std:: not required?

[pdbarnes]

This surprised me:

std:: namespace qualifier doesn't seem to be required when using std::algorithms, such as for_each.

Example code below.

Notes:
1. I use C printf from stdio.h, since iostream includes way too much.
2. There is no "using namespace std" statement, so everything should have to be fully qualified.
3. for_each running over char * needs to be qualified by std::
4. for_each running over std::vector does not.
5. I've checked the preprocessor output (g++ -E ...) and for_each is only defined in std::, by algorithm.

So what's going on here? How does the compiler know to look in std:: for for_each?

#include <stdio.h>
#include <vector>
#include <algorithm>  // for_each

void
print_char(const char c)
{
  printf("%c", c);
}

	
int
main (int argc, char ** argv)
{
  
  printf("\n") ;

  printf("%s", "C string:     ");
  char  cs[15] = "I've got std::";  // extra slot for null
  std::for_each(cs, cs + 14, print_char);
  printf("\n") ;

  printf("%s", "std::vector:  ");
  char vcs[9] = "I don't!";
  std::vector<char> vs;
  vs.assign(vcs, vcs +8);
  for_each(vs.begin(), vs.end(), print_char);
  printf("\n") ;

  return 0;
}

Compiled with g++ -Wall for_each_odd.cc -o for_each_odd

For the record, the output should be:

C string:     I've got std::
std::vector:  I don't!

[R0mai]

My guess :
http://en.wikipedia.org/wiki/Argument_dependent_name_lookup

[helios]

Oh, wow.
http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=286
All I can say is "I never would've guessed" and "I now hate using namespace more than ever".

[jsmith]

Never would have guessed what?

The entire article is just an emotional argument. The only "fact" he gives -- about operator<<
and cout isn't quite right, because operator<< is a member function of the stream.

The very last line of the article tells me that he is obviously not a proponent of C++, so that
alone says that the article is biased.

[helios]

operator<< is a member function

Oh, right.
Never mind, I think I misread something.

[pdbarnes]

Thanks, R0mai. ADL explains why the C-string version has to be qualified (char * is not a struct/class in std), but vector does not.