What is the difference between const_iterator and non-const iterator in the C++ STL?

Question

What is the difference between a const_iterator and an iterator and where would you use one over the other?

Answer 1

Minimal runnable examples

Non-const iterators allow you to modify what they point to:

std::vector v{0};
std::vector::iterator it = v.begin();
*it = 1;
assert(v[0] == 1);

Const iterators don't:

const std::vector v{0};
std::vector::const_iterator cit = v.begin();
// Compile time error: cannot modify container with const_iterator.
//*cit = 1;

As shown above, v.begin() is const overloaded, and returns either iterator or const_iterator depending on the const-ness of the container variable:

• How does begin() know which return type to return (const or non-const)?
• how does overloading of const and non-const functions work?

A common case where const_iterator pops up is when this is used inside a const method:

class C {
    public:
        std::vector v;
        void f() const {
            std::vector::const_iterator it = this->v.begin();
        }
        void g(std::vector::const_iterator& it) {}
};

const makes this const, which makes this->v const.

You can usually forget about it with auto, but if you starting passing those iterators around, you will need to think about them for the method signatures.

Much like const and non-const, you can convert easily from non-const to const, but not the other way around:

std::vector v{0};
std::vector::iterator it = v.begin();

// non-const to const.
std::vector::const_iterator cit = it;

// Compile time error: cannot modify container with const_iterator.
//*cit = 1;

// Compile time error: no conversion from const to no-const.
//it = ci1;

Which one to use: analogous to const int vs int: prefer const iterators whenever you can use them (when you don't need to modify the container with them), to better document your intention of reading without modifying.