I'm finishing up a C++ assignment and I'm running into an issue with connecting all the classes together. The classes have basic constructors, getters, setters, etc. so I'm confident the abundance of compiler errors I'm getting are due to me mix and matching all of my Classes incorrectly...
My classes are split up into cpp and h files and I've basically used the include statements every time that class needs something from another class. A basic diagram of which class needs what is shown below:
Basically what I'm trying to figure out is if having #include "Product.h" in both CustomerOrder and Retailer is messing things up or if I'm doing this all the wrong way.
Any help would be lovely!
Thanks!
EDIT:
Here's one instance of what each class is basically doing. The Customer class holds an array of CustomerOrders and CustomerOrders holds an array of Products etc. etc..
...
class Customer
{
private:
string customerName;
string customerID;
float balance;
static const int ORDERSIZE = 2;
CustomerOrder* orderList[ORDERSIZE];
Retailer retailer;
...
and Customer Order:
...
class CustomerOrder
{
private:
static const int SHOPSIZE = 20;
Product* shoppingList[SHOPSIZE];
...
First of all, your diagram is confusing. In OOP, arrows like the one you use indicate public inheritance, but that doesn't seem to be the case here (nor that it should).
Your question cannot be answered generally. You #include another class if you need the complete type. Otherwise, a forward declaration will do. That only has an impact on compilation speed. Nevertheless, the rule is to use forward declarations if you can and #includes if you must.
Here are some cases where you can get away with a forward declaration:
1.) Pointers:
class AnotherClass; // <-- forward declaration
class Example
{
// ...
AnotherClass *another_class;
};
2.) References:
class AnotherClass; // <-- forward declaration
class Example
{
// ...
void f(AnotherClass &another_class);
};
3.) Return values:
class AnotherClass; // <-- forward declaration
class Example
{
// ...
AnotherClass Get();
};
As soon as you are actually using the object of the forward-declared class, you need to have the #include. If you forget, the compiler will remind you.
Caveat 1: Pay attention when you use forward-declared (i.e. so-called "incomplete") types in standard containers. Your compiler may allow this, but it's undefined behaviour to do so!
class Example; // forward declaration
std::vector<Example> v; // undefined behaviour!
Caveat 2: Don't attempt to forward-declare standard classes. Just do #include <vector>, #include <string> and so on and let the compiler figure out how to optimize compile time.
Edit: Actually, the linker, rather than the compiler, will remind you if you forget to include a forward-declared class you cannot forward-declare, but that's just a nitpick :)
Related
I got this syntax I don't really understand:
class USphereComponent* ProxSphere;
I think this means create a class, but is this class a pointer?
But the result is just creating an object called ProxSphere from an existing class USphereComponent.
What does this syntax actually mean, and what is its usage?
class Someotherclass; // That has not been defined yet
class HelloWorld
{
Someotherclass* my_pointer;
};
Or an alternative:
class HelloWorld
{
class Someotherclass* my_pointer;
};
The first one is obviously the correct one if you have multiple pointers (or references) to such class that has not been defined yet.
Is the second better? (I don't know) if you only need to do it once, otherwise doing
class HelloWorld
{
class Someotherclass* my_pointer;
class Someotherclass* my_pointer2;
class Someotherclass* my_pointer3;
void func(class Someotherclass* my_pointer, class Someotherclass& my_ref);
};
may not be the best.
Jts's answer is correct. I'd like to add a use case for it:
This is mostly used when you have a circular class dependency.
Like:
class A { B* binst; };
class B { A* ainst; };
That wouldn't compile since B isn't previously known.
Therefore you would first declare class B.
class B;
class A { B* binst; };
class B { A* ainst; };
Or as mentioned, you can use syntactic sugar:
class A { class B* binst; };
class B { A* ainst; };
Such a dependency might be a code smell. It also might be ok or even necessary. If you have it, you should carefully think if you can't do it in some other yet convenient way.
That particular syntax is called a "forward declaration". It is used to declare a type that has not been defined yet.
This is basically telling the compiler "There exists a class type named USphereComponent that you haven't seen yet that will come up later in the code. Please don't yell at me if you see pointers of that type". This allows you to declare pointer and reference for that forward-declared type.
Writing:
class USphereComponent* ProxSphere;
Is really just the equivalent of writing this:
class USphereComponent;
USphereComponent* ProxSphere;
The only difference with the second syntax, is that you only need to forward-declare the type once when you do it like this class USphereComponent;, otherwise you need to use the first syntax and add the class keyword before each usage of USphereComponent.
There are two main reasons why you may want to use a forward declaration:
This is probably the most common usage of forward-declaration in Unreal Engine. In header (.h) files, forward-declaration allows you to use pointer of classes for which you did not #include the corresponding header file. In our particular example that means that forward-declaring USphereComponent means that we don't need a #include "SphereComponent.h" statement (if we're just trying to pass a USphereComponent around that is).
Typically when that happens, the #include statement is simply done in the .cpp file. There are mainly two advantages of reducing the number of includes in your header files:
Compilation times are faster. Mind you, this mostly has significant impact on a codebase as big as Unreal's.
This reduces the number of public dependencies of you module (by making them "private" since your includes are now in your .cpp). This makes your module easier to be depended upon and also makes its interface cleaner.
Like other answers have said, forward-declaration can be used to break circular dependencies when you have two types that depends on each other in the same file:
class B;
class A
{
B* foo;
};
class B
{
A* bar;
};
From what i've read, i should use forward declarations whenever I can. I have classes like this ( where every fields are pointers because of forward declarations ) :
class A
{
// ...
A* a;
B* b;
C* c;
D* d;
E* e;
};
But there is problems with that.
1- This implies to call new and delete ( or at least new with smart pointers ) for every fields in constructor, while stack allocated fields don't need this.
2- I've read that stack allocation was faster than heap allocation.
3- Also that means that almost every fields on every classes should be pointers.
Am I doing the right way doing like my example class? Or am I missing something with forward declarations?
The example you've shown is an overkill. The suggestion to use forward declarations doesn't mean your code design is driven by forward declaration practices. Forward declarations are just implementation detail, the design prevails.
First decide whether you need aggregation or composition, then whether the forward declaration is appropriate.
Do prefer forward declaration over #include when the forward-declared type is part of your method signature, i.e. a parameter's type.
#include "OtherClass.h" // 'bad' practice
class OtherClass; // this is better than #include
....
class MyClass
{
void method(OtherClass *ptr);
}
It's not an absolute rule anyway as it's not always possible/convenient to use forward decls instead of includes.
The implication is inverse - you're not supposed to use pointers just in order to use forward declarations, but you're suppose to use forward declarations after you've taken a design decision (such as using pointers instead of objects as members) when you can.
So if it makes more sense to use objects, do so, and include the files you need. Don't use pointers just so you can forward-declare the classes.
If you are using pointers as members, prefer forward declaration than exposing complete class definition. Don't use pointers just to meet some rule blindly.
Technically spoken, you can (and should!) use a forward declaration, if the interface of your class doesn't depend on a fully qualified type. The compiler has to reserve enough space for members and add management functions at compile time - just using pointers or references in your class does not introduce dependencies on types.
BTW: Forward declaration isn't that new: In some C standard libraries, FILE is a typedef for a forward declared struct, which makes sense since FILE is always used for pointers in the whole public file API.
Use pointers or references for objects that the class doesn't own. But for objects that are owned by this class don't use forward declarations as a reason for choosing pointers.
If you really want to minimize compile time dependencies consider the PIMPL idom rather than turning all your members into pointers:
MyClass.h:
#include <memory>
class MyClassImpl;
class MyClass {
public:
MyClass();
~MyClass();
void doThing();
private:
std::unique_ptr<MyClassImpl> pimpl_;
};
MyClass.cpp
#include "MyClass.h"
#include "MyClassImpl.h"
MyClass::MyClass() { } // in .cpp so unique_ptr constructor has complete type
MyClass::~MyClass() { } // in .cpp so unique_ptr destructor has complete type
void MyClass::doThing(){
pimpl_->doThing();
}
MyClassImpl.h:
#include "A.h"
#include "B.h"
class MyClassImpl {
private:
A a_;
B b_;
public:
void doThing();
};
MyClassImpl.cpp:
#include "MyClassImpl.h"
void MyClassImpl::doThing() {
// Do stuff with a_, b_, etc...
}
This might not address performance concerns as you still have dynamic memory allocation but you would have to measure it to see.
In addition to the good answers already given: In cases where your class doesn't create an object but uses it privately (for instance some utility class), references can be used instead of pointers.
class UtilityClass; // forward declaration (even interfaces make sense here)
class MyClass {
public:
/// takes an UtilityClass for implementing some of its functions
MyClass(UtilityClass& u): util(u) {}
private:
UtilityClass& util;
// ...more details
};
These are cases, where forward declaration doesn't mean that objects have to be created on heap (as for your problems #1 and #2).
I have been going through the code of a project and have encountered the statement
class foo;
.
.
.
.
foo* f1;
in various places. The class has not been declared in any of the headers included either. Can anybody tell me what that means.
It is a forward declaration. It can be used for classes, structs and functions, and it tells compiler that this is defined elsewhere or later.
For classes, there are (at least) two use cases.
1. Full definition not needed
After forward declaration, compiler does not know size or members of class, only name. That is enough for pointers to the class (and references which are basically syntactic sugar around pointers). But often pointer is enough, and then you can avoid including entire header file in another. This helps compilation speed, by avoiding need to recompile everything when one header changes.
myfuncs.h
class MyClass; // forward declaration
void helpMyClass(MyClass &needy);
// here this would give compiler error about incomplete type:
//void badHelp(MyClass needy); // value needs definition
myfuncs.cpp:
#include "myclass.h" // entire MyClass definition
void helpMyClass(MyClass &needy) {
needy.helpMe(false); // needs full definition
}
Important use case for this is the so called PIMPL idiom, also well covered here at SO under pimpl-idiom tag.
2. Two classes need to refer to each others
class node; // forward declarion
class collection {
node *frist; // pointer enabled by forward declaration
}
class node {
collection *owner; // already defined above so works too
}
In this case forward declaration is required to make this work nicely. Just saying in case you see it in the wild, there's the ugly way of using void pointer and casts, sometimes used when novice programmer does not know how this should be done.
I think you're referring to a forward declaration. It tells the compiler that a class named foo will be defined later. Until then it is an "incomplete type", meaning that pointers and references to the class can be defined. Instances of the class cannot be created until it is fully defined.
Your declaration is incorrect? I'm not sure.. I do know that you can't have "any" space "name".. Perhaps you missed an underscore?
I believe you meant:
class foo any_name();
In that case, it's forward declaring a function called any_name that returns a class instance of foo.
Example:
#include <iostream>
class foo any_name(); //The forward declaration..
class foo //the foo class implementation.
{
public:
foo(){std::cout<<"hey";}
};
class foo any_name() //The implementation of the function..
{
std::cout<<"any_name";
//return {}; //Can be used to return a constructed instance of foo.
};
int main()
{
any_name();
}
This question already has answers here:
When can I use a forward declaration?
(13 answers)
Closed 8 years ago.
I'm analying an Operating Systems project for school and came across this header file:
//kernelev.h
#ifndef _KERNELEV_H
#define _EVENT_H_
typedef unsigned char IVTNo;
class Thread;
class PCB;
class KernelSem;
class KernelEv {
public:
KernelEv (IVTNo ivtNo);
~KernelEv();
int wait(int MaxTimeToWait);
void signal();
[...]
Now, when writing the complete definitions of these methods (KernelEv, ~KernelEv, wait and signal), they used the attributes of the classes Thread, PCB and KernelSem. What would be the difference between generally introducing for instance #include Thread.h; #include KernelSem.h; and just declaring the classes like this: class Thread; Are there differences in data access rights? Or it's somehow completely different?
Thanks for your help, I hope my question is clear enough.
First, note that if you only introduce the classes, you won't be able to use the methods;
class Thread;
Thread x; // compile error: size of x unknown
Thread* x; // this is ok
// set x to some valid thread, maybe as a parameter
x->signal(); // compile error
But it makes no difference whether your declarations are in a header or included in your file. That is, you could replace the include line with a copy of the header and everything would work perfectly fine (every line in the above example would be valid). There are many reasons not to, however. Ease of maintenance would be the top issue, along with readability and modularity. It would also be less applicable to compiler caching (so would generally take longer to compile)
If you only have a declaration class A; and not the full class definition, then the type is said to be incomplete. This can be used in limited ways; you can do things that only need knowledge that the class exists, for example:
Declare a pointer or reference to it;
Declare a function using it as a parameter or return type;
Declare (but not define) an external variable of that type.
You can't do anything that requires knowledge of the class members, size, or other details given by the definition, for example:
Define a variable of that type;
Define a function using it;
Access any members or nested declarations;
Inherit from it;
Apply sizeof to it.
If I am not wrong, you are asking about Declaration vs Definition. They are different but related.
I forward you to this two posts googled as "c++ declaration definition", since they will explain much better than me :)
What is the difference between a definition and a declaration?
http://www.cprogramming.com/declare_vs_define.html
Just comment that your class A; is a declaration, while the includes Thread.h surely will have plenty definitions inside (and maybe declarations too).
--------------------- edit:
About forward declarations as commented by #Dave bellow:
class B ; // needed declaration
class A {
B field ;
} ;
class B {
A field ;
};
I am working on a codebase that is not my own, that has the following layout:
object.h:
// Objects are defined
// #include "tickets.h" (functions to access the objects)
// An access-handler object is defined
I want to introduce a class that knows about the objects, can be accessed from functions in tickets.h, but can also use the access-handler object. The functions are separate, i.e. class functions that are called in tickets.h do not use the access-handler (I wouldn't know where to start if that weren't the case).
Therefore my class needs to be defined before tickets.h, but some of its functions need to be defined after the access-handler. Is there a way to do this without splitting it up into two header files something like the following:
// Objects are defined
// -- include declaration of class, and definition of functions that tickets.h needs
// #include "tickets.h"
// An access-handler object is defined
// -- include functions of class that need the access-handler
This seems very messy splitting things up like this into two separate files, I was hoping to keep everything contained.
Thanks for any help, I clearly only have a very rudimentary understanding of declarations/definitions in c++.
EDIT: If I use forward declaration and include it before tickets.h (with the class declared in mynewclass.h and functions defined in mynewclass.cc) will mynewclass.cc be able to use objects declared after the inclusion of mynewclass.h? Namely the access-handler object.
EDIT2: Something like this:
object.h:
class obj { // definition }
#include "tickets.h"
class obj_handler {
public void handle { // code }
}
tickets.h:
void do_something(obj o){
communicator.foo();
}
My object (communicator):
class communicator {
public:
void foo() { // code }
void bar() { // use handle() from obj_handler }
}
As you can see, my communicator needs to be used in tickets.h, but can't be defined until after obj_handler. So where should I include it?
If I correctly understand your question - you can use forward declaration to solve this problem. This will allow you to declare some class before defining it's methods. For example:
// this is forward declaration of class A, no definition provided
class A;
class B
{
// uses A
A * a_;
};
// class A definition
class A
{
// may use class B now
B * b_;
};
I'm not quite sure whether I understand this right and don't have enough reputation here yet to make this a comment, so let me try to answer your question this way, please feel free to follow up if I'm guessing wrong:
I believe what you are referring to is an entire class definition, i.e., one including all function definitions within the class declaration. Other than that, it is not very common to see object definitions followed by preprocessor directives. What is typical though is a forward declaration of functions and a class prototype.
So, for example, you could declare in some header.h:
class C
{
public:
void method1(void);
int method2(void);
};
And in some implementation.cpp the definition of the functions like:
void C::method1(void) { /*...*/ }
In the other file preceded in the inclusion chain by your access-handler you then define the other function:
int C::method2(void) { /*...*/ }
What do you mean by access-handler, by the way?
Oh, and your linker likely will yell somewhat at you if you do function definition in a header file.
With regard to your addenda: everywhere you put a forward declaration, loosely speaking, the compiler will insert a copy of the declaration in question, consider it a soft link in the context of file systems. There are negative implications associated with it, like increased duration and the memory load of compilation if you have many forward declarations of the function signature or class. It's impossible to tell whether this will word in your particular situation since only you know the actual code in question. But most likely it would work.
Take a look at these pages:
http://en.wikipedia.org/wiki/Forward_declaration
When can I use a forward declaration?