Why is const reference parameter compatible to value parameter?

[JUAN DENT]

I have this code:

void f_source(const int& val)
{}

void CopyFn()
{
	std::function<void(const int&)> fn_source = f_source;
	std::function<void(int)> fn = f_source;
}

Why is the last assignment inside CopyFn() legal C++? f_source takes a const reference to int while the fn object takes an int by value...

Seems to me they are different and should not compile.

Regards,
Juan

[Peter87]

Because there is no reason not to allow it. When f_source's operator() receives an int, by value, there is nothing preventing it from passing that int as argument to a function with a const int& parameter. The return and parameter types doesn't have to match exactly as long as they are compatible.