However, a normal function pointer worked:

A conversion to function pointer exists for the function object types of lambdas that don't capture any variables. It should be pretty obvious why lambdas that do capture variables don't have that conversion.

#include <cstdio>
#include <functional>
 
int main ()
{
    volatile int a = 5;
    std::function<void ()> func = [a] () { printf("%i\n", a); };
    func();
    return 0;
}

However, a normal function pointer worked:

A conversion to function pointer exists for the function object types of lambdas that don't capture any variables. It should be pretty obvious why lambdas that do capture variables don't have that conversion.

#include <cstdio>
#include <functional>

int main ()
{
		volatile int a = 5;
		std::function<void ()> func = [a] () { printf("%i\n", a); };
	    func();
		return 0;
}

However, a normal function pointer worked:

A conversion to function pointer exists for the function object types of lambdas that don't capture any variables. It should be pretty obvious why lambdas that do capture variables don't have that conversion.

#include <cstdio>
#include <functional>

int main ()
{
	    volatile int a = 5;
	    std::function<void ()> func = [a] () { printf("%i\n", a); };
func();
	    return 0;
}