Why doesn't ANSI C have namespaces?

C

10

134

Having namespaces seems like no-brainer for most languages. But as far as I can tell, ANSI C doesn't support it. Why not? Any plans to include it in a future standard?

Concordat answered 9/12, 2010 at 8:14 Comment(8)

Use C++ as C-with-namespace! – Tosha 9/12, 2010 at 8:20

I can of course, but I'd still like to know – Concordat 9/12, 2010 at 8:23

@Chris Becke What is wrong with C++ namespaces? – Tosha 9/12, 2010 at 8:30

2 things. An unnecessary distinctive syntax: All other languages with namespaces just use '.' as separator as its not ambiguous with other uses of '.'. And, more critically, c++ never introduced a scoped using directive. Which meant that programmers overused using directives to import namespaces into global scope. Which meant that the c++ standards committee now can't add new features to std:: ever as the amount of code that would break as a result has rendered the partitioning redundant. – Mellow 9/12, 2010 at 8:40

@Chris Becke: I like distinctive syntax. I like to know whether I'm looking at a class in a name space or a member in a class. – Finecut 9/12, 2010 at 9:3

I think the folks best qualified to answer this question were/are on an ANSI C committee. Hopefully someone from one of the committees is here. Or ask a mailing list with someone who was, maybe? – Elegiac 9/12, 2010 at 19:26

@ChrisBecke, this is a few years late, but it's interesting that you argue that C++ namespaces were poorly implemented, so they shouldn't be implemented in C. Then you note that other languages implement them without the hangups of C++. If other languages can do it, why not introduce them to C? – Leavelle 22/10, 2013 at 14:54

never use using namespace ... in C++ headers, at least not when you are writing public headers, like for a library ;-) – Playacting 7/3, 2019 at 10:28

W

88

C does have namespaces. One for structure tags, and one for other types. Consider the following definition:

struct foo
{
    int a;
};

typedef struct bar
{
    int a;
} foo;

The first one has tag foo, and the later is made into type foo with a typedef. Still no name-clashing happens. This is because structure tags and types (built-in types and typedef'ed types) live in separate namespaces.

What C doesn't allow is to create new namespace by will. C was standardized before this was deemed important in a language, and adding namespaces would also threaten backwards-compatibility, because it requires name mangling to work right. I think this can be attributed due to technicalities, not philosophy.

EDIT: JeremyP fortunately corrected me and mentioned the namespaces I missed. There are namespaces for labels and for struct/union members as well.

Whitten answered 9/12, 2010 at 8:50 Comment(8)

There are actually more than two name spaces. In addition to the two you mention, there is a name space for labels and name spaces for the members of each struct and union. – Finecut 9/12, 2010 at 9:0

@Finecut : Many thanks for the correction. I only wrote this off memory, I didn't check the standard :-) – Whitten 9/12, 2010 at 19:22

what about namespace for functions? – Lammas 3/7, 2016 at 12:42

@Finecut Do we refer to the namespace of members of a struct foo as "the struct foo namespace"? – Consuela 7/12, 2016 at 10:45

@MinhTran Not normally, but that doesn't alter the fact that each struct has its owen namespace. – Finecut 7/12, 2016 at 10:51

This may well be called namespaces, but I believe these are not the sort of namespaces the OP was asking about. – Kilkenny 18/8, 2017 at 12:53

@Finecut you're obscuring terminology. You can solve similar scoping problems this way but these are namespaces with a lot of extra baggage since these are member objects and variables that belong to an instantiated object. Yes you can use static to get around that but then you're doing just that. Hacking around to mimic namespace behavior. – Riven 16/8, 2018 at 19:17

@Riven Nope. I'm not advocating hacking C features, merely stating the facts. Each struct definition declares a new namespace for its members. I'm not advocating exploiting that fact, nor am I aware of any means of exploiting it since structs cannot have static members. – Finecut 28/8, 2018 at 17:16

A

139

For completeness there are several ways to achieve the "benefits" you might get from namespaces, in C.

One of my favorite methods is using a structure to house a bunch of method pointers which are the interface to your library/etc..

You then use an extern instance of this structure which you initialize inside your library pointing to all your functions. This allows you to keep your names simple in your library without stepping on the clients namespace (other than the extern variable at global scope, 1 variable vs possibly hundreds of methods..)

There is some additional maintenance involved but I feel that it is minimal.

Here is an example:

/* interface.h */

struct library {
    const int some_value;
    void (*method1)(void);
    void (*method2)(int);
    /* ... */
};

extern const struct library Library;
/* end interface.h */

/* interface.c */
#include "interface.h"

void method1(void)
{
   ...
}
void method2(int arg)
{
   ...
}

const struct library Library = {
    .method1 = method1,
    .method2 = method2,
    .some_value = 36
};
/* end interface.c */

/* client code */
#include "interface.h"

int main(void)
{
    Library.method1();
    Library.method2(5);
    printf("%d\n", Library.some_value);
    return 0;
}
/* end client code */

The use of . syntax creates a strong association over the classic Library_function(), Library_some_value method. There are some limitations however, for one you can't use macros as functions.

Ailis answered 16/2, 2015 at 6:19 Comment(11)

... and are compilers smart enough to "dereference" the function pointer at compile time when you do library.method1()? – Zaneta 1/10, 2015 at 11:37

This is so awesome. One thing I might add, I'm trying out making all of my functions in my .c files static by default, thus the only functions exposed are the ones explicitly exposed in the const struct definition in the .c file. – Informed 1/2, 2017 at 19:53

That's a great idea, but how do you deal with constants and enums? – Bastien 19/7, 2017 at 14:43

@Zaneta - sorry to necro, but at least as of version 6.3.0, gcc will compute the actual address of function1/method2 when compiling with both -O2 and -flto. Unless you compile such libraries along with your own source, this approach will add some overhead to its function calls. – Permian 4/3, 2018 at 8:11

@AlexReinking: Well, that's nice, but we would never get these functions inlined. And - necro'ing is great, no apology necessary. – Zaneta 4/3, 2018 at 10:0

I ran your code. In struct definition, function1 should be named method1. – Esme 23/12, 2018 at 17:39

It's adding a lot of overhead. – Haymaker 5/5, 2019 at 18:31

Has Library been initialized when I initialize my own values using Library? – Sinless 28/5, 2019 at 7:12

From my limited C knowledge: Shouldn't the functions be declared static so that they are not global? – Uncircumcision 4/12, 2021 at 21:8

@Zaneta clang is able to inline them godbolt.org/z/4EanMrMrd – Fike 6/6, 2023 at 11:49

@OleMorud: Interesting, but modules and namespaces are not quite the same thing. – Zaneta 6/6, 2023 at 13:23

W

88

C does have namespaces. One for structure tags, and one for other types. Consider the following definition:

struct foo
{
    int a;
};

typedef struct bar
{
    int a;
} foo;