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5.22 Designated Initializers

Standard C89 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.

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 C89 mode as well. This extension is not implemented in GNU C++.

To specify an array index, write `[index] =' before the element value. For example,

int a[6] = { [4] = 29, [2] = 15 };

is equivalent to

int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being initialized is automatic.

An alternative syntax for this which has been obsolete since GCC 2.5 but GCC still accepts is to write `[index]' before the element value, with no `='.

To initialize a range of elements to the same value, write `[first ... last] = value'. This is a GNU extension. For example,

int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

If the value in it has side-effects, the side-effects will happen only once, not for each initialized field by the range initializer.

Note that the length of the array is the highest value specified plus one.

In a structure initializer, specify the name of a field to initialize with `.fieldname =' before the element value. For example, given the following structure,

struct point { int x, y; };

the following initialization

struct point p = { .y = yvalue, .x = xvalue };

is equivalent to

struct point p = { xvalue, yvalue };

Another syntax which has the same meaning, obsolete since GCC 2.5, is `fieldname:', as shown here:

struct point p = { y: yvalue, x: xvalue };

The `[index]' or `.fieldname' is known as a designator. You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used. For example,

union foo { int i; double d; };

union foo f = { .d = 4 };

will convert 4 to a double to store it in the union using the second element. By contrast, casting 4 to type union foo would store it into the union as the integer i, since it is an integer. (See section 5.24 Cast to a Union Type.)

You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example,

int a[6] = { [1] = v1, v2, [4] = v4 };

is equivalent to

int a[6] = { 0, v1, v2, 0, v4, 0 };

Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an enum type. For example:

int whitespace[256]

  = { [' '] = 1, ['\t'] = 1, ['\h'] = 1,

      ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

You can also write a series of `.fieldname' and `[index]' designators before an `=' to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair. For example, with the `struct point' declaration above:

struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };

If the same field is initialized multiple times, it will have value from the last initialization. If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, gcc will discard them and issue a warning.

[링크 : https://gcc.gnu.org/onlinedocs/gcc-3.0.1/gcc_5.html]

pi@raspberrypi ~/src/pch $ g++ -c stdafx.h -o stdafx.h.gch

pi@raspberrypi ~/src/pch $ ls -al

합계 4244

drwxr-xr-x 2 pi pi    4096  7월 30 00:58 .

drwxr-xr-x 9 pi pi    4096  7월 30 00:58 ..

-rw-r--r-- 1 pi pi      37  7월 30 00:58 1

-rw-r--r-- 1 pi pi      91  7월 30 00:58 a.cpp

-rw-r--r-- 1 pi pi      37  7월 30 00:58 stdafx.h

-rw-r--r-- 1 pi pi 4324036  7월 30 00:59 stdafx.h.gch

pi@raspberrypi ~/src/pch $ file stdafx.h.gch

stdafx.h.gch: GCC precompiled header (version 013) for C++

pi@raspberrypi ~/src/pch $ mv stdafx.h stdafx.h.bak

pi@raspberrypi ~/src/pch $ g++ a.cpp 

pi@raspberrypi ~/src/pch $ mv stdafx.h.gch stdafx.h.gch.bak

pi@raspberrypi ~/src/pch $ ls -al

합계 4248

drwxr-xr-x 2 pi pi    4096  7월 30 01:01 .

drwxr-xr-x 9 pi pi    4096  7월 30 00:58 ..

-rw-r--r-- 1 pi pi      91  7월 30 00:58 a.cpp

-rwxr-xr-x 1 pi pi    6492  7월 30 01:00 a.out

-rw-r--r-- 1 pi pi      37  7월 30 00:58 stdafx.h.bak

-rw-r--r-- 1 pi pi 4324036  7월 30 00:59 stdafx.h.gch.bak

pi@raspberrypi ~/src/pch $ g++ a.cpp

a.cpp:1:20: fatal error: stdafx.h: 그런 파일이나 디렉터리가 없습니다

compilation terminated.

pi@raspberrypi ~/src/pch $ gcc -x c++-header stdafx.h -o stdafx.h.gch

pi@raspberrypi ~/src/pch $ ls -al

합계 4248

drwxr-xr-x 2 pi pi    4096  7월 30 01:01 .

drwxr-xr-x 9 pi pi    4096  7월 30 00:58 ..

-rw-r--r-- 1 pi pi      91  7월 30 00:58 a.cpp

-rwxr-xr-x 1 pi pi    6492  7월 30 01:00 a.out

-rw-r--r-- 1 pi pi      37  7월 30 00:58 stdafx.h

-rw-r--r-- 1 pi pi 4324036  7월 30 01:01 stdafx.h.gch  

-x language

Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. Possible values for language are:

c  c-header  c-cpp-output

c++  c++-header  c++-cpp-output

objective-c  objective-c-header  objective-c-cpp-output

objective-c++ objective-c++-header objective-c++-cpp-output

assembler  assembler-with-cpp

ada

f77  f77-cpp-input f95  f95-cpp-input

java

-o file

Place output in file file. This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.

[링크 : http://linux.die.net/man/1/gcc] 

pi@raspberrypi ~/src/pch $ gcc stdafx.h -o stdafx.h.gch

stdafx.h:1:18: fatal error: string: 그런 파일이나 디렉터리가 없습니다

compilation terminated.