when to use new in C++?

Question

What's a good policy for when to use "new" to make an instance of a class? I've been hobby programming C++ for a while but I'm still not for sure when is the best time to do this:

MyClass thing(param1, param2);

over this:

MyClass* thing;
thing = new MyClass(param1, param2);

Any advice?

Answer 1

Design-wise, use automatic (stack) allocation as much as possible. Whenever you need to extend the lifetime of an object beyond a certain scope, then dynamically allocate it.

And even so, never dynamically allocate things raw. Always keep them wrapped into some sort of wrapper that implements Scope-Bound Resource Management (SBRM, first known under the dumb/awkward name Resource-Acquisition Is Initialization or RAII.) That is, dynamic allocations should be kept in automatic objects that will clean up automatically!

A good example of this is std::vector: you cannot leak the memory internal to a vector, because it's destructor is run in every scenario when memory should be free'd, and it will free it for you. auto_ptr is the first and only smart pointer available in the standard library, but it's pretty bad. Better is to use shared_ptr, or many of the other popular smart pointers available in Boost and/or TR1 and/or C++0x.

Performance-wise, objects allocated on the stack can be done so very quickly (the stack size is increased per-function-call, so all the required memory has been allocated up-front by a simple move of a pointer.) Contrarily, dynamic allocation generally requires much more time. It's quite possible to get speedy dynamic allocations with custom allocation schemes, but even the best will still be slower than stack allocation.

Occasionally, you might find you spend too much time copying objects around. In this case, it may be worth it to dynamically allocate it and merely move pointers around. However, please note I said "find". This kind of change is something you find by profiling and measuring, never guessing.

So: Automatic allocation when possible, dynamic allocation when needed.