When should I use the new keyword in C++?

Question

I\'ve been using C++ for a short while, and I\'ve been wondering about the new keyword. Simply, should I be using it, or not?

1) With the new keywo

Answer 1

If your variable is used only within the context of a single function, you're better off using a stack variable, i.e., Option 2. As others have said, you do not have to manage the lifetime of stack variables - they are constructed and destructed automatically. Also, allocating/deallocating a variable on the heap is slow by comparison. If your function is called often enough, you'll see a tremendous performance improvement if use stack variables versus heap variables.

That said, there are a couple of obvious instances where stack variables are insufficient.

If the stack variable has a large memory footprint, then you run the risk of overflowing the stack. By default, the stack size of each thread is 1 MB on Windows. It is unlikely that you'll create a stack variable that is 1 MB in size, but you have to keep in mind that stack utilization is cumulative. If your function calls a function which calls another function which calls another function which..., the stack variables in all of these functions take up space on the same stack. Recursive functions can run into this problem quickly, depending on how deep the recursion is. If this is a problem, you can increase the size of the stack (not recommended) or allocate the variable on the heap using the new operator (recommended).

The other, more likely condition is that your variable needs to "live" beyond the scope of your function. In this case, you'd allocate the variable on the heap so that it can be reached outside the scope of any given function.