Creating array with constant

Question

I was working on a program in Netbeans on Linux using a gcc compiler when, upon switching to Visual C++ on Windows 7, the code failed to compile as Visual C++ says it expected constant expression on several lines. On Netbeans, I simply did something similar to char name[fullName.size()];, while on Visual C++, I tried, among other things,

const int position = fullName.size();
char Name[position];

How can I create a constant to use for the array?

Note: I know about this question, but is there any way I can get this working without using vectors, since that would require a re-write of many parts of the program?

The actual question here is "How can I create an array of a non-predetermined size on the stack?" And the answer is, you can't in VC++. — Michael Myers, Mar 19 '10 at 21:41
what is fullname.size() ? looks like a function call, that would be runtime call and thus not a constant expression. — John Knoeller, Mar 19 '10 at 21:42
fullname.size() is the length of the string. It's a function call — wrongusername, Mar 19 '10 at 21:45
Could you give this more context? I don't see how using vector here would effect other parts of the program. — GManNickG, Mar 19 '10 at 21:56

score 2 · Answer 1 · answered Mar 19 '10 at 21:43

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This is not possible in VC++. I know, pretty sad :(

The solutions include:

Create it on the heap
Make it constant

The new C++ standard (C++0x) proposes a 'constant expression' feature to deal with this. For more info, check this out.

answered Mar 19 '10 at 21:43

Nathan Osman

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Thanks! I can't make it constant since I have no idea what the size of the string will be. What do you mean by "creating it on the heap?" Sorry, I'm still a novice at programming. – wrongusername Mar 19 '10 at 21:46
Allocate memory using `new` and free with `delete` – Nathan Osman Mar 19 '10 at 21:49
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Which really means use `vector`. There is *no* reason to have raw memory allocations hanging about, they leak and make your life more difficult. – GManNickG Mar 19 '10 at 22:03
I'm not saying use `new` and `delete` directly - I'm just explaining what the heap is. – Nathan Osman Mar 19 '10 at 22:08

score 1 · Answer 2 · answered Mar 19 '10 at 21:46

C++ requires that the size of the array be known at compile time. If you don't mind using a non-standard extension, gcc does allow code like you're doing (note that while it's not standard C++, it is standard in C, as of C99).

I'd also guess that you could use a vector (in this particular place) with less trouble than you believe though -- quite a bit of code that's written for an array can work with a vector with only a re-compile, and little or no rewriting at all.

score 1 · Accepted Answer · answered Mar 19 '10 at 21:46

1

In VC++ you can't do runtime declarations of stack array sizes, but you can do stack allocation via _alloca

so this:

const int position = fullName.size();
char Name[position];

becomes this:

const int position = fullName.size();
char * Name = (char*)_alloca(position * sizeof(char));

It's not quite the same thing, but it's as close as you are going to get in VC++.

answered Mar 19 '10 at 21:46

John Knoeller

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I think we should steer the asker toward the heap... I don't want to see a beginner start writing code with _alloca – Nathan Osman Mar 19 '10 at 21:49
@George: The heap might be a better choice for an array of objects, but for a simple string buffer, it's a worse choice. – John Knoeller Mar 19 '10 at 21:57
Why not just use std::string? He says he's a beginner, that's the easiest thing to use. – Nathan Osman Mar 19 '10 at 22:07
Never mind guys, I found out a way to skip using arrays/vectors entirely. Thanks for the help! :) – wrongusername Mar 19 '10 at 22:08
@George: Because he asked how to solve a specific problem, not how to redesign his code using a completely different API. – John Knoeller Mar 19 '10 at 22:16

score 0 · Answer 4 · answered Mar 19 '10 at 22:58

0

Your char name[fullName.size()]; is an example of a variable-length array which - as far as I know - are not standardized in C++ so you're at the mercy of the compiler. [Slightly off-topic they're part of the C99 standard]

answered Mar 19 '10 at 22:58

Eugen Constantin Dinca

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Creating array with constant

4 Answers4