code
@463035818_is_not_a_number Yes, it depends on how the check is performed and what should be performed when check fails.

Pepijn Kramer · Accepted Answer · 2021-09-08 12:25:50Z

Is this what you are looking for? It is the generic solution, not making any assumptions on the signature of Fn

#include <iostream> #include <utility> template <typename Fn> class K { public: explicit K(const Fn& fn) : m_fn{ fn } { }; template<typename... args_t> auto do_thing(args_t&&... args) { return m_fn(std::forward<args_t>(args)...); } private: Fn m_fn; }; int main() { auto f = [](int x) {std::cout << x << std::endl; }; K kl{ f }; kl.do_thing(5); }

Collectives™ on Stack Overflow

Initialize a lambda function in the body of a class constructor

2 Answers 2

2 Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Related