Array initialization with const variable

Question

I was coding a mini GTK+2.0 game when i had a problem. When i write this :

const unsigned LABEL_NUMBER = 4;
const char *LABEL_TEXT[4] = {
                              "Five or More",
                              "By ... "
                              "& ...",
                              "April 2016",
                              "~~ Thanks for playing ~~"
                           };

There is no problem. But when i write this :

const unsigned LABEL_NUMBER = 4;
const char *LABEL_TEXT[LABEL_NUMBER] = {
                                          "Five or More",
                                          "By ... "
                                          "& ...",
                                          "April 2016",
                                          "~~ Thanks for playing ~~"
                                       };

gcc answers :

source/gui.c: In function ‘create_about_window’:
source/gui.c:202:4: error: variable-sized object may not be initialized
    const char *LABEL_TEXT[LABEL_NUMBER] = {
    ^
source/gui.c:203:34: error: excess elements in array initializer [-Werror]
                                  "Five or More",
                                  ^
source/gui.c:203:34: note: (near initialization for ‘LABEL_TEXT’)
source/gui.c:204:34: error: excess elements in array initializer [-Werror]
                                  "By ... & ..."
                                  ^
source/gui.c:204:34: note: (near initialization for ‘LABEL_TEXT’)
source/gui.c:206:34: error: excess elements in array initializer [-Werror]
                                  "April 2016",
                                  ^
source/gui.c:206:34: note: (near initialization for ‘LABEL_TEXT’)
source/gui.c:207:34: error: excess elements in array initializer [-Werror]
                                  "~~ Thanks for playing ~~"
                                  ^
source/gui.c:207:34: note: (near initialization for ‘LABEL_TEXT’)

So i just want to know why gcc displays this errors while i use a constant unsigned integer to set the array size ?

It's required behaviour for C compilers; const unsigned LABEL_NUMBER = 4; is a variable — albeit one that doesn't change value. Arrays come in two flavours; those with a size fixed by a compile time integer constant (which can be initialized), and those with a variable size (which cannot be initialized). Because, in the terms of the C compiler (C standard), the latter is a variable, you have a variably-modified array and can't use initializers. In case of doubt, use enum { LABEL_NUMBER = 4 };. That will appear in your symbol table but can be used in array dimensions. — Jonathan Leffler
– Jonathan Leffler, Commented Apr 16, 2016 at 21:29
It's curious to use string concatenation on the two shortest strings in the initializers. ("By ... " "& ...", is a single string because there's no comma after the second double quote.) — Jonathan Leffler
– Jonathan Leffler, Commented Apr 16, 2016 at 21:31
@MooingDuck this is a C question; you're using a C++ compiler — M.M
– M.M, Commented Apr 17, 2016 at 5:56

haccks · Accepted Answer · 2016-04-17 05:27:29Z

1

Variable length arrays can't be initialised using initializers.

C11 - §6.7.9/3:

The type of the entity to be initialized shall be an array of unknown size or a complete object type that is not a variable length array type.

edited Apr 17, 2016 at 5:27

answered Apr 16, 2016 at 21:26

haccks

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4 Comments

Mooing Duck Over a year ago

I'm sort of shocked that it compiles: coliru.stacked-crooked.com/a/bb6ad14a4c89e222

haccks Over a year ago

@MooingDuck; In c++, const qualified objects are treated as real constant literals (with some exceptions). You are compiling using g++. It's a C++ compiler.

Maxime Over a year ago

But i don't understand why LABEL_TEXT[] is a VLA, char *LABEL_TEXT[const ...] == char *(LABEL_TEXT[const ...]) no ?

haccks Over a year ago

@Maxime; In C, const qualified objects are not constant literals.

Nafel Hekun · Accepted Answer · 2016-04-16 21:38:37Z

0

You can define LABEL_NUMBER as macro:

#define LABEL_NUMBER 4

That way the LABEL_NUMBER

const char *LABEL_TEXT[LABEL_NUMBER]

will be replaced at pre-processing phase by defined macro value which will produce:

const char *LABEL_TEXT[4]

for a compiler to compile.

answered Apr 16, 2016 at 21:38

Nafel Hekun

1

1 Comment

Maxime Over a year ago

Thank you for your answer but it isn't really what i was asking :)

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