Struct vs Class, take II

[nuzzle]

Besides the visibility of members,

Structs are value types and classes are reference types.

As you know, value types are allocated on the stack or inline as a part of another object while classes are allocated on the heap.

Struct is often a small object that needs to be treated like a primitive type while classes are normally used for implementation requiring more processing power. In other words, classes would normally be used to implement a piece of business logic, rather than to support data related or primitive-type objects.

I hope this clears it up a bit.

Although your description roughly follows common usage I think it's worth mentioning that the roles you assign to the class and the struct are a historical convention and nothing that's stipulated by the language.

In fact the struct and the class are conceptually equivalent in C++. Nothing prevents anyone from using them in the exact opposite manner from what you state.

I mention this because making struct and class to be equals was a very important design decision as explained by Stroustrup in The Design and Evolution of C++ on page 76.

[ninja9578]

There is one more major difference between structs and classes. structs can exist inside of an extern "C" block, classes may not.

Most people use structs to just store data instead of classes, simply because it's faster to write a struct that a class. It doesn't take long to type "public:" but why bother if you don't have to.

[laserlight]

structs can exist inside of an extern "C" block, classes may not.

That is not true.

[Paul McKenzie]

There is one more major difference between structs and classes. structs can exist inside of an extern "C" block, classes may not.

extern "C"
{
   class x
   {
     int y;
   };
}

Thank you for testing your code with Comeau C/C++!
Tell others about http://www.comeaucomputing.com/tryitout ! 

Your Comeau C/C++ test results are as follows: 


Comeau C/C++ 4.3.10.1 (Oct  6 2008 11:28:09) for ONLINE_EVALUATION_BETA2
Copyright 1988-2008 Comeau Computing.  All rights reserved.
MODE:strict errors C++ C++0x_extensions


In strict mode, with -tused, Compile succeeded (but remember, the Comeau online compiler does not link). 
Compiled with C++0x extensions enabled.

Regards,

Paul McKenzie

[Chris_F]

Besides the visibility of members,

Structs are value types and classes are reference types.

As you know, value types are allocated on the stack or inline as a part of another object while classes are allocated on the heap.

Struct is often a small object that needs to be treated like a primitive type while classes are normally used for implementation requiring more processing power. In other words, classes would normally be used to implement a piece of business logic, rather than to support data related or primitive-type objects.

I hope this clears it up a bit.

This is not true. None of it. amira7 is clearly a C# programmer or has somehow gotten confused, reading C# literature.

[ninja9578]

Okay, it shouldn't be used in a extern "C" block. Because if you do, it completely breaks all C compatibility.

[nuzzle]

This is not true. None of it. amira7 is clearly a C# programmer or has somehow gotten confused, reading C# literature.

It's true in the sense that it describes common usage fairly well. But, as I noted in my first post, this usage is not stipulated by the C++ language.

In the book I mentioned (The Design and Evolution of C++, p. 76), Stroustrup explains why he did not keep the C-struct in C++. The reason is that he didn't want one simple and one advanced version of the same concept. He dropped the C-struct and made struct and class equals in order to keep the C++ language together.

Anyone is free to assign different roles to the struct and the class but that's just a question of convention and personal taste. In C++ "a struct is a class" and according to Stroustrup this notion has "stopped C++ from drifting into becoming a much higher-level language with a disconnected low-level subset".

So Stroustrup didn't want C++ to become two languages in one and that's why struct and class are now equals. But that doesn't prevent anyone, including me, from treating the struct as if it was the little brother of the class. Even Stroustrup himself subscribes to this usage. (The C++ Programming Language, 3'rd ed., p. 234).

[Speedo]

Okay, it shouldn't be used in a extern "C" block. Because if you do, it completely breaks all C compatibility.

Should only be a problem if you use a ctor/dtor, operator overloading, virtual functions, etc. All of which can be used with either struct or class.