Class members that are objects - Pointers or not? C++

c++ memory pointers class-design member
Mark picture Mark · Oct 6, 2010 · Viewed 21.3k times · Source

If I create a class MyClass and it has some private member say MyOtherClass, is it better to make MyOtherClass a pointer or not? What does it mean also to have it as not a pointer in terms of where it is stored in memory? Will the object be created when the class is created?

I noticed that the examples in QT usually declare class members as pointers when they are classes.

Answer

justin picture justin · Oct 6, 2010

If I create a class MyClass and it has some private member say MyOtherClass, is it better to make MyOtherClass a pointer or not?

you should generally declare it as a value in your class. it will be local, there will be less chance for errors, fewer allocations -- ultimately fewer things that could go wrong, and the compiler can always know it is there at a specified offset so... it helps optimization and binary reduction at a few levels. there will be a few cases where you know you'll have to deal with pointer (i.e. polymorphic, shared, requires reallocation), it is typically best to use a pointer only when necessary - especially when it is private/encapsulated.

What does it mean also to have it as not a pointer in terms of where it is stored in memory?

its address will be close to (or equal to) this -- gcc (for example) has some advanced options to dump class data (sizes, vtables, offsets)

Will the object be created when the class is created?

yes - the size of MyClass will grow by sizeof(MyOtherClass), or more if the compiler realigns it (e.g. to its natural alignment)

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