Question

I’ve just started working on a new codebase where each class contains a shared_ptr typedef (similar to this) like:

typedef boost::shared_ptr<MyClass> Ptr;

Is the only purpose to save typing boost::shared_ptr?

If that is the case, is the only reason not to do

#define Ptr boost::shared_ptr 

in one common header the general problems with #define? Then you can do:

Ptr<MyClass> myClass(new MyClass);

which is no more typing than

MyClass::Ptr myClass(new MyClass);

and saves the Ptr definition in each class.

Answer 1

A macro (#define) is always defined globally.
This means that every use of the ‘string’ Ptr (even a variable) will be replaced by the macro.

The typedef can be placed in a class, in a namespace, … so you have much better control over it.

EDIT:
another advantage is that you can haver different Ptr types in different classes, e.g.

• ClassX::Ptr is a boost shared_ptr
• ClassY::Ptr can be another shared_ptr
• ClassZ::Ptr can be a simple “Class Z *” pointer

If these classes are then used in templated code, you can use T::Ptr as a type of pointer to the class, and the template will use the most-appropriate pointer for the class.