How does STL return 2 iterators with the same function?

I noticed that vec开发者_如何学编程tor.begin() will return a const iterator, or an iterator based on what is given on the left. How is something like this implemented since the arguments given to the function are the same.

Thanks

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Its return-type is based on whether the vector itself is being accessed through a const reference (or pointer) or not. Also, an iterator can be implicitly converted to a const_iterator, which is why something like this works: std::vector<T> v; std::vector<T>::const_iterator it = v.begin();.

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begin() and end() are overloaded on the const-ness of *this, something like:

iterator begin();
const_iterator begin() const;

iterator end();
const_iterator end() const;

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They are overloaded on the const-ness of the member function:

struct Foo
{
    int bar() { return 1; }
    int bar() const { return 2; }
};

int main()
{
    Foo a;
    const Foo b;
    assert(a.bar() == 1);
    assert(b.bar() == 2);
}