Struct into a C++ class

[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:

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

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

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[tbscope]

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.

[squidge]

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.

[franco.amato]

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

[Zlatomir]

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

Yes:

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

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

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This is the only difference between class and struct

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.

[franco.amato]

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

[wysota]

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):

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

// class, private by default:
class SomeClass; // forward declare in header file

// struct, public by default:
struct SomeClass {
 //...
}; // define in implementation file

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[franco.amato]

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):

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

// class, private by default:
class SomeClass; // forward declare in header file

// struct, public by default:
struct SomeClass {
 //...
}; // define in implementation file

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

[wysota]

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.

[franco.amato]

Yes:

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

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

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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

[Zlatomir]

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:

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

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

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[aamer4yu]

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 )

[franco.amato]

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:

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

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

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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

[tbscope]

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.

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;
}

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;
}

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[wysota]

// 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; }

// 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; }

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[franco.amato]

// 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; }

// 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; }

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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

[wysota]

We have shown you two different approaches to the same problem. In tbscope's code MyStruct is just a public structure, in my code it is defined in the scope of the MyClass class, so its full name is MyClass::MyStruct. My code is better encapsulated, his is more reusable.

[franco.amato]

We have shown you two different approaches to the same problem. In tbscope's code MyStruct is just a public structure, in my code it is defined in the scope of the MyClass class, so its full name is MyClass::MyStruct. My code is better encapsulated, his is more reusable.

Ok thank you.

[zack]

there are of course use cases for a struct in a class.
When the struct is in the private section you can bundle some internal data.
And the struct can't be created from outside.

e.g. when I have to store more then two variables in one vector I use a struct for it, thats better then a boost::tuple, because when you acces the data you can better read your code.

[squidge]

You could put a class in the private section of a class and bundle your internal data there too, and it couldn't be created from outside.

The only real difference betwen struct and class is that a struct is by default public, whereas a class is by default private.