Struct into a C++ class

Good morning,
I'm not a very C++ expert.
What I need is a class containing a struct. I tried to write it but I get linker errors.
Can I have a little example of a Class containing a struct?

Best Regards,
Franco

Code:

---

class MyClass
{
public:
    MyClass();
    ~MyClass();
 
    struct MyStruct
    {
        MyStruct() : a(0), b(0) {}
        int a;
        int b;
    };
 
    MyStruct *getMyStruct();
 
private:
    MyStruct *mystruct;
};
 
MyClass::MyClass()
{
    mystruct = new MyStruct;
}
 
MyClass::~MyClass()
{
    delete mystruct;
}
 
MyStruct *MyClass::getMyStruct()
{
    return mystruct;
}

---

Not checked for syntax errors.

@tbscope: should that code compile?
!!!i thought not!!!
EDITED:
I was wrong
It compiles... I didn't knew that it is possible to have a class declaration inside another class

As far as i know this is not possible, you should declare the struct separately, and use the pointer (or instance) in the class:

Code:

---

struct MyStruct
{
//...
};
 
class MyClass
{
public:
//...
private:
    MyStruct *mystruct; //or MyStruct myStructInstance;
};

---

A better design is to code the struct in it's own .h, .cpp files and include MyStruct.h in the MyClass.cpp file (of course with the use of a pointer and forward declaration in MyClass.h)

And just for clarification - a "struct" in C++ is also a class so instances of it can be treated as any other C++/Qt object (i.e. like QString or QObejct).

The code needs one correction, you have to return properly prefixed type:

Code:

---

MyClass::MyStruct *MyClass::getMyStruct()
{
  return mystruct;
}

---

Oops, thanks for the correction.

I never had a use for a class in a class. I don't think it makes sense to do this.

How can I use the famous typedef?

As far I remember, the only difference between a struct and a class in C++ is that a struct defaults to public, where a class defaults to non-public (not sure if it's protected or private).

There's no need to typedef a struct in C++. You can use it's name directly, without the "struct" keyword.

tbscope: It makes sense when you want a class for encapsulation purposes, but don't want it visible from outside of the main class.

So I create an instance of the struct as it was a class? Should I use the new operator? ( sorry for the foo questions )

Quote:

---

Originally Posted by franco.amato

So I create an instance of the struct as it was a class?

---

Yes:

Code:

---

class MyClass {
    int member; //by "default" member is  private
};
struct MyStruct{
    int member; //by "default" member is public
};

---

This is the only difference between class and struct

Quote:

---

Originally Posted by franco.amato

Should I use the new operator? ( sorry for the foo questions )

---

Only if you want to allocate on the heap (just like you do when you are creating your own types by using the "class" keyword.

Thank you very much for the explanation.
A last question: how the compiler know if is a C++ or C struct?

Quote:

---

Originally Posted by Zlatomir

Yes:

Code:

---

class MyClass {
    int member; //by "default" member is  private
};
struct MyStruct{
    int member; //by "default" member is public
};

---

This is the only difference between class and struct

Only if you want to allocate on the heap (just like you do when you are creating your own types by using the "class" keyword.

---

In my old 'C language' course I remember that when I worked with c struct I had to create it with a malloc, so in C++ I imagine I have to do the same with the new, but seems I'm wrong

You don't have to use new (you use it only if you really need to allocate on the heap)
You can create an instance of a struct just like the one of a class:

Code:

---

class MyClass {
    int member; //by "default" member is  private
};
struct MyStruct{
    int member; //by "default" member is public
};
MyClass classInstance;    //both are compiling ok
MyStruct structInstance;
 
classInstance.member = 0;  //error because class "member" is private 
structInstance.member = 10; //ok because struct "member" is public

---

Quote:

---

Originally Posted by franco.amato

A last question: how the compiler know if is a C++ or C struct?

---

If you use a C++ compiler then every struct is a class for it. In other words in C++ there is no difference between a class and a struct, you can even do this (although msvc issues a warning for it):

Code:

---

// class, private by default:
class SomeClass; // forward declare in header file
 
// struct, public by default:
struct SomeClass {
 //...
}; // define in implementation file

---

Quote:

---

A last question: how the compiler know if is a C++ or C struct?

---

From the file extension I guess.. try saving the file as .c and see if it compiles..
Also there might be some option to the compiler to tell if its C or C++.. but am not sure of such setting.

Another difference of struct in C and C++ is that in C++ its same as class and you can even inherit it. While in C you cannot have inheritance as far as I know. ( One can easily verify )

Quote:

---

Originally Posted by wysota

If you use a C++ compiler then every struct is a class for it. In other words in C++ there is no difference between a class and a struct, you can even do this (although msvc issues a warning for it):

Code:

---

// class, private by default:
class SomeClass; // forward declare in header file
 
// struct, public by default:
struct SomeClass {
 //...
}; // define in implementation file

---

---

So my question comes naturally:
Why does exist in C++ the struct if it has no difference with a class?

Quote:

---

Originally Posted by franco.amato

Why does exist in C++ the struct if it has no difference with a class?

---

So that you can take C code and compile it with a C++ compiler retaining the same functionality as if you compiled it with a C compiler.

Quote:

---

Originally Posted by wysota

And just for clarification - a "struct" in C++ is also a class so instances of it can be treated as any other C++/Qt object (i.e. like QString or QObejct).

The code needs one correction, you have to return properly prefixed type:

Code:

---

MyClass::MyStruct *MyClass::getMyStruct()
{
  return mystruct;
}

---

---

Wysota but if I declare the struct in a .h file outside the class, how I have to change the code of the getMyStruct?

Regards,
Franco

Quote:

---

Originally Posted by franco.amato

Wysota but if I declare the struct in a .h file outside the class, how I have to change the code of the getMyStruct?

---

Try to find some good books about c++ and object oriented programming.

Code:

---

struct MyStruct
{
    MyStruct() : a(0), b(0) {}
    int a;
    int b;
};
 
class MyClass
{
public:
    MyClass();
    ~MyClass();
 
    MyStruct *getMyStruct();
 
private:
    MyStruct *mystruct;
};
 
MyClass::MyClass()
{
    mystruct = new MyStruct;
}
 
MyClass::~MyClass()
{
    delete mystruct;
}
 
MyStruct *MyClass::getMyStruct()
{
    return mystruct;
}

---

Code:

---

// h
class MyClass {
public:
  struct MyStruct {
     int a, b;
     MyStruct() : a(0), b(0){}
  };
 
  MyStruct* getPtr() const;
  MyStruct getObj() const;
  MyClass();
  ~MyClass();
private:
  MyStruct *structPtr;
  MyStruct structObj;
};
 
// cpp
 
MyClass::MyClass() { structPtr = new MyStruct; }
MyClass::~MyClass() { delete structPtr; }
 
MyClass::MyStruct* MyClass::getPtr() const { return structPtr; }
MyClass::MyStruct MyClass::getObj() const { return structObj; }

---

Quote:

---

Originally Posted by wysota

Code:

---

// h
class MyClass {
public:
  struct MyStruct {
     int a, b;
     MyStruct() : a(0), b(0){}
  };
 
  MyStruct* getPtr() const;
  MyStruct getObj() const;
  MyClass();
  ~MyClass();
private:
  MyStruct *structPtr;
  MyStruct structObj;
};
 
// cpp
 
MyClass::MyClass() { structPtr = new MyStruct; }
MyClass::~MyClass() { delete structPtr; }
 
MyClass::MyStruct* MyClass::getPtr() const { return structPtr; }
MyClass::MyStruct MyClass::getObj() const { return structObj; }

---

---

Well I still don't understand as the code of wysota is different from the code of tbscope.
Meanwhile thankx to all for the precious help.
Best Regards,
Franco