Why is it allowed to declare an automatic array with size depending on user input? [duplicate]

Question

I'm using MinGW to compile for C++11 and I found out that this doesn't throw an error:

int S;
cin>>S;
char array[S];

While this does ("storage size of 'array' isn't known"):

char array[];

To me, the size is also unknown in the first case, as it depends on what the user input is.

As far as I knew, automatic arrays are allocated at compile time in stack memory. So why wouldn't the first example fail?

Answer 1

You are apparently not aware of the GNU GCC extension Arrays of Variable Length. Therefore your first code compiles.

The error message is something different. You have to specify the array length.

gcc has the -pedantic switch - enabling this switch the compiler will report your first code as invalid:

warning: ISO C++ forbids variable length array ‘array’

Read also this thread What is the purpose of using -pedantic in GCC/G++ compiler?

Use very carefully compiler extensions because should you port your code to another compiler then you are in big trouble.

Answer 2

It's not. C++ doesn't have variable-length arrays, though some compilers allow it as an extension of the language.