Array of size defined by not constant variable

Question

There is such code:

#include <iostream>

int main()
{
  int size;
  std::cin >> size;

  size = size + 1;
  int tab3[size];

  tab3[0] = 5;
  std::cout << tab3[0] << " " << sizeof(tab3) << std::endl;
  return 0;
}

The result is:

$ g++ prog.cpp -o prog -Wall -W 
$ ./prog
5
5 24

Why does this code even compile? Shouldn't be length of array a constant variable?

I used g++ version 4.4.5.

Kerrek SB · Accepted Answer · 2011-11-14 17:51:51Z

13

Variable-length arrays in C++ are available as an extension in GCC. Compiling with all warnings should have alerted you to that fact (include -pedantic).

answered Nov 14, 2011 at 17:51

Kerrek SB

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14 Comments

Michael Krelin - hacker Over a year ago

+1, tho I had no idea sizeof() can also be non-constant. If I ever thought of using sizeof() on that I'd expect it to fail for some reason.

Kerrek SB Over a year ago

@MichaelKrelin-hacker: again, as an extension... VLAs require a runtime sizeof(), so it's part of the C99 standard. It's just not a very C++-type of thing. Think about decltype and all this...

Kerrek SB Over a year ago

It is interesting though that there doesn't appear to exist any standardized method of allocating memory on the stack. We could handle object construction with placement-new, but it's just impossible in C++ to get a variable amount of raw memory on the stack.

Michael Krelin - hacker Over a year ago

Sure, I don't doubt it's a standard, it's just that I had no idea and never thought of it. Probably alloca() is not standardized, but it's pretty common, not?

Alok Save Over a year ago

There was a proposal to consider VLA's in C++ as well but the Standards committee dropped it because they considered the modifications to the type system for its support far outweighed the flexibility they would provide.

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Kleist · Accepted Answer · 2011-11-14 17:58:44Z

7

It is a C99 feature, not a part of C++. They are commonly refered to as VLAs(Variable Length Arrays.

If you run g++ with -pedantic it will be rejected.

See GCC docs for more info.

Comments

viraj · Accepted Answer · 2011-11-14 17:52:37Z

2

GCC provide's VLA's or variable length arrays. A better practice is to create a pointer and use the new keyword to allocate space. VLA's are not available in MSVC, so the second option is better for cross platform code

answered Nov 14, 2011 at 17:52

viraj

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