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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/../iterator/ranges/distance.html below:

std::ranges::distance - cppreference.com

1,2) Returns the number of hops from first to last.

3) Returns the size of r as a signed integer.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

1) The number of increments needed to go from first to last.

1) Linear.

2) Constant.

#include <cassert>
#include <forward_list>
#include <iterator>
#include <vector>
 
int main() 
{
    std::vector<int> v{3, 1, 4};
    assert(std::ranges::distance(v.begin(), v.end()) == 3);
    assert(std::ranges::distance(v.end(), v.begin()) == -3);
    assert(std::ranges::distance(v) == 3);
 
    std::forward_list<int> l{2, 7, 1};
    // auto size = std::ranges::size(l); // error: not a sizable range
    auto size = std::ranges::distance(l); // OK, but aware O(N) complexity
    assert(size == 3);
}

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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