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

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:

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

@KerrekSB moreover, one is C and the other is C++. C++ is not a superset of C. — Quentin, Apr 27 '15 at 13:52
See this answer: http://stackoverflow.com/questions/4404725/why-do-some-includes-need-the-h-and-others-not/4404753#4404753 — Ayushi Jha, Apr 27 '15 at 13:52
(1) Yes, However it depends. (2) `` wraps functions in `std` namespace. The first one exists for backward compatibility. — masoud, Apr 27 '15 at 13:52

score 14 · Accepted Answer · edited Aug 01 '15 at 18:07

14

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.

edited Aug 01 '15 at 18:07

Emil Laine

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answered Apr 27 '15 at 13:59

Alper

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Also note that the C++ standard allows the C++ counterparts to the C std lib headers to put the names in the global namespace. – juanchopanza Apr 27 '15 at 14:17
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Allows. Not requires. – Lightness Races in Orbit Apr 27 '15 at 14:23
Inclusion in the global namespace for identifiers declared in ``, and inclusion in `std` for identifiers declared in `` is *optional*. See my answer. – Keith Thompson Apr 27 '15 at 15:56

Keith Thompson · Answer 2 · 2015-04-27T17:28:59.230

The C++ standard library incorporates the C standard library (with a few minor tweaks).

Each C header with a name like <foo.h> has a corresponding C++ header <cfoo>. For example, the C++ header <cstdio> corresponds to the C header <stdio.h>.

Quoting the 2011 ISO C++ standard, 17.6.1.2 [headers] paragraph 4:

In the C ++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope (3.3.6) of the namespace std. It is unspecified whether these names are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations (7.3.3).

So given #include <cstdio>, the printf function definitely can be referred to as std::printf, and optionally may be visible as printf in the global namespace. (This option is up to the implementation, not the programmer.)

Of course you can refer to it as just printf within the scope of using namespace std.

In my opinion, this is unfortunate; it seems to be for the convenience of implementers rather than programmers. It's safest to assume that printf is declared only with in the std namespace. If you use #include <cstdio> and then refer to printf in the global namespace, your code might compile today and fail to compile on a different implementation.

Conversely, as a deprecated feature, the C++ standard library also includes the C standard headers with their original names, such as <stdio.h>. Quoting the standard, section D.5 [depr.c.header]:

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).

So given #include <stdio.h>, the name printf is definitely visible in the global namespace, and optionally (again, this is the implementation's option, not yours) visible as std::printf.

``? Do you mean "given `#include `" in your last sentence? – zmb Apr 27 '15 at 16:36 — zmb, Apr 27 '15 at 16:36
@zmb: Indeed I do, thanks (fixed). – Keith Thompson Apr 27 '15 at 17:29 — Keith Thompson, Apr 27 '15 at 17:29

score 1 · Answer 3 · edited May 23 '17 at 12:10

1

std is the namespace and by using :: (after the std) you explicitly using the functions of the namespace std. Now, imagine that you create your own namespace and some of the functions that you have created there have the same name as the functions in std namespace. This could be a problem, but by using the std::func1 and YourNameSpace::func1 you are preventing this issue.
Look in here. Thanks @karma_geek

edited May 23 '17 at 12:10

Community

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answered Apr 27 '15 at 14:03

OhadM

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"Look in here" does not answer the question. Bring the content here please. – Lightness Races in Orbit Apr 27 '15 at 14:23

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

3 Answers3