Why is std:: not needed in front of C functions?

[john.c]

Is it standards-conforming that you don't need to put std:: in front of functions from the C library in gcc? E.g.,

// g++ -std=c++14 -Wall -Wextra -pedantic -o wtf wtf.cpp
#include <iostream>
#include <cstdlib>
#include <ctime>
int main() {
    srand(time(0));
    std::cout << rand() << '\n';
}

It seems that clang doesn't complain, either.

[jimblumberg]

Yes it is standards conforming. See this section of the C++17 standard (from the N4594 draft):

D.3 C standard library headers [depr.c.headers]
1 For compatibility with the C standard library and the C Unicode TR, the C ++ standard library provides the
26 C headers, as shown in Table 172.
2 Except for the functions declared in 26.10, every C header, each of which has a name of the form name.h,
behaves as if each name placed in the standard library namespace by the corresponding cname header is
placed within the global namespace scope. It is unspecified whether these names are first declared or defined
within namespace scope (3.3.6) of the namespace std and are then injected into the global namespace scope
by explicit using-declarations (7.3.3).

3 [ Example: The header <cstdlib> assuredly provides its declarations and definitions within the namespace
std. It may also provide these names within the global namespace. The header <stdlib.h> assuredly
provides the same declarations and definitions within the global namespace, much as in the C Standard. It
may also provide these names within the namespace std. — end example ]

[john.c]

I was just coming back to post that same quote (but from section D.5 in the C++14 n4296 draft).
That just seems idiotic, though.
I wonder if it's just a concession to ease compiler implementation.
At any rate, it does mean that if you use, e.g., cstdlib, then you need to say std::rand for portability.

[christop]

I wonder if it's just a concession to ease compiler implementation.

I imagine it's more to make it easier for people to migrate from C to C++. Most valid C programs can compile as C++ without any changes.

[john.c]

I imagine it's more to make it easier for people to migrate from C to C++. Most valid C programs can compile as C++ without any changes.

That can't be the reason since including a C header as <stdlib.h> is guaranteed to dump the identifiers into the global namespace (but can additionally put them in std). Including it as <cstdlib> is guaranteed to put the identifiers in std (but can additionally dump them globally).

[christop]

That can't be the reason since including a C header as <stdlib.h> is guaranteed to dump the identifiers into the global namespace (but can additionally put them in std). Including it as <cstdlib> is guaranteed to put the identifiers in std (but can additionally dump them globally).

Oh, I see what you mean. I must have skimmed the part about including cstdlib vs stdlib.h. In that case, yes, it might be to ease compiler implementation, or it could be written that way in the standard because existing compilers do it that way. It's basically a warning to the application writer that those function names might be defined at the global namespace, so the application writer cannot use those function names.