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Template cross declaration ??

Started by GhostAce Apr 21, 2006 at 7:36 AM 4 replies 950+ views

Original Post

Apr 21, 2006 at 7:36 AM

I have a problem with templates. Please check this code:


template<typename _TPtr, typename _TObj>
struct test_t
{
    _TPtr *ptr;
    _TObj obj;
};

typedef test_t<two_t, int>  one_t;
typedef test_t<int, one_t>  two_t;

I obviously need to declare 'two_t' before 'one_t', but 'one_t' also needs 'two_t' declared before. Code for this simple test-case can be easily written without templates, but how to make this WITH templates? I would be grateful for any help.

MrEvil

110

MrEvil

Apr 21, 2006 at 7:43 AM

Wouldn't two_t expand to test_t which would expand to test_t > which would expand to test_t, int> > until your compiler slaps you?

I don't get it.

tychon

Apr 21, 2006 at 7:47 AM

Quote:
Original post by MrEvil
Wouldn't two_t expand to test_t which would expand to test_t > which would expand to test_t, int> > until your compiler slaps you?

I don't get it.

I was puzzling over this myself.

GhostAce, could you paste the non-templated code?

GhostAce

Author

GhostAce

Apr 21, 2006 at 7:52 AM

Notice that there is a pointer, so you won't get infinite structures recursion.

struct two_t;struct one_t{    two_t *ptr;    int obj;};struct two_t{    int *ptr;    one_t obj;};

Emmanuel Deloget

431

Emmanuel Deloget

Apr 21, 2006 at 7:55 AM

struct FirstType;typedef std::pair<FirstType*, int> SecondType;struct FirstType : public std::pair<int*, SecondType> { };

But it's awfull. Really. I smell a design weirdness [smile]

GhostAce

Author

GhostAce

Apr 21, 2006 at 8:08 AM

Quote:
Original post by Emmanuel Deloget
struct FirstType;typedef std::pair<FirstType*, int> SecondType;struct FirstType : public std::pair<int*, SecondType> { };
But it's awfull. Really. I smell a design weirdness [smile]

Great! [Why didn't I think of this before? :)]
Yes it's awful, but quite interesting.

Here is the corrected code if anyone is interested:

template<typename _TPtr, typename _TObj>struct test_t{    _TPtr *ptr;    _TObj obj;};struct two_t;typedef test_t<two_t, int>  one_t;typedef test_t<int, one_t>  _helper_two_t;struct two_t : public _helper_two_t {/*empty*/};

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