C++ specify array indexes in initializer, like C

Question

I used C before (embedded stuff), and I can initialize my arrays like that:

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

i.e. I can specify indexes inside initializer. Currently I'm learning Qt/C++, and I can't believe this isn't supported in C++.

I have this option: -std=gnu++0x, but anyway it isn't supported. (I don't know if it is supported in C++11, because Qt works buggy with gcc 4.7.x)

So, is it really not supported in C++? Or maybe there's a way to enable it?

UPD: currently I want to initialize const array, so std::fill won't work.

"Or maybe there's a way to enable it?" - use C? (:D) Good question, +1. — user529758
– user529758, Commented Nov 26, 2012 at 10:30
Your code is neither C (including C11). C11 does not support initializing ranges this way ([a...b]=1), only single elements ([a]=1). — Yakov Galka
– Yakov Galka, Commented Nov 26, 2012 at 10:33
hmm... can you use extern "C" { ................ } around declaration? But I believe this syntax is your embedded compiler's addition. — anishsane
– anishsane, Commented Nov 26, 2012 at 10:33
@anishsane "I believe this syntax is your embedded compiler's addition" - no, it's a GNU extension to C99. — user529758
– user529758, Commented Nov 26, 2012 at 10:38

Community · Accepted Answer · 2017-05-23 12:19:20Z

7

hm, you should use std::fill_n() for that task...

as stated here http://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html the designated inits (extention) are not implemented in GNU C++

edit: taken from here: initialize a const array in a class initializer in C++

as a comment said, you can use std:vector to get the result desired. You could still enforce the const another way around and use fill_n.

int* a = new int[N];
// fill a

class C {
  const std::vector<int> v;
public:
  C():v(a, a+N) {}
};

edited May 23, 2017 at 12:19

CommunityBot

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answered Nov 26, 2012 at 10:32

Najzero

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4 Comments

Dmitry Frank Over a year ago

Thanks for the answer. Actually, I need to initialize const array, so, std::fill_n() won't work for me.

Yakov Galka Over a year ago

@DmitryFrank: Use std::vector, write a function that initializes the vector using fill_n and returns the result. Use this function to initialize a const vector.

iammilind Over a year ago

You need delete a; also. Two dynamic allocation for filling array, isn't it an overkill?

Dmitry Frank Over a year ago

After years, it works in g++ 4.8.2. :) when compiling with -std=c++11

Dmitry Frank · Accepted Answer · 2015-08-23 14:34:31Z

4

After years, I tested it just by chance, and I can confirm that it works in g++ -std=c++11, g++ version is 4.8.2.

answered Aug 23, 2015 at 14:34

Dmitry Frank

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Comments

Juraj Blaho · Accepted Answer · 2012-11-26 10:33:14Z

2

No, it is not possible to do it in C++ like that. But you can use std::fill algorithm to assign the values:

int widths[101];
fill(widths, widths+10, 1);
fill(widths+10, widths+100, 2);
fill(widths+100, widths+101, 3);

It is not as elegant, but it works.

answered Nov 26, 2012 at 10:33

Juraj Blaho

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