I have seen this type of error everywhere and, although I have looked at the answers, none seem to help.
I get the following error with the following piece of code:
error: 'A' has not been declared
B.h:
#include "A.h"
class B{
public:
static bool doX(A *a);
};
A.h:
include "B.h"
class A{};
To run off a checklist of things I've already tried:
- Names are spelled correctly
- A is in A.h
- There are no namespaces
- No templates
- No macros
I have other classes with can find A just fine. The only thing I can think of is that 'static' is causing a problem.
Replace the include with a forward declaration:
//B.h
class A;
class B{
public:
static bool doX(A *a);
};
Include files only when you have to.
Also, use include guards. This will prevent other nasty issues like re-definitions & such.
If you have two headers including each other you end up with a circular dependency, and due to the way the preprocessor works it means one will be defined before the other.
To fix, I would avoid including A.h in B.h, and just forward declare instead:
class A;
class B{
public:
static bool doX(A *a);
};
You can then include A.h in B.cpp
Related
The title probably isn't very clear, what I'm talking about is:
Say you have a class A depending on B and a class B depending on A.
This causes a circular dependency, but can be solved pretty easily:
/// a.h
#ifndef __A_H
#define __A_H
struct B;
#include "b.h"
struct A
{
B *bptr;
A();
void doStuffWithB();
};
#endif
/// b.h
#ifndef __B_H
#define __B_H
struct A;
#include "a.h"
struct B
{
A *aptr;
B();
void doStuffWithA();
};
#endif
Boom! Circular dependency resolved.
However, what if I now wanted to use an enum declared in B as an argument in A ?
/// a.h ...
struct A
{
B *bptr;
A();
void doStuffWithB(B::Type type); // <-- Compiler smells something funny here
};
/// b.h ...
struct B
{
enum Type
{
// Whatever...
}
B();
void doStuffWithA();
};
Obviously it will not work because the enum isn't yet declared.
The solution for this would be to just put the enum in it's own header file, but it's really nice to have it attached to the class, so would there be any funny way to do something like that ? I searched around but couldn't find a satisfactory answer.
struct B;
#include "b.h"
In the original version of the two header files this #include is completely unnecessary. You'll be happy to learn that the struct B; forward declaration is completely sufficient for declaring the B * class member. The analogous thing applies for the struct A; and the #include in the other header file: the #include is unnecessary.
And once you made this fortunate discovery, the solution in this specific case becomes obvious: simply put back the #include "b.h" back wher eit was.
Only a.h needs to #include the other file, and fully declare the B object in order to make use of its inner enum. The reason for the original compilation error is the unneeded include from b.h to a.h.
When compiling the b.h, its #include of a.h happens before the class and its inner enum gets declared. Then, the #include from a.h to b.h does nothing, because it gets blocked by the include guards. And then the A class gets compiled without the B class, and its inner enum, getting declared.
So, this specific case can be solved, because the dependency on an inner class member is from one of the classes only. What is, actually, an insolvable problem is when two classes have a mutually dependency on each other's inner class member. The only solution is a refactoring, pulling out the inner class type/enums into the global scope, and perhaps using a using or a typedef to alias an inner class to it.
//A.h
class B;
class A{
void Stuff();
B* FOO():
B* _b;
}
extern A* A_A();
//A.cpp
#include "A.h"
#include "B.h"
B* A::FOO(){
return(_b);
}
//B.h
class B{
void BOO();
}
//B.cpp
#include "A.h"
#include "B.h"
void B::BOO(){
A_A->Stuff();
}
Here there is a cross include of the .h files from the .cpp files. So they both depend on one another. Though using the forward declaration and pointers it seems like that would break the cycle. So my question is: Is this a circular dependency? Why?
When A depends on B and vice versa, you have a circular dependency, by definition. The fact that you can get it to work with a forward declaration doesn't change that fact.
You've broken the circular include chain with the forward declarations, but you still have a logical circular dependency between A and B. They each require things provided by the other class.
Regarding your comment above, you should never feel ashamed to bring questions to your team. If find out you were wrong, then you've learned something.
I have this situation (two classes with two different header files):
b.h
#include "a.h"
class B {
friend void B* A::create(void);
private:
int x;
};
a.h
#include "b.h"
class A {
public:
void B* create(void);
...
};
basically class A creates "B" objects. I want to give the creation function create() access to modify the private member 'x' of class B. I want to simulate the 'internal' keyword in C#, but at a more granular level.
The problem I have is that I end up with a circular header reference. b.h wants to come after a.h, but a.h wants to come after b.h. What is the proper way of doing this? Do I need to resort to void* ptrs? Since I only create one type of object, i don't think I need the factory pattern. Hopefully I'm missing something simple.
What you need is a "forward declaration". Basically you don't include the classes' headers in other classes' headers (unless you want to use composition, then you have to). In your case, it'd look like this:
// B.h
class A;
class B {
// declare pointers to A freely
};
And the other way round in A.h.
You'd probably need to include A.h in B.cpp (and the other way round) to be able to actually dereference those pointers to access fields, but your problem with circular header inclusion is now gone.
I have class A (in A.h) which depends on class B in (B.h) and vice versa. Forward declaring the used functions works, but this means I have to update everywhere where I forward declared those functions in the future, ex, if I remove, or change argument in those functions they must all be updated to reflect change. I don't feel this is good practice. Is there a way around this?
Thanks
If you only need to work with pointers or references to a class at the declaration level, you can do it like this:
A.h
class B; // forward class declaration
class A {
A(B &);
};
B.h
class A;
class B {
B(A &);
};
B.cpp
#include "B.h"
#include "A.h" // now we get the full declaration of A
B::B(A &a) {
a.foo(5);
}
Mutual dependencies like this are tough to deal with but sometimes unavoidable.
If A and B depend on the implementations of each other, then you've got a system design problem that you need to resolve before proceeding further.
The best way is to have a forward declaration header:
a.fwd.h
#pragma once
class A;
a.h
#pragma once
#include "a.fwd.h"
#include "b.fwd.h"
class A
{
A(B*);
};
etc.
This way, each class provides its own forward declarations - localised alongside the header where it belongs - checked for consistency with the real declarations and definitions by including the forward declaration header in the header, and the header in the implementation.
Consider the following two scenarios (Edited just to complete the whole question and make it clearer)
Case 1: (doesnt compile as rightly mentioned below)
//B.h
#ifndef B_H
#define B_H
#include "B.h"
class A;
class B {
A obj;
public:
void printA_thruB();
};
#endif
//B.cpp
#include "B.h"
#include <iostream>
void B::printA_thruB(){
obj.printA();
}
//A.h;
#ifndef A_H
#define A_H
#include "A.h"
class A {
int a;
public:
A();
void printA();
};
#endif
//A.cpp
#include "A.h"
#include <iostream>
A::A(){
a=10;
}
void A::printA()
{
std::cout<<"A:"<<a<<std::endl;
}
//main.cpp
#include "B.h"
#include<iostream>
using namespace std;
int main()
{
B obj;
obj.printA_thruB();
}
Case 2: (the only modifications...works without compiliation error)
//B.h
#include "A.h" //Add this line
//class A; //comment out this line
Let us assume both the A.cpp and B.cpp are complied together. Do the above two scenarios make any differences? Is there a reason to prefer one method over the other?
Edit:
So how do I make scenario 1 work.
Forward declaration is not a substitute for Header file inclusion.
As the name itself implies, forward declaration is just a Declaration and not a definition.
So, you will declare saying the compiler that it is a class and I just declaring it here and will provide you the definition when am gonna use it. So, normally you forward declare in the Header file and #include in the .cpp file where you will use the members of the forward declared class.
By doing so, what you make is, wherever you are including the header file there will just be a declaration for the class instead of the entire contents #included...
But having said that, when the compiler requires the definition of the class, it should be #included..
So, in your case A obj; requires the definition of class A and hence you should #include..
I myself asked a similar question here and another similar question which has also a nice answer...
Hope it helps..
Case 1 will produce an "incomplete type" error when you compile B.cpp. Because class B contains a class A object, the definition (and in particular the size) of class A is required to be complete before the definition of class B.
Alternatively, you could choose to make some_variable a pointer or reference to class A, and in that case your forward declaration would be sufficient in B.h. You'd still need a full definition of A in B.cpp (assuming you made actual use of the A member functions/data).
You need to use forward declarations in cases where you have classes that refer to each other.
//A.h
class B;
class A {
B* someVar;
}
//B.h
#include <A.h>
class B {
A* someVar;
}
But there's no benefit for doing it in the case you laid out.
Think like a compiler. In order to create an A inside of B, the compiler has to know how to build an A, and the only way to do that is to have the complete definition. The forward declaration tells the compiler that class A exists without describing what it looks like; this is adequate for defining a pointer or a reference. When it comes time to use that pointer or reference, the complete class definition will be required.
If you meant to portray some_variable as a pointer then the frequently recommended practice is to use forward declarations whenever possible to avoid the overhead of includes and longer compile times.
I'm all for best practices but I really like using IDEs that have nice code navigation features and forwards cause a problem there, at least with Netbeans. Whenever I try to navigate to a type declaration, I always end up at the forward and not the .h file containing the actual declaration. I'm willing to accept some extra compile time for the ease of navigation. Maybe this is just a problem with Netbeans :)
.. oh yeah.. If you look at the related questions to the right of your question, you will find lots of additional information of forward declarations.
For case 1, compiler will complain with "incomplete type" for class B because class B contains a class A object, and you did not tell B any detail of class A， so compiler cann't decide the size of object B.
For your case, you can use A& obj or A* obj instead of A obj, since the size of a reference/pointer is const(4/8 for 32bit/64bit CPU).