Operator comma overloading

Question

I'm trying to learn more about how operator overloading works.

I understand that overloading the comma operator may not be the best idea, but this is for instructional purposes only.

I'm expecting the following code to use my overloaded operator (I used brackets as I'm aware the comma operator has the lowest precedence) to construct a vector containing (1,2) and then call the vector's assignment operator.

However, I get an error:

no known conversion from argument 1 from 'int' to 'const std::vector<int>&'

I don't understand why this is happening. (1,2) should construct a vector so it shouldn't be trying to convert from an int to a vector<int>?

#include <vector>
#include <utility>

using std::vector;
using std::move;

template <typename T>
vector<T> operator,(const T& v1, const T& v2)
{
    vector<T> v;
    v.push_back(v1);
    v.push_back(v2);
    return move(v);
}

int main()
{
    vector<int> a;
    a = (1,2);
    return 0;
}

Answer 1

There is already a built-in definition for the comma operator applied to ints. Your template isn't even in the running for overload resolution, since you can't overload operators unless at least one of the arguments is a user defined type.

You could do something like this:

template<typename T>
struct vector_maker
{
    std::vector<T> vec;
    vector_maker& operator,(T const& rhs) {
        vec.push_back(rhs);
        return *this;
    }

    std::vector<T> finalize() {
        return std::move(vec);
    }
};

int main() {
    auto a = (vector_maker<int>(),1,2,3,4,5).finalize();
}

Or take a look at Boost.Assign, which allows constructions like this:

std::vector<int> a;
a += 1,2,3,4,5,6,7,8;