When using standard C functions in C++, is the "std::" prefix required? [duplicate]

Question

When use standard C function in C++, should we prefix every function with std::?

for example (file name: std.C):

#include <cstdio>

int main() {
  std::printf("hello\n");
  printf("hello\n");
}

This file can be compiled with:

g++ -Wall -Werror -std=c++11 std.C

without any error.

my questions are:

1. Should we always place std:: before all the standard C library functions when they are used in C++?
2. What's the main difference between header files like <stdio.h> and <cstdio>?

Answer 1

The C++ library includes the same definitions as the C language library organized in the same structure of header files, with the following differences:

• Each header file has the same name as the C language version but with a "c" prefix and no extension. For example, the C++ equivalent for the C language header file <stdlib.h> is <cstdlib>.
• Every element of the library is defined within the std namespace.

Nevertheless, for compatibility with C, the traditional header names name.h (like stdlib.h) are also provided with the same definitions within the global namespace although its use is deprecated in C++.

(source)

The std:: part of the std::printf() call is the standard way to use names in the standard library, therefore, I suggest to use it.