One of the places you’d like to use a const for constant expressions is inside classes. The typical example is when you’re creating an array inside a class and you want to use a const instead of a #define to establish the array size and to use in calculations involving the array. The array size is something you’d like to keep hidden inside the class, so if you used a name like size, for example, you could use that name in another class without a clash. The preprocessor treats all #defines as global from the point they are defined, so this will not achieve the desired effect.

You might assume that the logical choice is to place a const inside the class. This doesn’t produce the desired result. Inside a class, const partially reverts to its meaning in C. It allocates storage within each object and represents a value that is initialized once and then cannot change. The use of const inside a class means “This is constant for the lifetime of the object.” However, each different object may contain a different value for that constant.

Thus, when you create an ordinary (non-static) const inside a class, you cannot give it an initial value. This initialization must occur in the constructor, of course, but in a special place in the constructor. Because a const must be initialized at the point it is created, inside the main body of the constructor the const must already be initialized. Otherwise you’re left with the choice of waiting until some point later in the constructor body, which means the const would be un-initialized for a while. Also, there would be nothing to keep you from changing the value of the const at various places in the constructor body.