Non-const declaration of array

Question

I have been teaching myself programming for couple of years, and I was sure that if you need array declaration of a variable number you need to use malloc or

Answer 1

You can get this functionality in C or C++ with alloca (_alloca on Windows). std::vector is NOT a substitute: it is allocated on the heap, with new, which calls malloc, which is potentially expensive.

There is a good reason why you might want to have an array whose length is determined at runtime allocated on the stack: it's really fast. Suppose you have loop that executes frequently but has an array that depends on something at runtime (say, the size of your canvas widget). You can't just hard-code a number: your program will crash when we all get 36" 300 dpi Retina-display monitors and pixels[2400] is no longer safe. But you don't want new, or your loop hits a malloc and gets slow.

Although, for large arrays, it might be better to have a std::vector that is static to the function an only gets resized (larger) when necessary since your stack has limited size.

(See http://msdn.microsoft.com/en-us/library/wb1s57t5(VS.71).aspx)