General min and max - C++

Question

Writing a general minimum function, Two questions came to my mind. The code works fine with any input type and different argument number:

namespace xyz
{

template <typename T1, typename T2>
auto min(const T1 &a, const T2 &b) -> decltype(a+b)
{
    return a < b ? a : b;
}

template <typename T1, typename T2, typename ... Args>
auto min(const T1 &a, const T2 &b, Args ... args) -> decltype(a+b)
{
    return min(min(a, b), args...);
}

}

int main()
{
    cout << xyz::min(4, 5.8f, 3, 1.8, 3, 1.1, 9) << endl;
    //                   ^        ^           ^
    //                   |        |           |
    //                 float    double       int
}

• Is there a better replacement for decltype(a+b) ? I thing there's a standard class which I can't remember, something like decltype(std::THE_RESULT<a,b>::type) .

• The returned type of that decltype(std::THE_RESULT<a,b>::type) is const & or not ?

Answer 1

std::common_type ( c++11 ):

For non-specialized std::common_type , the rules for determining the common type between every pair T1 , T2 are exactly the rules for determining the return type of the ternary conditional operator where T1 and T2 are the types of its second and the third operands.

and

For arithmetic types, the common type may also be viewed as the type of the (possibly mixed-mode) arithmetic expression such as T0() + T1() + ... + Tn().

Not sure about const& , but you could play with std::remove_cv and std::remove_reference (and std::is_reference to find out the answer).

In fact, here's a list of type support utilities. Knock yourself out.

Answer 2

After the answer and worth comments I did it as below:

template <typename T1, typename T2>
auto min(const T1 &a, const T2 &b) 
-> typename std::common_type<const T1&, const T2&>::type
{
    return a < b ? a : b;
}

template <typename T1, typename T2, typename ... Args>
auto min(const T1 &a, const T2 &b, const Args& ... args)
-> typename std::common_type<const T1&, const T2&, const Args& ...>::type
{
    return min(min(a, b), args...);
}