Question

Following is excerpted from vs 2010 map implementation, this member function is defined in parent class _Tree which is in file xtree .

template<class _Valty>
    typename _STD tr1::enable_if<!_STD tr1::is_same<const_iterator,
        typename _STD tr1::remove_reference<_Valty>::type>::value,
            iterator>::type
    insert(const_iterator _Where,
        _Valty&& _Val)
    {   // try to insert node with value _Val using _Where as a hint
    return (_Insert(_Where,
        this->_Buynode(_STD forward<_Valty>(_Val))));
    }

Actually,this function is iterator insert ( iterator position, const value_type& x ),the interesting part of it is its return type! What does it mean? From what i understand, it disable template instantiation if value_type(i.e. _Valty) is same as const_iterator.

But, in such case, since there is no implicit conversion from const_iterator to value_type, this looks like redundant.

I believe I have something that i don’t understand, what is it? what does the return type template prohibit?

Answer 1

There are other insert members that take a range of objects, [iterator, iterator).

The enable_if is used to avoid problems with overload resolution by enabling this function only when the types involved are different.