Vector中的C ++ const迭代器

Question

I am a newbie to C++. So, please bear with me. I was looking into the implementation of the std::vector class. I found the following 2 different implementation of the begin() method. I understand that the first one returns a RW iterator and the second one returns a read-only iterator. I thought that mere difference in return type is not enough for function overloading. How does this work then?

iterator
begin()
{ return iterator(this->_M_impl._M_start); }

const_iterator
begin() const
{ return const_iterator(this->_M_impl._M_start); }

Answer 1

One is const and the other isn't. The const version will be called for const std::vector objects while the other is called for non-const std::vector objects. Also note that this also applies to const and non-const references and pointers.

More info on const methods and overloading:

• Meaning of "const" last in a C++ method declaration?
• What's the deal with "const-overloading"?

Also relevant:

• How many and which are the uses of "const" in C++?

Answer 2

The implicit "this" parameter is const in the second method. This is enough to distinguish them in overloading.