explicit indexes in C array literals?

Question

The Linux kernel source has a lot of array literals like this:

enum {
  FOO,
  BAR
};

static const char* const names[] = {
  [FOO] = "foo", /* wtf is this? */
  [BAR] = "bar",
};

Here each line explicitly indicates the index within the array of the supplied value instead of relying on ordering.

I don't know the phrase to search for - what is this called? What standard defines it? (Or is it a GNU extension?) Can I do this in C++ or just plain C? Experimenting with gcc, I find with the above in test.c,

$ gcc --version
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE

these commands return success:

$ gcc -Wall -c test.c
$ gcc -Wall -c --std=c90 test.c
$ gcc -Wall -c --std=gnu90 test.c
$ gcc -Wall -c --std=iso9899:1990 test.c
$ gcc -Wall -c --std=c1x test.c

and these commands fail with various complaints about lambdas and operator=:

$ g++ -Wall -c test.c
$ g++ -Wall -c --std=c++98 test.c
$ g++ -Wall -c --std=gnu++98 test.c
$ g++ -Wall -c --std=c++0x test.c
$ g++ -Wall -c --std=gnu++0x test.c

That suggests this is valid C (in just about any dialect) but not C++. But I'm skeptical. I don't remember seeing this used anywhere but the Linux kernel. I also don't see it described in, for example, this list of constructs valid in C but not C++.

Answer 1

It's part of standard C (C99 and newer), called "designated initialization".

From 6.7.9 Initialization, paragraph 6:

If a designator has the form

[ constant-expression ]

then the current object ... shall have array type and the expression shall be an integer constant expression. If the array is of unknown size, any nonnegative value is valid.

And paragraph 33:

EXAMPLE 9 Arrays can be initialized to correspond to the elements of an enumeration by using designators:

enum { member_one, member_two };
const char *nm[] = {
      [member_two] = "member two",
      [member_one] = "member one",
};

According to answers at this question, C++ doesn't support the same behaviour. Your compiler may provide extensions.

Perhaps more helpful to you (and a direct answer to your question) is the GCC documentation, which says:

In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C90 mode as well. This extension is not implemented in GNU C++.

Answer 2

It's a c99 designation initializer.

Designation initializers allow you initialize an array or a struct in any order. Omitted elements are initialized as if they are static objects.

int bla[16] = {[5] = 42, [9] = 42};

this initializes element bla[5] and bla[9] to 42 and all the remaining elements to 0. The designation is an integer constant expression between [].

enum {
  FOO,
  BAR
};

static const char* const names[] = {
  [FOO] = "foo", /* wtf is this? */
  [BAR] = "bar",
};

Here the designation is an enum constant. This is allowed as an enum constant is considered as an integer constant expression in C.

This feature is a C feature and it is not present in C++.