c++ standard two different declarations

Question

Why in c++ standard ( I look at cpp reference site) two variants with the same signature are allowed?

For example:

reference front();
const_reference front() const; 

Answer 1

That trailing const is part of the signature. Pretend the implicit this is explicit:

reference       front(      This *this);
const_reference front(const This *this);

Clearly the argument lists are different.

Answer 2

two variants with the same signature

That's a common misconception. The const at the end of the signature is part of the signature.

It can be useful, for example, with container classes that return references to the contained data. The const version returns a const reference so the datum cannot be modified through that reference.