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Asked: 2026-05-27T05:42:37+00:00

Possible Duplicate: why a[n] is accepted in c during runtime? Declaring the array size

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Possible Duplicate:
why a[n] is accepted in c during runtime?
Declaring the array size with a non-constant variable

I just wrote some code to test some other code and I needed an array as input data. As the size of the input data may differ, I declared the variable as follows:

float input[num_pixels_row][num_pixels_col][3];

where num_pixels_row and num_pixels_col are non-const variables which are set using input from the command line. I ran the code and it worked.

Then after a little while I noticed what I had just done and thought “Hey, wait a minute! This shouldn’t work!!” But the strange thing is that it does. Since input is declared inside a function it should be allocated on the stack, but how can the compiler determine the stack frame if the size of the array isn’t known?

I asked two colleagues and they were just as puzzled. By the way, I compiled the code using g++ 4.6.1

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1 Answer

  1. Editorial Team

    Variable length arrays are a part of the C99 specification, which gcc also allows in C++ programs.

    I don’t think this has been added to C++11 though, unfortunately. Though I’d suspect that since many C++ compilers also strive for C compliance that they’ll end up supporting this as well.

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