Read-Only Iterators

In our CheckList class—which used pointers as iterators—we declared two different begin/end functions:

// We provide two overloaded begin/end functions: one for writable checklists
// and one for read-only ones.

std::string* begin();
std::string* end();

const std::string* begin() const;
const std::string* end() const;

What should std::list<double> do if the list is read-only? Would it be okay to return a const std::list<double>::iterator?

C++'s solution is to provide two distinct classes that are almost identical, the only difference being whether operator* returns a reference or a const reference. So

• std::list<double>::iterator is an iterator that lets you write the data (and read it, too).
• std::list<double>::const_iterator is an iterator that only lets you read the data.

All the C++ standard-library data structures work similarly, with two types, an iterator type and a const_iterator type.

One positive thing about using auto i = myList.begin() is that you don't have to worry about what the right type to write for a particular situation, C++ will automatically pick the right type. So, when we aren't allowed to write to the data, we'll have a const_iterator, and otherwise we'll get an iterator back.

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