Mastering Class Member Initialization-4 : Page 4

Defining Class Constants
Sometimes, you need an internal constant that all instances of the same class share. In earlier stages of C++, an anonymous enum was used for this purpose:

class Allocator
{
 enum { PAGE_SIZE=1024 };
//
};

Although this form is still in use, standard-compliant compilers offer a preferable method of defining class constants:

class Allocator
{
 static const int PAGE_SIZE=1024;
public:
 Allocator(int pages){ p=new char[pages*PAGE_SIZE];}
//
};
const int Allocator::PAGE_SIZE; //no initializer here

This kind of initialization is permitted only with const static members of integral types. For any other type, use a member initialization list:

class Math
{
 const static double PI;
//
};
const double Math::PI= 3.14159265358979;

Knowing When and How
The primary form of initializing members is a member initialization list. For const data, references and subobjects whose constructors take arguments, the use of a member initialization list is mandatory. As a special case, const static members of an integral type are initialized inside the class body. Other static data members are initialized when defined, outside the class body.

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