Forward Declaration with Vector of Class Type - Pointer to Incomplete Class Type Not Allowed

forward declaration with vector of class type - pointer to incomplete class type not allowed

You can't forward declare members.

Instead, bar.cpp should #include both foo.h and bar.h. Problem solved.

In general, if you use the sequence:

• Forward declare all class types
• Define all class types
• Bodies of class members

everything will be fine.

Pointer to incomplete class type is not allowed

An "incomplete class" is one declared but not defined. E.g.

class Wielrenner;

as opposed to

class Wielrenner
{
    /* class members */
};

You need to #include "wielrenner.h" in dokter.ccp

pointer to incomplete class type is not allowed in a partial template class

You probably forgot to include a header file with definition of a parameter class somewhere. This is the most common cause of the error.

Forward declaration works when you do not use fields from the declared class. Once you start using fields, you need a definition of this class.

C++ pointer to incomplete class type is not allowed

You need to add this to Dog.cpp:

#include "AnimalCare.h"

Also, you are missing the ; after your class declaration for Dog, so it should be:

class Dog
{
public:
Dog(AnimalCare* parent);

std::string GetParentName();
void Feed(void*);
private:
AnimalCare* g_parent;
};

Error: Pointer to incomplete class type is not allowed. How do I go about this?

You are misreading the error. It is not complaining about having a pointer to an incomplete type, but about dereferencing it.

if(foo->foobar()==true)

At this point the type of foo is an incomplete type, so the compiler cannot check whether it will have a foobar member function (or member functor).

Basically for an incomplete type you can declare and define pointers or references, declare interfaces (functions that take or return the type). But other than that you cannot create objects of the type, or use the pointers/references for anything other than copying the pointer/reference.

Regarding what you are doing wrong, you need to look to your real files in more details. Either you are not including the header that defines Foo, or you have multiple Foo types (different namespaces? one defines the type, another has the forward declaration) or your include guards are wrong and even if you include the header the guard discards the contents of the header. Note that after inclusion of the header that defines Foo you don't need to (and should not) provide a forward declaration of the type, as that can easily lead to multiple declarations of Foo in different contexts. If removing the forward declaration fails to compile, figure out why and fix that.

c++ Incomplete type not allowed error accessing class reference information (Circular dependency with forward declaration)

If you will place your definitions in this order then the code will be compiled

class Ball;

class Player {
public:
    void doSomething(Ball& ball);
private:
};

class Ball {
public:
    Player& PlayerB;
    float ballPosX = 800;
private:

};

void Player::doSomething(Ball& ball) {
    ball.ballPosX += 10;                   // incomplete type error occurs here.
}

int main()
{
}

The definition of function doSomething requires the complete definition of class Ball because it access its data member.

In your code example module Player.cpp has no access to the definition of class Ball so the compiler issues an error.

forward declarations, Incomplete type

Include in file Foo.cpp header Bar.h

Foo.cpp:

#include "Foo.h"
#include "Bar.h"

Foo::Foo(const Bar &bar) : mBar(bar) {}

int Foo::getVal() const {
    return mBar.getVal();
}

Or include header Bar.h in header Foo.h

Foo.h:

#pragma once
#include "Bar.h"

class Foo {
    const Bar &mBar;

public:
    Foo(const Bar &bar);

    int getVal() const;
};

Take into account that function Bar::getVal must have qualifier const

int getVal() const;

Otherwise you will get one more compilation error because this non-const function is called from a const function of class Foo.

int Foo::getVal() const {
    return mBar.getVal();
    //     ^^^^^^^^^^^ 
}

incomplete type not allowed, function dec/def and no classes

This:

 int main(){
      void out( fib( input() ) );  // error here
 }

is interpreted to something like an instantiation of a variable of type void calling the constructor that accepts a int as first parameter,
instead you probably want this:

   int main(){
       out( fib( input() ) );  
   } 

that is just a function call

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