Range-for循环识别错误的类型（C2440）

Question

The C2440 error (cannot convert from 'char' to 'Text_iterator' is occurring on this line):

void print(Document& d)
{
    for (Text_iterator p : d) cout << *p;
}

Replacing 'Text_iterator' with 'auto' gives the 'illegal indirection' error (dereferenced type must be a pointer).

Class Document has begin() and end() functions defined (returning Text_iterator), and Text_iterator has the usual iterator operators. Here is the code:

class Text_iterator
{
    list<Line>::iterator ln;
    Line::iterator pos;
public:
    // . . .

    char& operator*() { return *pos; }
    Text_iterator& operator++();

    // . . .
};

And Document:

struct Document
{
    list<Line> line;

    Text_iterator begin()
    {
        // . . .
    }

    Text_iterator end()
    {
        // . . .
    }

    // . . .
};

Answer 1

You are using the incorrect type for the loop variable. The variable is not the iterator of the container but it is what the iterator of that container points to.

If you had

std::vector<int> foo(10);

And you want to use a ranged based for loop you would use

for (int e : foo) // or for (auto e : foo) 
    std::cout << e << " ";

You would not use

for (std::vector<int>::iterator e : foo)
    std::cout << *e << " ";

So you need to use whatever Text_iterator points to instead of Text_iterator . In this case

void print(Document& d)
{
    // if p is cheap to copy
    for (auto p : d) cout << p;
    // if p is expensive to copy
    for (const auto& p : d) cout << p;
}

should work.

Answer 2

The range-for iterates over the content of a container. So it's not about iterators, but dereferenced iterators.

Your code should be:

void print(Document& d)
{
    for (auto c : d) cout << c;
}

or if you really want to keep control over the iterators:

void print(Document& d)
{
    for (auto p = d.begin(); p!=d.end(); p++) cout << *p;
}