Question

I am trying to write a function that will set the first argument to the value of the second argument. However, when the second argument is a container class, I want it to set the first argument to the first element of the container. I found this question which answers a similar problem, however, I can not seem to get it to work in my case.

When I compile this code I get an error saying that SetVar is ambiguous. Is there anyway to get the functionality working?

Here is my code so far…

#include <iostream>
#include <vector>
template<typename T1,typename T2>
static void SetVar(T1& a, const T2 &b, typename  T2::const_iterator= T2().begin()){
    //Check to make sure b contains an element.
    if(b.begin()!=b.end())  a=*b.begin();
}
template<typename T1,typename T2>
static void SetVar(T1& a, const T2 &b,...){
    a=b;
}
int main(int argc, const char * argv[])
{
    int x;
    int y=5;
    std::vector<int> z;
    z.push_back(1);
    z.push_back(3);
    SetVar(x, y);
    //Should print 5
    std::cout<<x<<"\n";

    SetVar(x, z);//<---SetVar is ambiguous
    //Should print 1
    std::cout<<x<<"\n";
    return 0;
}

Answer 1

Passing argument to an ellipsis does make a function overload less preferred than one that has an actual parameter for the same argument, but that doesn’t come up here because there is no third argument in your calls.

I would use enable_if for both:

#include <type_traits>

// Enabled if T2 has `const_iterator` and `begin()`:
template<typename T1,typename T2>
auto SetVar(T1& a, const T2 &b)
  -> typename std::enable_if<std::is_convertible<
         decltype(std::declval<const T2&>().begin()),
         typename T2::const_iterator>::value
     >::type
{
    //Check to make sure b contains an element.
    if(b.begin()!=b.end())  a=*b.begin();
}

// Enabled if expression 'a=b' is valid.
template<typename T1,typename T2>
auto SetVar(T1& a, const T2 &b)
  -> typename std::enable_if<std::is_assignable<T1, const T2&>::value>::type
{
    a=b;
}

It’s still possible for the above to be ambiguous, but only if both conditions are true, which means a weird container or implicit conversion is going on – and in that case I would want the compiler to warn me of the confusion.