在C ++中转发“预先声明”一个类

Question

I have a situaion in which I want to declare a class member function returning a type that depends on the class itself. Let me give you an example:

class Substring {
    private:
        string the_substring_;
    public:
        // (...)
        static SubstringTree getAllSubstring(string main_string, int min_size);
};

And SubstringTree is defined as follows:

typedef set<Substring, Substring::Comparator> SubstringTree;

My problem is that if I put the SubstringTree definition after the Substring definition, the static method says it doesn't know SubstringTree. If I reverse the declarations, then the typedef says it doesn't know Substring.

How can I do it? Thanks in advance.

Answer 1

As you've written it, the short answer is you can't.

You do have a few close alternatives:

1) Declare SubstringTree in Substring

class Substring {
public:
    class Comparator;
    typedef set< Substring, Comparator> Tree;

private:
    string the_substring_;
public:
    // (...)
    static Tree getAllSubstring(string main_string, int min_size);
};

typedef Substring::Tree SubstringTree;

2) Define the Comparator outside of Substring:

class Substring;
class SubstringComparator;
typedef set< Substring, SubstringComparator> SubstringTree;

class Substring {
public:

private:
    string the_substring_;
public:
    // (...)
    static SubstringTree getAllSubstring(string main_string, int min_size);
};

3) You can use a template to delay the lookup until you have more declarations:

template <typename String>
struct TreeHelper
{
  typedef set< String, typename String::Comparator> Tree;
};

class Substring {
public:
  class Comparator;

private:
  string the_substring_;
public:
  // (...)
  static TreeHelper<Substring>::Tree getAllSubstring(string main_string
                                             , int min_size);
};

typedef TreeHelper<Substring>::Tree SubstringTree;

Answer 2

You could define it inside the class:

class Substring {
    private:
        string the_substring_;
    public:
        // (...)
        typedef set<Substring, Substring::Comparator> SubstringTree;
        static SubstringTree getAllSubstring(string main_string, int min_size);
};

Answer 3

You can predeclare a class with this:

class Foo;

Keep in mind that before the class is actually defined, you can only declare pointers to it, not instances.

Answer 4

forward declaration

class Substring;

I don't know if that will work for non pointer uses of Substring though.