Using undefined type

Question

//file list.h
#include "stdafx.h"

namespace st
{
    struct My_List;
    typedef My_List list;
    list* create(const char* name);
}

//file list.cpp
#include "stdafx.h"
#include "list.h"
namespace st
{
    struct My_List
    {
        const char* name_;
        My_List* left_;
        My_List* right_;

        My_List(const char* name):name_(name),
            left_(nullptr),
            right_(nullptr)
        {}
        My_List(const My_List&);

        ~My_List()
        {

        }
        void insert(My_List*);

        void set_name(char* name)
        {

            name_ = name;
        }

        const char* get_name()const
        {
            return name_;
        }
    };
    typedef My_List list;

    /*helper class for optor+ */
    struct MyChar
    {
        const char* my_data_;
        MyChar(const char* c_string):my_data_(c_string){}
        operator const char*()
        {
            return my_data_;
        }
        operator char*()
        {
            return const_cast<char*>(my_data_);
        }

    };

    char* operator+(MyChar left_, MyChar right_)
    {
        if (!left_.my_data_ || !right_.my_data_)
        {
            return 0;
        }
        size_t size = 1;//size is set to one for final '\0' char in an array
        char* p = "";//if both c_strings are empty this is returned
        bool has_left_ = false;
        bool has_right_ = false;
        if (strlen(left_))
        {
            size += strlen(left_);
            has_left_ = true;
        }
        if (strlen(right_))
        {
            size += strlen(right_);
            has_right_ = true;
        }
        bool both = has_left_ && has_right_ ? true : false;
        if (both)
        {
            p = new char[size]();
            const void* p_v = p;//just to keep address of beginning of p
            const char* tmp = left_;
            /*copying first c_string*/
            while (*p++ = *tmp++);

            tmp = right_;
            /*one too far after last loop*/
            --p;
            while (*p++ = *tmp++);

            *p = '\0';
            /*go back to the beginning of an array*/
            p = static_cast<char*>(const_cast<void*>(p_v));
            return p;
        }
        else if (has_left_)
        {
            return left_;
        }
        else if (has_right_)
        {
            return right_;
        }
        return p;//returns "" if both c_strings were empty
    }


    My_List::My_List(const My_List& pat):left_(nullptr),right_(nullptr)
    {
        name_ = pat.name_ + MyChar("_cpy");
        My_List* pattern = const_cast<My_List*>(&pat);
        My_List* target = this;
        while (pattern->right_)
        {

            target->right_ = static_cast<My_List*>(malloc(sizeof(My_List)));
            *target->right_ = *pattern->right_;
            target->right_->set_name(pattern->right_->get_name() + MyChar("_cpy"));
            target->right_->left_ = static_cast<My_List*>(malloc(sizeof(My_List)));
            *target->right_->left_ = *pattern->right_->left_;
            target->right_->left_->set_name(pattern->right_->left_->get_name() + MyChar("_cpy"));
            pattern = pattern->right_;
            target = target->right_;
        }
    }

    void My_List::insert(My_List* obj)
    {
        /*to catch first branch*/
        My_List* tmp = this;
        if (tmp->right_)
        {
            /*go to the end of right side*/
            while (tmp->right_)
            {
                tmp = tmp->right_;
            }

            tmp->right_ = obj;
            obj->left_ = tmp;
        }
        else
        {
            tmp->right_ = obj;
            obj->left_= this;
        }
    }
    My_List* create(const char* name)
    {
        return new My_List(name);
    }
}

//file main.cpp
#include "stdafx.h"
#include "list.h"
using namespace st;

int _tmain(int argc, _TCHAR* argv[])
{

    list* my = create("a");
    list* b = create("b");
    my->insert(b);//HERE I'M GETTING ERROR
    return 0;
}

err msg: 'Error 1 error C2027: use of undefined type 'st::My_List' 13'

Why? Especially that if I comment this line it will get compiled and create() is using this type.

Answer 1

You have done only a forward declaration in header file list.h. The actual definition is in list.cpp. While compiling main.cpp you are including list.h, so compiler doesn't know what methods are there in the class My_List. You need to move the struct definition to header file list.h, you can still keep the method implementation in list.cpp