Using C++20 makes some compilations errors disappear

March 21, 2024

Why does the following main.cpp:

#include <vector>

int main() {

    std::vector<size_t> order;
    reverse(order.begin(), order.end());

    return EXIT_SUCCESS;
}

compile without errors with g++ -std=c++20 main.cpp but fails (as expected) without C++20 enabled (g++ main.cpp)?

main.cpp: In function ‘int main()’:
main.cpp:5:17: error: ‘size_t’ was not declared in this scope; did you mean ‘std::size_t’?
    5 |     std::vector<size_t> order;
      |                 ^~~~~~
      |                 std::size_t
In file included from /usr/include/c++/11/bits/stl_algobase.h:59,
                 from /usr/include/c++/11/vector:60,
                 from main.cpp:1:
/usr/include/x86_64-linux-gnu/c++/11/bits/c++config.h:280:33: note: ‘std::size_t’ declared here
  280 |   typedef __SIZE_TYPE__         size_t;
      |                                 ^~~~~~
main.cpp:5:23: error: template argument 1 is invalid
    5 |     std::vector<size_t> order;
      |                       ^
main.cpp:5:23: error: template argument 2 is invalid
main.cpp:6:19: error: request for member ‘begin’ in ‘order’, which is of non-class type ‘int’
    6 |     reverse(order.begin(), order.end());
      |                   ^~~~~
main.cpp:6:34: error: request for member ‘end’ in ‘order’, which is of non-class type ‘int’
    6 |     reverse(order.begin(), order.end());
      |                                  ^~~
main.cpp:6:5: error: ‘reverse’ was not declared in this scope
    6 |     reverse(order.begin(), order.end());
      |     ^~~~~~~
main.cpp:8:12: error: ‘EXIT_SUCCESS’ was not declared in this scope
    8 |     return EXIT_SUCCESS;
      |            ^~~~~~~~~~~~

For the record, I’m using g++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0.

EDIT: There are similar questions about size_t vs std::size_t, but I’m more puzzled about the reverse function.

>Solution :

std::reverse is found through argument dependent lookup as std::vector::iterator is in the same namespace.

Presumably the missing <algorithm> include is found indirectly through <vector>, adding c++20 will add more features to <vector> which presumably requires the inclusion of <algorithm>.

Note that this compiles with MSVC and clang with libc++ without needing c++20: https://godbolt.org/z/hGoj4shr3 and GCC compiles with the missing include: https://godbolt.org/z/b4jKWEo1f