7.11 Smart Pointers to const and const Smart Pointers

Raw pointers allow two kinds of constness: the constness of the pointed-to object and that of the pointer itself. The following is an illustration of these two attributes:

const Something* pc = new Something; // points to const object


pc->ConstMemberFunction(); // ok


pc->NonConstMemberFunction(); // error


delete pc; // ok (surprisingly)[4]


Something* const cp = new Something; // const pointer


cp->NonConstMemberFunction(); // ok


cp = new Something; // error, can't assign to const pointer


const Something* const cpc = new Something; // const, points to const


cpc->ConstMemberFunction(); // ok


cpc->NonConstMemberFunction(); // error


cpc = new Something; // error, can't assign to const pointer

^[4] Every once in a while, the question "Why can you apply the delete operator to pointers to const?" starts a fierce debate in the comp.std.c++ newsgroup. The fact is, for better or worse, the language allows it.

The corresponding uses of SmartPtr look like this:

// Smart pointer to const object


SmartPtr<const Something> spc(new Something);


// const smart pointer


const SmartPtr<Something> scp(new Something);


// const smart pointer to const object


const SmartPtr<const Something> scpc(new Something);

The SmartPtr class template can detect the constness of the pointed-to object either through partial specialization or by using the TypeTraits template defined in Chapter 2. The latter method is preferable because it does not incur source-code duplication as partial specialization does.

SmartPtr imitates the semantics of pointers to const objects, const pointers, and the combinations thereof.