How to forward declare a template class in namespace std

Question

ifndef   TEST    define   TEST    namespace std       template lt typename T gt      class list     template lt typename T gt  void Pop std  list lt T gt    l        while  l- gt empty            l- gt pop        endif   and used that function in my main  I get errors  Of course  I know that there are more template params for std  list  allocator I think   But  that is beside the point  Do I have to know the full template declaration of a template class to be able to forward declare it   EDIT  I wasn t using a pointer before - it was a reference  I ll try it out with the pointer

User · Answer

there is a limited alternative you can use

header:

class std_int_vector;

class A{
    std_int_vector* vector;
public:
    A();
    virtual ~A();
};

cpp:

#include "header.h"
#include <vector>
class std_int_vector: public std::vectror<int> {}

A::A() : vector(new std_int_vector()) {}
[...]

not tested in real programs, so expect it to be non-perfect.

User · Answer

Forward declaration should have complete template arguments list specified

User · Answer

I solved that problem    I was implementing an OSI Layer  slider window  Level 2  for a network simulation in C    Eclipse Juno   I had frames  template  lt class T gt   and its states  state pattern  forward declaration    The solution is as follows   In the   cpp file  you must include the Header file that you forward  i e   ifndef STATE H   define STATE H   include  lt stdlib h gt   include  Frame h   template  lt class T gt  class LinkFrame   using namespace std   template  lt class T gt  class State      protected      LinkFrame lt int gt   myFrame       Its cpp    include  State h   include  Frame h   include   LinkFrame h   template  lt class T gt  bool State lt T gt   replace Frame lt T gt   f     And    another class

User · Answer

The problem is not that you can t forward-declare a template class  Yes  you do need to know all of the template parameters and their defaults to be able to forward-declare it correctly   namespace std     template lt class T  class Allocator   std  allocator lt T gt  gt    class list      But to make even such a forward declaration in namespace std is explicitly prohibited by the standard  the only thing you re allowed to put in std is a template specialisation  commonly std  less on a user-defined type  Someone else can cite the relevant text if necessary   Just  include  lt list gt  and don t worry about it   Oh  incidentally  any name containing double-underscores is reserved for use by the implementation  so you should use something like TEST H instead of   TEST    It s not going to generate a warning or an error  but if your program has a clash with an implementation-defined identifier  then it s not guaranteed to compile or run correctly  it s ill-formed  Also prohibited are names beginning with an underscore followed by a capital letter  among others  In general  don t start things with underscores unless you know what magic you re dealing with

[c++] How to forward declare a template class in namespace std?

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