Forward declaration of a typedef in C

Question

Why won t the compiler let me forward declare a typedef   Assuming it s impossible  what s the best practice for keeping my inclusion tree small

User · Answer

Using forward declarations instead of a full  includes is possible only when you are not intending on using the type itself  in this file s scope  but a pointer or reference to it     To use the type itself  the compiler must know its size - hence its full declaration must be seen - hence a full  include is needed     However  the size of a pointer or reference is known to the compiler  regardless of the size of the pointee  so a forward declaration is sufficient - it declares a type identifier name     Interestingly  when using pointer or reference to class or struct types  the compiler can handle incomplete types saving you the need to forward declare the pointee types as well        header h     Look Ma  No forward declarations  typedef class A  APtr     class A is an incomplete type - no fwd  decl  anywhere typedef class A amp  ARef   typedef struct B  BPtr     struct B is an incomplete type - no fwd  decl  anywhere typedef struct B amp  BRef      Using the name without the class struct specifier requires fwd  decl  the type itself      class C             fwd  decl  type typedef C  CPtr     no class struct specifier  typedef C amp  CRef     no class struct specifier   struct D            fwd  decl  type typedef D  DPtr     no class struct specifier  typedef D amp  DRef     no class struct specifier

User · Answer

Like  BillKotsias  I used inheritance  and it worked for me   I changed this mess  which required all the boost headers in my declaration   h    include  lt boost accumulators accumulators hpp gt   include  lt boost accumulators statistics hpp gt   include  lt boost accumulators statistics stats hpp gt   include  lt boost accumulators statistics mean hpp gt   include  lt boost accumulators statistics moment hpp gt   include  lt boost accumulators statistics min hpp gt   include  lt boost accumulators statistics max hpp gt   typedef boost  accumulators  accumulator set lt float   boost  accumulators  features lt    boost  accumulators  tag  median    boost  accumulators  tag  mean    boost  accumulators  tag  min    boost  accumulators  tag  max   gt  gt  VanillaAccumulator t   std  unique ptr lt VanillaAccumulator t gt  acc    into this declaration    h   class VanillaAccumulator  std  unique ptr lt VanillaAccumulator gt  acc    and the implementation    cpp  was   include  lt boost accumulators accumulators hpp gt   include  lt boost accumulators statistics hpp gt   include  lt boost accumulators statistics stats hpp gt   include  lt boost accumulators statistics mean hpp gt   include  lt boost accumulators statistics moment hpp gt   include  lt boost accumulators statistics min hpp gt   include  lt boost accumulators statistics max hpp gt   class VanillaAccumulator   public   boost  accumulators  accumulator set lt float      boost  accumulators  features lt        boost  accumulators  tag  median        boost  accumulators  tag  mean        boost  accumulators  tag  min        boost  accumulators  tag  max  gt  gt

User · Answer

As Bill Kotsias noted  the only reasonable way to keep the typedef details of your point private  and forward declare them is with inheritance  You can do it a bit nicer with C  11 though  Consider this      LibraryPublicHeader h  class Implementation   class Library       private      Implementation  impl            LibraryPrivateImplementation cpp     This annoyingly does not work            typedef std  shared ptr lt Foo gt  Implementation      However this does  and is almost as good  class Implementation   public std  shared ptr lt Foo gt    public         C  11 allows us to easily copy all the constructors      using shared ptr  shared ptr

User · Answer

You can do forward typedef  But to do  typedef A B    you must first forward declare A   class A   typedef A B

User · Answer

To  fwd declare a typedef  you need to fwd declare a class or a struct and then you can typedef declared type  Multiple identical typedefs are acceptable by compiler   long form   class MyClass  typedef MyClass myclass t    short form   typedef class MyClass myclass t

User · Answer

Because to declare a type  its size needs to be known  You can forward declare a pointer to the type  or typedef a pointer to the type   If you really want to  you can use the pimpl idiom to keep the includes down  But if you want to use a type  rather than a pointer  the compiler has to know its size   Edit  j random hacker adds an important qualification to this answer  basically that the size needs to be know to use the type  but a forward declaration can be made if we only need to know the type exists  in order to create pointers or references to the type  Since the OP didn t show code  but complained it wouldn t compile  I assumed  probably correctly  that the OP was trying to use the type  not just refer to it

User · Answer

For those of you like me  who are looking to forward declare a C-style struct that was defined using typedef  in some c   code  I have found a solution that goes as follows        a h  typedef struct  bah       int a      int b     bah      b h  struct  bah   typedef  bah bah    class foo      foo bah   b      foo bah b      bah   mBah          b cpp   include  b h    include  a h    foo  foo bah   b       mBah   b       foo  foo bah b       mBah    amp b

User · Answer

In C    but not plain C   it s perfectly legal to typedef a type twice  so long as both definitions are completely identical      foo h struct A    typedef A  PA      bar h struct A      forward declare A typedef A  PA  void func PA x       baz cc  include  bar h   include  foo h     We ve now included the definition for PA twice  but it s ok since they re the same     A x  func  amp x

User · Answer

I replaced the typedef  using to be specific  with inheritance and constructor inheritance       Original  using CallStack   std  array lt StackFrame  MAX CALLSTACK DEPTH gt     Replaced  struct CallStack    Not a typedef to allow forward declaration      public std  array lt StackFrame  MAX CALLSTACK DEPTH gt      typedef std  array lt StackFrame  MAX CALLSTACK DEPTH gt  Base    using Base  Base       This way I was able to forward declare CallStack with   class CallStack

User · Answer

I had the same issue  didn t want to mess with multiple typedefs in different files  so I resolved it with inheritance   was   class BurstBoss    public       typedef std  pair lt Ogre  ParticleSystem   bool gt  ParticleSystem     removed this with      did   class ParticleSystem   public std  pair lt Ogre  ParticleSystem   bool gt     public       ParticleSystem Ogre  ParticleSystem  system  bool enabled    std  pair lt Ogre  ParticleSystem   bool gt  system  enabled                Worked like a charm  Of course  I had to change any references from  BurstBoss  ParticleSystem   to simply  ParticleSystem

[c++] Forward declaration of a typedef in C++

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