std::minmax

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minmax
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Defined in header <algorithm>
template< class T >
std::pair<const T&, const T&> minmax( const T& a, const T& b );
 (1) (since C++11)
(constexpr since C++14)
template< class T, class Compare >

std::pair<const T&, const T&> minmax( const T& a, const T& b,

                                      Compare comp );
 (2) (since C++11)
(constexpr since C++14)
template< class T >
std::pair<T, T> minmax( std::initializer_list<T> ilist );
 (3) (since C++11)
(constexpr since C++14)
template< class T, class Compare >

std::pair<T, T> minmax( std::initializer_list<T> ilist,

                        Compare comp );
 (4) (since C++11)
(constexpr since C++14)

Returns the lowest and the greatest of the given values.

1,2) Returns references to the smaller and the greater of a and b.
1) Uses operator< to compare the values.
If T is not LessThanComparable, the behavior is undefined.
2) Use the comparison function comp to compare the values.
3,4) Returns the smallest and the greatest of the values in initializer list ilist.
If ilist.size() is zero, or T is not CopyConstructible, the behavior is undefined.
3) Uses operator< to compare the values.
If T is not LessThanComparable, the behavior is undefined.
4) Use the comparison function comp to compare the values.

Parameters

a, b - the values to compare
ilist - initializer list with the values to compare
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns true if the first argument is less than the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(const Type1& a, const Type2& b);

While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
The types Type1 and Type2 must be such that an object of type T can be implicitly converted to both of them.

Return value

1,2) Returns the result of std::pair<const T&, const T&>(a, b) if a < b or if a is equivalent to b. Returns the result of std::pair<const T&, const T&>(b, a) if b < a.
3,4) A pair with the smallest value in ilist as the first element and the greatest as the second. If several elements are equivalent to the smallest, the leftmost such element is returned. If several elements are equivalent to the largest, the rightmost such element is returned.

Complexity

1) Exactly one comparison using operator<.
2) Exactly one application of the comparison function comp.
3,4) Given N as ilist.size():
3) At most
3N
2
 comparisons using operator<.
4) At most
3N
2
 applications of the comparison function comp.

Possible implementation

minmax (1) 
template<class T>
constexpr std::pair<const T&, const T&> minmax(const T& a, const T& b)
{
    return (b < a) ? std::pair<const T&, const T&>(b, a)
                   : std::pair<const T&, const T&>(a, b);
}
minmax (2) 
template<class T, class Compare>
constexpr std::pair<const T&, const T&> minmax(const T& a, const T& b, Compare comp)
{
    return comp(b, a) ? std::pair<const T&, const T&>(b, a)
                      : std::pair<const T&, const T&>(a, b);
}
minmax (3) 
template<class T>
constexpr std::pair<T, T> minmax(std::initializer_list<T> ilist)
{
    auto p = std::minmax_element(ilist.begin(), ilist.end());
    return std::pair(*p.first, *p.second);
}
minmax (4) 
template<class T, class Compare>
constexpr std::pair<T, T> minmax(std::initializer_list<T> ilist, Compare comp)
{
    auto p = std::minmax_element(ilist.begin(), ilist.end(), comp);
    return std::pair(*p.first, *p.second);
}

Notes

For overloads (1,2), if one of the parameters is a temporary, the reference returned becomes a dangling reference at the end of the full expression that contains the call to minmax: 

int n = 1;
auto p = std::minmax(n, n + 1);
int m = p.first; // ok
int x = p.second; // undefined behavior
 
// Note that structured bindings have the same issue
auto [mm, xx] = std::minmax(n, n + 1);
xx; // undefined behavior

Example

Run this code
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v{3, 1, 4, 1, 5, 9, 2, 6};
    std::srand(std::time(0));
    std::pair<int, int> bounds = std::minmax(std::rand() % v.size(),
                                             std::rand() % v.size());
 
    std::cout << "v[" << bounds.first << "," << bounds.second << "]: ";
    for (int i = bounds.first; i < bounds.second; ++i)
        std::cout << v[i] << ' ';
    std::cout << '\n';
}

Possible output: 

v[2,7]: 4 1 5 9 2

See also

 returns the smaller of the given values
(function template)
returns the greater of the given values
(function template)
(C++11)
 returns the smallest and the largest elements in a range
(function template)
(C++20)
 returns the smaller and larger of two elements
(niebloid)
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