When should you use a class vs a struct in C++?

0 votes
asked Sep 10, 2008 by alan-hinchcliffe

In what scenarios is it better to use a struct vs a class in C++?

21 Answers

0 votes
answered Jan 10, 2008 by andy

For C++, there really isn't much of a difference between structs and classes. The main functional difference is that members of a struct are public by default, while they are private by default in classes. Otherwise, as far as the language is concerned, they are equivalent.

That said, I tend to use structs in C++ like I do in C#, similar to what Brian has said. Structs are simple data containers, while classes are used for objects that need to act on the data in addition to just holding on to it.

0 votes
answered Jan 10, 2008 by enigmatic

They are pretty much the same thing. Thanks to the magic of C++, a struct can hold functions, use inheritance, created using "new" and so on just like a class

The only functional difference is that a class begins with private access rights, while a struct begins with public. This is the maintain backwards compatibility with C.

In practice, I've always used structs as data holders and classes as objects.

0 votes
answered Jan 10, 2008 by mbyrne215

One place where a struct has been helpful for me is when I have a system that's receiving fixed format messages (over say, a serial port) from another system. You can cast the stream of bytes into a struct that defines your fields, and then easily access the fields.

typedef struct
{
    int messageId;
    int messageCounter;
    int messageData;
} tMessageType;

void processMessage(unsigned char *rawMessage)
{
    tMessageType *messageFields = (tMessageType *)rawMessage;
    printf("MessageId is %d\n", messageFields->messageId);
}

Obviously, this is the same thing you would do in C, but I find that the overhead of having to decode the message into a class is usually not worth it.

0 votes
answered Jan 10, 2008 by alan-hinchcliffe

To answer my own question (shamelessly), As already mentioned, access privileges are the only difference between them in C++.

I tend to use a struct for data-storage only. I'll allow it to get a few helper functions if it makes working with the data easier. However as soon as the data requires flow control (i.e. getters/setters that maintain or protect an internal state) or starts acquring any major functionality (basically more object-like), it will get 'upgraded' to a class to better communicate intent.

0 votes
answered Jan 10, 2008 by argv0

Structs (PODs, more generally) are handy when you're providing a C-compatible interface with a C++ implementation, since they're portable across language borders and linker formats.

If that's not a concern to you, then I suppose the use of the "struct" instead of "class" is a good communicator of intent (as @ZeroSignal said above). Structs also have more predictable copying semantics, so they're useful for data you intend to write to external media or send across the wire.

Structs are also handy for various metaprogramming tasks, like traits templates that just expose a bunch of dependent typedefs:

template <typename T> struct type_traits {
  typedef T type;
  typedef T::iterator_type iterator_type;
  ...
};

...But that's really just taking advantage of struct's default protection level being public...

0 votes
answered Jan 10, 2008 by baltimark

I never use "struct" in C++.

I can't ever imagine a scenario where you would use a struct when you want private members, unless you're willfully trying to be confusing.

It seems that using structs is more of a syntactic indication of how the data will be used, but I'd rather just make a class and try to make that explicit in the name of the class, or through comments.

E.g.

class PublicInputData {
    //data members
 };
0 votes
answered Sep 10, 2008 by commodore-jaeger

Differences between a class and a struct in C++ are that structs have default public members and bases and classes have default private members and bases. Both classes and structs can have a mixture of public and private members, can use inheritance and can have member functions.

I would recommend using structs as plain-old-data structures without any class-like features, and using classes as aggregate data structures with private data and member functions.

0 votes
answered Sep 10, 2008 by ferruccio

The only time I use a struct instead of a class is when declaring a functor right before using it in a function call and want to minimize syntax for the sake of clarity. e.g.:

struct Compare { bool operator() { ... } };
std::sort(collection.begin(), collection.end(), Compare()); 
0 votes
answered Jan 27, 2009 by adisak

You can use "struct" in C++ if you are writing a library whose internals are C++ but the API can be called by either C or C++ code. You simply make a single header that contains structs and global API functions that you expose to both C and C++ code as this:

// C access Header to a C++ library
#ifdef __cpp
extern "C" {
#endif

// Put your C struct's here
struct foo
{
    ...
};
// NOTE: the typedef is used because C does not automatically generate
// a typedef with the same name as a struct like C++.
typedef struct foo foo;

// Put your C API functions here
void bar(foo *fun);

#ifdef __cpp
}
#endif

Then you can write a function bar() in a C++ file using C++ code and make it callable from C and the two worlds can share data through the declared struct's. There are other caveats of course when mixing C and C++ but this is a simplified example.

0 votes
answered Jan 28, 2009 by javier

they're the same thing with different defaults (private by default for class, and public by default for struct), so in theory they're totally interchangeable.

so, if I just want to package some info to move around, I use a struct, even if i put a few methods there (but not many). If it's a mostly-opaque thing, where the main use would be via methods, and not directly to the data members, i use a full class.

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