Struct into a C++ class

[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 )