How to Implement Interfaces in C++

Interface/Implementation in ANSI C

It sounds like you are already doing the right thing: good C code also organizes interfaces in .h-files and implementations in .c-files.

Example a.h file:

void f(int a);

Example a.c file:

#include "a.h"
static void helper(void) {...}
void f(int a) {... use helper()...}

Example main.c file:

#include "a.h"
int main(void) { f(123); return 0; }

You get modularity because helper-functions are not declared in headers so other modules dont know about them (you can declare them at the top of the .c file if you want). Having this modularity reduces the number of recompiles needed and reduces how much has to be recompiled. (The linking has to be done every time though). Note that if you are not declaring helper-functions in the header then you are already pretty safe, however having the static in front of them also hides them from other modules during linking so there is no conflict if multiple modules use the same helper-function-names.

If you are working with only primitive types then that is all you need to know and you can stop reading here. However if your module needs to work with a struct then it gets just a little more complicated.

Problematic example header b.h:

typedef struct Obj {
    int data;
}*Obj;
Obj make(void);
void work(Obj o);

Your module wants to pass objects in and out. The problem here is, that internals are leaked to other modules that depend on this header. If the representation is changed to float data then all using modules have to recompile. One way to fix this is to only use void*. That is how many programs do it. However that is cumbersome because every function getting the void* as argument has to cast it to Obj. Another way is to do this:

Header c.h:

typedef struct Obj*Obj;
Obj make(void);
void work(Obj);

Implementation c.c:

#include "c.h"
typedef struct Obj {
    int data;
}*Obj;

The reason why this works is, that Obj is a pointer (as opposed to a struct by value/copy). Other modules that depend on this module only need to know that a pointer is being passed in and out, not what it points to.

how to implement Interfaces in C++?

C++ has no built-in concepts of interfaces. You can implement it using abstract classes which contains only pure virtual functions. Since it allows multiple inheritance, you can inherit this class to create another class which will then contain this interface (I mean, object interface :) ) in it.

An example would be something like this -

class Interface
{
public:
    Interface(){}
    virtual ~Interface(){}
    virtual void method1() = 0;    // "= 0" part makes this method pure virtual, and
                                   // also makes this class abstract.
    virtual void method2() = 0;
};

class Concrete : public Interface
{
private:
    int myMember;

public:
    Concrete(){}
    ~Concrete(){}
    void method1();
    void method2();
};

// Provide implementation for the first method
void Concrete::method1()
{
    // Your implementation
}

// Provide implementation for the second method
void Concrete::method2()
{
    // Your implementation
}

int main(void)
{
    Interface *f = new Concrete();

    f->method1();
    f->method2();

    delete f;

    return 0;
}

How do you declare an interface in C++?

To expand on the answer by bradtgmurray, you may want to make one exception to the pure virtual method list of your interface by adding a virtual destructor. This allows you to pass pointer ownership to another party without exposing the concrete derived class. The destructor doesn't have to do anything, because the interface doesn't have any concrete members. It might seem contradictory to define a function as both virtual and inline, but trust me - it isn't.

class IDemo
{
    public:
        virtual ~IDemo() {}
        virtual void OverrideMe() = 0;
};

class Parent
{
    public:
        virtual ~Parent();
};

class Child : public Parent, public IDemo
{
    public:
        virtual void OverrideMe()
        {
            //do stuff
        }
};

You don't have to include a body for the virtual destructor - it turns out some compilers have trouble optimizing an empty destructor and you're better off using the default.

How can I simulate interfaces in C++?

Since C++ has multiple inheritance unlike C# and Java, yes you can make a series of abstract classes.

As for convention, it is up to you; however, I like to precede the class names with an I.

class IStringNotifier
{
public:
  virtual void sendMessage(std::string &strMessage) = 0;
  virtual ~IStringNotifier() { }
};

The performance is nothing to worry about in terms of comparison between C# and Java. Basically you will just have the overhead of having a lookup table for your functions or a vtable just like any sort of inheritance with virtual methods would have given.

Implement an interface in C++?

C++ does not have interfaces. You create an abstract class with a pure virtual method.

class MonoBehaviour 
{
    virtual void Update() = 0;
};

class your_class : MonoBehaviour 
{
    void Update() 
    { 
        // implement here. 
    }
};

How to implement a property in an interface

In the interface, you specify the property:

public interface IResourcePolicy
{
   string Version { get; set; }
}

In the implementing class, you need to implement it:

public class ResourcePolicy : IResourcePolicy
{
   public string Version { get; set; }
}

This looks similar, but it is something completely different. In the interface, there is no code. You just specify that there is a property with a getter and a setter, whatever they will do.

In the class, you actually implement them. The shortest way to do this is using this { get; set; } syntax. The compiler will create a field and generate the getter and setter implementation for it.

How do you auto-implement an interface in C#?

That hasn't changed. All you need to do is hover over the interface name in the class, and the blue underline bar should show up, giving you options to implement the interface. If the blue underline is not appearing, check to make sure the interface is accessible with what assemblies and namespaces you are referencing.

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