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 you are writing in C++ you are probably writing for performance. Using new and the free store is much slower than using the stack (especially when using threads) so only use it when you need it.

As others have said, you need new when your object needs to live outside the function or object scope, the object is really large or when you don't know the size of an array at compile time.

Also, try to avoid ever using delete. Wrap your new into a smart pointer instead. Let the smart pointer call delete for you.

There are some cases where a smart pointer isn't smart. Never store std::auto_ptr<> inside a STL container. It will delete the pointer too soon because of copy operations inside the container. Another case is when you have a really large STL container of pointers to objects. boost::shared_ptr<> will have a ton of speed overhead as it bumps the reference counts up and down. The better way to go in that case is to put the STL container into another object and give that object a destructor that will call delete on every pointer in the container.