CODEWITHSUNDEEP
c++algorithmc++14minmax
TemplateRex
TemplateRex

Reputation: 70526

Comparing std::minmax against a pair

The standard algorithms min and max can be compared against a single value. However, the minmax algorithm return value cannot be compared against a pair of values: 

#include <algorithm>
#include <utility>

template<class T1, class T2>
constexpr auto make_cref_pair(T1&& t1, T2&& t2)
{
    return std::pair<T1 const&, T2 const&>(std::forward<T1>(t1), std::forward<T2>(t2));
}

int main()
{
    static_assert(std::min(2, 1) == 1); // OK
    static_assert(std::max(2, 1) == 2); // OK
    //static_assert(std::minmax(2, 1) == std::make_pair(1, 2)); // ERROR, const int& vs int pair comparison
    static_assert(std::minmax(2, 1) == std::pair<const int&, const int&>(1, 2)); // OK
    static_assert(std::minmax(2, 1) == make_cref_pair(1, 2)); // OK
}

Live Example

The reason is that make_pair(2, 1) returns a pair<int, int> and minmax(1, 2) returns a pair<const int&, const int&>. There are no referenceness-mixing operator== overloads for pair.

The fix is then to explicitly write std::pair<const int&, const int&>(int, int) or to wrap this in a home-made make_cref_pair function.

Questions: is there a cleaner way to compare the minmax return value against a pair of values? And did I correctly handle the references in my make_cref_pair?

Upvotes: 11

Views: 900

Answers (3)

mindriot
mindriot

Reputation: 5668

One option is to explicitly convert the left-hand side to std::pair<int,int>:

#include <algorithm>
#include <utility>

template <typename T1, typename T2>
constexpr std::pair<T1,T2> myminmax(const T1& t1, const T2& t2)
{
    return std::minmax(t1,t2);
}

int main()
{
    static_assert(myminmax(2, 1) == std::make_pair(1, 2));
}

Upvotes: 3

NathanOliver
NathanOliver

Reputation: 180720

One thing you could do is take advantage of the std::minmax overload that takes a std::initializer_list<T> and returns a std::pair<T,T>. Using that you could have

int main()
{
    const int a = 10, b = 20;
    static_assert(std::minmax({2, 1}) == std::make_pair(1, 2));
    static_assert(std::minmax({a, b}) == std::make_pair(a, b));
}

Which will compile and allows you to get rid of make_cref_pair. It does call std::minmax_element so I am not sure if this decreases the efficiency or not.

Upvotes: 5

user6362820
user6362820

Reputation: 136

std::minmax has an initializer_list overload. This returns a non-const non-reference pair:

static_assert(std::minmax({2, 1}) == std::make_pair(1, 2));

Unfortunately this may be less performant, since the complexities respectively are "exactly one comparison" and "at most (3/2) * t.size() applications of the corresponding predicate".

Upvotes: 12

Related Questions