-> first / second for an empty map iterator to start

Question

I can not understand what is happening in this fragment.
The map link says: "If the container is empty, the return value of the iterator should not be dereferenced."
But what about some_map->begin()->second? on a blank card.
I thought this would be invalid, but this code prints "0". Can anyone explain why?

int main()
{
 map<int,int> a;

 printf("%d",a.begin()->second);
 return 1;
}

Thank!

Answer 1

From this std::map::beginlink

If the container is empty, the returned iterator will be equal end()

Then look at this linkstd::map::end :

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Answer 2

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Answer 3

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template<typename U, typename V>
typename map<U,V>::iterator &map<U,V>::iterator::operator --()
{
    if ( node_ != nullptr )
    {
        // go to predecessor
        if ( node_->leftChild_ != nullptr )
        {
            for ( node_ = node_->leftChild_; node_->rightChild_ != nullptr; node_ = node_->rightChild_ );
        }
        else
        {
            node *n;
            do
            {
                n = node_;
                node_ = node->parent_;
                // may not go before begin()
                assert( node_ != nullptr );
            } while ( n == node_->leftChild_ );
        }
    }
    else
    {
        // point to last node - map must not be empty
        assert( container_->root_ != nullptr );
        for ( node_ = container_->root_; node_->rightChild_ != nullptr; node = node->rightChild_ );
    }
    return *this;
}

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template<typename U, typename V>
typename map<U,V>::reference map<U,V>::iterator::operator *() const
{
    // cannot dereference end iterator
    assert( hasSuccessor(node_) );
    return *node_;
}

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