NAME

std::ranges::max - std::ranges::max

Synopsis

Defined in header <algorithm>
Call signature
template< class T, class Proj = std::identity,

std::indirect_strict_weak_order< (since
std::projected<const T*, Proj>> Comp = ranges::less > C++20)

constexpr const T& max( const T& a, const T& b, Comp comp = {}, Proj proj =
{} );
template< std::copyable T, class Proj = std::identity,

std::indirect_strict_weak_order< (since
std::projected<const T*, Proj>> Comp = ranges::less > C++20)

constexpr const T max( std::initializer_list<T> r, Comp comp = {}, Proj
proj = {} );
template< ranges::input_range R, class Proj = std::identity,

std::indirect_strict_weak_order<
std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > (since
requires std::indirectly_copyable_storable<ranges::iterator_t<R>, C++20)
ranges::range_value_t<R>*>

constexpr ranges::range_value_t<R> max( R&& r, Comp comp = {}, Proj proj =
{} );

Returns the greater of the given projected values.

1) Returns the greater of a and b.
2) Returns the first greatest value in the initializer list r.
3) Returns the first greatest value in the range r.

The function-like entities described on this page are niebloids, that is:

* Explicit template argument lists may not be specified when calling any of them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the left
of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters

a, b - the values to compare
r - the range of values to compare
comp - comparison to apply to the projected elements
proj - projection to apply to the elements

Return value

1) The greater of a and b, according to their respective projected values. If they
are equivalent, returns a.
2-3) The greatest value in r, according to the projection. If several values are
equivalent to the greatest, returns the leftmost one. If the range is empty (as
determined by ranges::distance(r)), the behavior is undefined.

Complexity

1) Exactly one comparison.
2-3) Exactly ranges::distance(r) - 1 comparisons.

Possible implementation

struct max_fn {
template<class T, class Proj = std::identity,
std::indirect_strict_weak_order<
std::projected<const T*, Proj>> Comp = ranges::less>
constexpr
const T& operator()(const T& a, const T& b, Comp comp = {}, Proj proj = {}) const
{
return std::invoke(comp, std::invoke(proj, a), std::invoke(proj, b)) ? b : a;
}

template<std::copyable T, class Proj = std::identity,
std::indirect_strict_weak_order<
std::projected<const T*, Proj>> Comp = ranges::less>
constexpr
const T operator()(std::initializer_list<T> r, Comp comp = {}, Proj proj = {}) const
{
return *ranges::max_element(r, std::ref(comp), std::ref(proj));
}

template<ranges::input_range R, class Proj = std::identity,
std::indirect_strict_weak_order<
std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
requires std::indirectly_copyable_storable<ranges::iterator_t<R>,
ranges::range_value_t<R>*>
constexpr
ranges::range_value_t<R> operator()(R&& r, Comp comp = {}, Proj proj = {}) const
{
using V = ranges::range_value_t<R>;
if constexpr (ranges::forward_range<R>) {
return static_cast<V>(*ranges::max_element(r, std::ref(comp), std::ref(proj)));
}
else {
auto i = ranges::begin(r);
auto s = ranges::end(r);
V m(*i);
while (++i != s) {
if (std::invoke(comp, std::invoke(proj, m), std::invoke(proj, *i))) {
m = *i;
}
}
return m;
}
}
};

inline constexpr max_fn max;

Notes

Capturing the result of std::ranges::max by reference produces a dangling reference
if one of the parameters is a temporary and that parameter is returned:

int n = 1;
const int& r = std::ranges::max(n-1, n+1);
// r is dangling

Example

// Run this code

#include <algorithm>
#include <iostream>
#include <string>

int main()
{
namespace ranges = std::ranges;
using namespace std::string_view_literals;
std::cout << "larger of 1 and 9999: " << ranges::max(1, 9999) << '\n'
<< "larger of 'a', and 'b': '" << ranges::max('a', 'b') << "'\n"
<< "longest of \"foo\", \"bar\", and \"hello\": \""
<< ranges::max({ "foo"sv, "bar"sv, "hello"sv }, {},
&std::string_view::size) << "\"\n";
}

Output:

larger of 1 and 9999: 9999
larger of 'a', and 'b': 'b'
longest of "foo", "bar", and "hello": "hello"