C++ creating arrays

Question

Why can't I do something like this:

int size = menu.size;
int list[size];

Is there anyway around this instead of using a vector? (arrays are faster, so I wanted to use arrays)

thanks

Answer 1

The size must be known at compile-time, since the compiler needs to know how much stack space will be needed to allocate enough memory for it. (Edit: I stand corrected. In C, variable length arrays can be allocated on the stack. C++ does not allow variable length arrays, however.)

But, you can create arrays on the heap at run-time:

int* list = new int[size];

Just make sure you free the memory when you're done, or you'll get a memory leak:

delete [] list;

Note that it's very easy to accidentally create memory leaks, and a vector is almost for sure easier to use and maintain. Vectors are quite fast (especially if you reserve() them to the right size first), and I strongly recommend using a vector instead of manual memory-management.

In general, it's a good idea to profile your code to find out where the real bottlenecks are than to micro-optimize up front (because the optimizations are not always optimizations).

Answer 2

As other have said, the C++ language designers have chosen not to allow variable length arrays, VLAs, in spite of them being available in C99. However, if you are prepared to do a bit more work yourself, and you are simply desperate to allocate memory on the stack, you can use alloca().

That said, I personally would use std::vector. It is simpler, safer, more maintainable and likely fast enough.