How to combine std::bind(), variadic templates, and perfect forwarding? - c++

Question

I want to invoke a method from another, through a third-party function; but both use variadic templates. For example:
void third_party(int n, std::function<void(int)> f)
{
f(n);
}
struct foo
{
template <typename... Args>
void invoke(int n, Args&&... args)
{
auto bound = std::bind(&foo::invoke_impl<Args...>, this,
std::placeholders::_1, std::forward<Args>(args)...);
third_party(n, bound);
}
template <typename... Args>
void invoke_impl(int, Args&&...)
{
}
};
foo f;
f.invoke(1, 2);
Problem is, I get a compilation error:
/usr/include/c++/4.7/functional:1206:35: error: cannot bind ‘int’ lvalue to ‘int&&’
I tried using a lambda, but maybe GCC 4.8 does not handle the syntax yet; here is what I tried:
auto bound = [this, &args...] (int k) { invoke_impl(k, std::foward<Args>(args)...); };
I get the following error:
error: expected ‘,’ before ‘...’ token
error: expected identifier before ‘...’ token
error: parameter packs not expanded with ‘...’:
note: ‘args’
From what I understand, the compiler wants to instantiate invoke_impl with type int&&, while I thought that using && in this case would preserve the actual argument type.
What am I doing wrong? Thanks,

Answer 1

Binding to &foo::invoke_impl<Args...> will create a bound function that takes an Args&& parameter, meaning an rvalue. The problem is that the parameter passed will be an lvalue because the argument is stored as a member function of some internal class.
To fix, utilize reference collapsing rules by changing &foo::invoke_impl<Args...> to &foo::invoke_impl<Args&...> so the member function will take an lvalue.
auto bound = std::bind(&foo::invoke_impl<Args&...>, this,
std::placeholders::_1, std::forward<Args>(args)...);
Here is a demo.