C++: Why does int array[size] work?

Question

I have started learning c++. I read that an array\'s size can only be set before run and dymanic arrays can be set during runtime. So I was expecting this to fail but it did

Answer 1

In all current and past standards, that code is ill-formed. Variable Length Arrays are a C99 feature not a C++ feature, although some compilers do provide it as an extension. In the upcoming standard (expected to be C++14, currently in the process of review) a similar feature by other name (and slightly different semantics) has been adopted, so expect this to become standard in the future.

Note that in general, that is, excluding arrays of runtime bound (as they are named in the upcoming standard), the size of the array is part of the static type of the object and known at compile time. In the case of VLAs or array of runtime bound, the size is not known at compile time and thus the type is somehow of a second class citizen among types. The implication is that you cannot use VLAs/ARB with templates (as code generation for the template depends on the type, which includes the size, which is unknown at compile time).

In the same way there are other limitations, sizeof is not a compile time operation for VLAs, and it is not even allowed for ARB, these form of arrays can only be used for objects with automatic storage duration (i.e. in the stack), you cannot take the address of the array (although you can take the address of the first member),...

Another important point to consider is that the standard does not guarantee that the memory for an ARB will be allocated on the stack, and allows implementations to call a global allocation function, although the intention is that compilers will catch up and provide the space from the stack.

Answer 2

Be careful! Undefined integers are not guarenteed to be any value at all. Some compilers will default to 0, while others will use whatever garbage bits where already in memory. For this reason Visual Studio wouldn't even let me compile. Step through your code to see the allocated memory for score before you set the size variable. Likely size as it is compiled is a random integer from garbage memory, which means it can change every time you execute!

A C style array needs to know how much continuous memory to allocate. This allows for direct indexing, and other optimizations. As others have suggested, std::vector is what is the standard dynamic container in C++, which uses an array under the hood.

Answer 3

Probably you are using GCC compiler, it has an extension called Arrays of Variable Length.

std::vector is the real dynamic arrays in C++.

To select this standard in GCC, use the option -std=c++11; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings).