CODEWITHSUNDEEP
c++arrays
scdmb
scdmb

Reputation: 15621

Array of size defined by not constant variable

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.

Upvotes: 10

Views: 792

Answers (3)

Kleist
Kleist

Reputation: 7985

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.

See also: VLAs are evil.

Upvotes: 7

viraj
viraj

Reputation: 1814

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

Upvotes: 2

Kerrek SB
Kerrek SB

Reputation: 476960

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).

Upvotes: 13

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