The code below doesn\'t compile on gcc 4.5 because the call to foo is ambiguous. What is the correct way to disambiguate it?
#include
#
The best way is to explicitly create a std::function
object of the correct type then pass that object to the function:
std::function<void(int, int)> func =
[](int a, int b) { cout << "a: " << a << " b: " << b << endl; }
foo(func);
or inline:
foo(
std::function<void(int, int)>(
[](int a, int b) { cout << "a: " << a << "b: " << b << endl; }
));
std::function
has a constructor template that accepts anything:
template<class F> function(F);
Because of this, there's no way for the compiler to know during overload resolution which foo
to select: both std::function<void(int)>
and std::function<void(int, int)>
have a constructor that can take your lambda expression as an argument.
When you pass a std::function
object directly, the std::function
copy constructor is preferred during overload resolution, so it is selected instead of the constructor template.
Answer for the future: If the capture list is guaranteed to be empty, you can also use ordinary function pointers. In C++0x, a captureless lambda is implicitly convertible to a function pointer. So, you can use something like
void foo(void (*t)(int, int)) { t(1, 2); }
void foo(void (*t)(int)) { t(1); }
and call foo
directly with the captureless lambda (or a function pointer with matching type).
Note that this conversion is a very recent addition to the draft language standard (it was added in February of this year), so it is not likely to be widely supported yet. Visual C++ 2010 doesn't support it yet; I don't know about the latest g++.
I've recently been thinking about a similar problem and when looking around for any known solutions I came across this post and lack of solutions for resolving
An alternative solution is to abstract over the functor as a template argument and use decltype to resolve its type. So, the above example would become:
#include <iostream>
#include <functional>
using namespace std;
template<class F>
auto foo(F t) -> decltype(t(1,2))
{
t(1, 2);
}
template<class F>
auto foo(F t) -> decltype(t(2))
{
t(2);
}
int main()
{
foo([](int a, int b){ cout << "a: " << a << " b: " << b << endl;});
}
This works as expected with gcc 4.5.