One simple way of doing this prior to C 20 is to do a nested loop:

template<typename Container>
constexpr bool has_duplicate(Container&& container)
{
    for (auto it1 = container.cbegin(); it1 != container.cend();   it1)
        for(auto it2 = container.cbegin(); it2 != it1;   it2)
            if (*it1 == *it2)
                return 1;
    return 0;
}

With the addition of init-statement in range-based for loop, and the introduction of ranges::subrange, I believe this function could be rewritten with range-based for loop:

template<std::ranges::input_range Container>
constexpr bool has_duplicate(Container&& container)
{
    for(auto it = container.cbegin(); const auto& obj1 : container)
        for(const auto& obj2 : std::ranges::subrange(container.cbegin(), it  ))
            if(obj1 == obj2) 
                return 1;
    return 0;
}

While it worked fine on gcc, it fails to compile with clang, unless I manually set it with libc : https://godbolt.org/z/KM4a9zazs, with the following error:

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.0/../../../../include/c  /12.0.0/bits/iterator_concepts.h:982:13: error: no matching function for call to '__begin'
        = decltype(ranges::__cust_access::__begin(std::declval<_Tp&>()));

• Am I doing something wrong, or was it clang?

I've also tried:

template<std::ranges::input_range Container>
constexpr bool has_duplicate(Container&& container)
{
    for(auto index = 0ull; const auto& obj1 : container)
        for(const auto& obj2 : container | std::ranges::views::take(index  ))
            if(obj1 == obj2) return 1;
    return 0;
}

Once again gcc worked with no problem, but clang failed with either libraries.

CodePudding user response：

Godbolt uses libstdc by default, even when compiling with Clang, which is somewhat confusing.

Whether Clang should actually fully support linking towards libstdc or not, I cannot say, but e.g. the following Clang bug report highlights the same issue:

• Bug 46746 - Clang can't compile libstdc ranges

Minimal example from the bug report:

#include <ranges>

void foo() {
    std::ranges::iota_view iota(2, 10);
    iota.begin(); 
}

error: no matching function for call to '__begin'