Question

I’m used to doing Java programming, where you never really have to think about pointers when programming. However, at the moment I’m writing a program in C++. When making classes that have members of other classes, when should I use pointers and when should I not? For example, when would I want to do this:

class Foo {     Bar b; } 

As opposed to this:

class Foo {     Bar* b; } 

Answer 1

Start by avoiding pointers.

Use them when:

• You want to use the Pimpl idiom, or an abstract factory.
• The Bar instance is actually managed by some other part of your program, whereas the Foo class just needs to be able to access it.
• You want to postpone the construction of the Bar object (i.e., you want to create it after constructing Foo).
• In your business logic, the Bar object may not exist at all; you would use null also in Java. However, check out boost::optional as well.
• Bar is actually a base class, and you need the instance to be polymorphic.
• You happen to be using a toolkit that prefers to present GUI widgets as pointers. Examples could include (but are certainly not limited to) wxWidgets and GLUI.

In any of these cases (*), start by using a smart pointer, such as boost::shared_ptr. Otherwise, you are likely to forget to deallocate the memory, sooner or later. Once you know what you are doing, consider case-by-case what pointer type is best.

(*) any case – except, probably, the bullet regarding GUI widgets; in this case, your toolkit would most probably manage the resources for you as well