the conversion from a reference to a value looks strange to me
Why?
Does this look strange too?
int foo(int i) { return i; }void bar(const int& ir) { foo(ir); }This is exactly the same. A function taking an int by value gets called by another function, taking an int by const-reference.
Inside bar the variable ir gets copied, and the return value gets ignored. That's exactly what happens inside the std::function<void(const int&)> when it has a target with the signature int(int).