Why is reverse() directly usable without std:: prefix?

[Jens Hirschmann]

Consider this code snippet:
live demo: http://cpp.sh/5mymy

#include <vector>
#include <algorithm>

int main() {
    std::vector<int> v {1,2,3,4};
    reverse(v.begin(), v.end()); // no need to use std::?
}

I can call reverse() without using the std:: prefix. How is this possible?

Thanks!

[George P]

It looks to work for the C++ containers. std::vector, std::list, etc. Not including std:: will NOT work for a regular array.

Why the difference? Don't have a clue. SWAG: maybe because the C++ containers have builtin iterators.

#include <array>
#include <iostream>
#include <iterator>
#include <algorithm>

[code]#include <array>
#include <iostream>
#include <iterator>
#include <algorithm>

int main()
{
   std::array<int, 3> v { 1,2,3 };

   reverse(std::begin(v), std::end(v));

   for (auto e : v)
   {
      std::cout << e;
   }
   std::cout << '\n';

   int a[] = { 4, 5, 6, 7 };

   std::reverse(std::begin(a), std::end(a));

   for (auto e : a)
   {
      std::cout << e;
   }
   std::cout << '\n';
}

This example works in Visual Studio 2019.

[dutch]

It looks in std because the parameter (v) is from std (std::vector).

[Jens Hirschmann]

Thanks for the replies!

I digged in the standard, although it's fairly cryptic to a beginner, my guess is that the section "[basic.lookup.argdep]" (section 6.5.2 in C++20 DIS N4860) describes the behaviour. Am I correct?

For each argument type T in the function call, there is a set of zero or more associated namespaces and a set of zero or more associated entities (other than namespaces) to be considered. The sets of namespaces and entities are determined entirely by the types of the function arguments.

Surely this is a pretty confusing feature! I imagine this might cause some unwanted side effects.

[Ganado]

Yes, ADL (Argument-Dependent Lookup) can be a confusing feature, and has caused headaches.
http://www.cplusplus.com/forum/lounge/263831/

mbozzi wrote:

See Herb Sutter's A Modest Proposal: Fixing ADL:
https://wg21.link/p0934

(This proposal might be dead in the water -- I don't know its status, but it gives details into some issues)

[Jens Hirschmann]

Very helpful links, many thanks!