成员结构的前向声明

Question

I want to write such a code, but it doesn't work:

struct ast;
struct parser;

struct parser
{
    struct node
    {
        node *parent;
    };

    ast build_ast() //here we want to use object of ast tyoe
    {
        node *ret = new node;
        return ast(ret);
    }
}; 

struct ast
{
    parser::node *root; //here we try to access member struct of y
    ast(parser::node *rt):root(rt){}
};

The problem is that I try to create in parser an object of incomplete type. If I swap implementations of structures, the problem is that I try to access member struct of incomplete type. Obviously, I can solve this problem, by making node an independent structure, but that seems bad, because I may want to have other node types in my program. The question is: can I somehow make a forward declaration of node struct outside the parser struct? For example:

struct ast;
struct parser;
struct parser::node; //making a forward declaration of member struct

If this cannot be done, what are the ways to solve the conflict?

Answer 1

You have to defer the definition of build_ast until ast is a complete type:

inline ast build_ast(); //here we want to use object of ast type

_{inline is here to make this declaration functionally identical to what you had in example. You may want to remove it and move build_ast to an implementation file}

Outside of the struct definition:

ast parser::build_ast()
{
    node *ret = new node;
    return ast(ret);
}

To forward-declare node , you need to do this within parser definition. In other words, you cannot forward-declare an inner class without defining the outer.

struct parser
{
    struct node;
    // ...
};

You can then proceed with a definition:

struct parser::node
{
    node *parent;
};