Why and when we can omit std keyword in C++?

Asked Dec 21 '20 at 11:33

Active Dec 21 '20 at 12:59

Viewed 69 times

Both this code:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <utility>
#include <vector>

int main() {
  std::vector<int> v;

  std::exchange(v, {1, 2, 3, 4, 5});

  copy(cbegin(v), cend(v) - 1, std::ostream_iterator<int>(std::cout, ", "));
  std::cout << *rbegin(v) << "\n\n";

  return 0;
}

and this code:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <utility>
#include <vector>

int main() {
  std::vector<int> v;

  std::exchange(v, {1, 2, 3, 4, 5});

  std::copy(std::cbegin(v), std::cend(v) - 1, std::ostream_iterator<int>(std::cout, ", "));
  std::cout << *std::rbegin(v) << "\n\n";

  return 0;
}

can be compiled (in C++14 and C++17) successfully and do the same job. But I wonder why I can omit std:: as well as in what circumstance?

edited Dec 21 '20 at 12:59

Jarod42

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asked Dec 21 '20 at 11:33

Gamcheong

7

https://en.cppreference.com/w/cpp/language/adl – rafix07 Dec 21 '20 at 11:35
3

The symbol `std` is not a keyword, it's a normal `namespace` identifier defined in standard C++ headers (with e.g. `namespace std { ... }`). – Some programmer dude Dec 21 '20 at 11:36
@tadman As you can see my code did not have that expression, but it just work. You may try with your compiler. – Gamcheong Dec 21 '20 at 11:38
@Someprogrammerdude okay that's my mistake. – Gamcheong Dec 21 '20 at 11:39
Whereas `std::vector` is in namespace `std`, it is not necessary the case for `std::vector::const_iterator` (which might even be `const int*`). – Jarod42 Dec 21 '20 at 13:02

Why and when we can omit std keyword in C++?

0 Answers0