Question

I want to create a structure which contains a list of same structure like this:

#include <list>
struct Url
{
    CString strUrl;
    std::list<Url> children;
};

int main()
{
    Url u1, u2;
    u1.children.push_back(u2);
}

This code is not compiling. But when I replace std::list with std::vector it is working fine. How can I make this working with std::list?

Output window contains the following error.

c:\program files\microsoft visual studio\vc98\include\list(29) : error C2079: '_Value' uses undefined struct 'Url'
        E:\test\Test.cpp(23) : see reference to class template instantiation 'std::list<struct Url,class std::allocator<struct Url> >' being compiled
c:\program files\microsoft visual studio\vc98\include\functional(185) : error C2079: 'value' uses undefined struct 'Url'
        c:\program files\microsoft visual studio\vc98\include\list(285) : see reference to class template instantiation 'std::binder2nd<struct std::not_equal_to<struct Url> >' being compiled
        E:\test\Test.cpp(23) : see reference to class template instantiation 'std::list<struct Url,class std::allocator<struct Url> >' being compiled

Answer 1

If you need a workround for what seems to be a VC6 bug, create the list dynamically:

#include <list>
#include <string>     // I don't use MFC

struct Url
{
    std::string strUrl;
    std::list<Url> * children;

    Url() {
       children = new std::list <Url>;
    }

    ~Url() {
        delete children;
    }
};

int  main()
{
    Url u1, u2;
    u1.children->push_back(u2);
}

Some have asked why lists of the same type as members are allowed (and in my view they are) when

Url array[5]; 

for example as a member would not be. I can’t find anything in the standard either, but sizeof( std:;list <T>) is not dependent on the thing it is a list of. Suppose list was implemented as (some pseudo C++ here):

list <T> {
   listEntry <T> * first;
};

then there is no unknown size to deal with. Consider the following minimal code that addresses the questioners problem:

template <typename T> struct A {
};

struct B {
    A <B> b;
};

I can’t see any possible reason that this should not be legal.