Only compiles as an array of pointers, not array of arrays

Question

Suppose I define two arrays, each of which have 2 elements (for theoretical purposes):

char const *arr1[] = { "i", "j" };
char const *arr2[] = { "m", "n" };

Is there a way to define a multidimensional array that contains these two arrays as elements? I was thinking of something like the following, but my compiler displays warnings about incompatible types:

char const *combine[][2] = { arr1, arr2 };

The only way it would compile was to make the compiler treat the arrays as pointers:

char const *const *combine[] = { arr1, arr2 };

Is that really the only way to do it or can I preserve the type somehow (in C++, the runtime type information would know it is an array) and treat combine as a multidimensional array? I realise it works because an array name is a const pointer, but I'm just wondering if there is a way to do what I'm asking in standard C/C++ rather than relying on compiler extensions. Perhaps I've gotten a bit too used to Python's lists where I could just throw anything in them...

Interesting problem. I don't know the solution, but I'll be watching. — Xavier Ho
– Xavier Ho, Commented May 14, 2010 at 1:19

jpalecek · Accepted Answer · 2010-05-14 01:24:49Z

6

No. First, this

char const *combine[][2] = { arr1, arr2 };

cannot work, because arr1 and arr2 cannot be used to initialize an array. But this:

char const *arr1[] = { "i", "j" };
char const *arr2[] = { "m", "n" };

char const *(*combine[])[2] = { &arr1, &arr2 };

works as well as this

char const *arr1[] = { "i", "j" };
char const *arr2[] = { "m", "n" };

char const *combine[][2] = { {"i", "j"}, {"m", "n"} };

answered May 14, 2010 at 1:24

jpalecek

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3 Comments

Dustin Over a year ago

Interesting solution... An array of pointers to arrays each of length 2 containing values of type char const * (or const char * if you prefer; they're the same thing). The extra level of indirection is a bit of a nuisance when I want to use the arrays contained in combine, but I like the idea because it does exactly what I asked! Not only that, you showed how to do it generically with any arrays of the same length. Thanks a bunch! ^_^

Artefacto Over a year ago

In const char *arr1[], arr1 is an array of pointers to const char, in char *const arr1[], arr1 is an array of const pointers to char, but char const *arr1[]... Is it a compiler extension?

Johannes Schaub - litb Over a year ago

Notice that you have to access the arrays as combine[i][0][j] or (*combine[i])[j] in the first case. In the second case it's just combine[i][j]. Another interesting fact is that in C you don't even need to specify the 2, because T[] is compatible with T[N]. You can just say char const *(*combine[])[] = { &arr1, &arr2 };. However you can then only use the (*combine[i])[j] form of accessing (because sizeof *combine[i] is unknown). That's a C speciality only, though. In C++ you need to specify the size.

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