引用未在 Visual Studio 2017 中编译的迭代器

Question

In my project in Visual c++ in visual studio 2015, I used references to iterators.

I updated to vs2017 today, but the compiler appears to have changed more than I thought. Hopefully I can resolve most problems, but some I don't know why it would change.

For example, I used the snippet

for (auto& it = stack.begin(); it != stack.end() /* not hoisted */; /* no increment */)
    {
        if (it->second->empty()) { stack.erase(it++); }    // or "it = m.erase(it)" since C++11
        else
        {
            auto obj = move(it->second);
            stack.erase(it);
            return move(obj);
        }
    }

where stack is a std::map<float, std::unique_ptr<StackableObj> .

It worked fine before, but in vs2017 does not compile, instead it produces the type error

Error   C2440   'initializing': cannot convert from 'std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<_Ty>>>' to 'std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<_Ty>>> &'.

What do I do about this, or where do I look in order to know what has been changed? Googling some terms didn't give any specific answers to what is going on.

Thanks in advance

Answer 1

Non-const reference to an rvalue is not valid. See this for more on the topic:

By default, the compiler cannot bind a non-const or volatile lvalue reference to an rvalue.

The most optimal solution here is dropping & , since copying iterators is cheap.