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

Alan Hinchcliffe
1#
Alan Hinchcliffe Published in 2008-09-10 16:29:54Z

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

HolyBlackCat
2#
HolyBlackCat Reply to 2017-02-07 19:00:59Z

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.

Community
3#
Community Reply to 2017-05-23 11:47:30Z

As everyone else notes there are really only two actual language differences:

  • struct defaults to public access and class defaults to private access.
  • When inheriting, struct defaults to public inheritance and class defaults to private inheritance. (Ironically, as with so many things in C++, the default is backwards: public inheritance is by far the more common choice, but people rarely declare structs just to save on typing the "public" keyword.

But the real difference in practice is between a class/struct that declares a constructor/destructor and one that doesn't. There are certain guarantees to a "plain-old-data" POD type, that no longer apply once you take over the class's construction. To keep this distinction clear, many people deliberately only use structs for POD types, and, if they are going to add any methods at all, use classes. The difference between the two fragments below is otherwise meaningless:

class X
{
  public:

  // ...
};

struct X
{
  // ...
};

(Incidentally, here's a thread with some good explanations about what "POD type" actually means: What are POD types in C++?)

Community
4#
Community Reply to 2017-05-23 12:02:49Z

There are lots of misconceptions in the existing answers.

Both class and struct declare a class.

Yes, you may have to rearrange your access modifying keywords inside the class definition, depending on which keyword you used to declare the class.

But, beyond syntax, the only reason to choose one over the other is convention/style/preference.

Some people like to stick with the struct keyword for classes without member functions, because the resulting definition "looks like" a simple structure from C.

Similarly, some people like to use the class keyword for classes with member functions and private data, because it says "class" on it and therefore looks like examples from their favourite book on object-oriented programming.

The reality is that this completely up to you and your team, and it'll make literally no difference whatsoever to your program.

The following two classes are absolutely equivalent in every way except their name:

struct Foo
{
   int x;
};

class Bar
{
public:
   int x;
};

You can even switch keywords when redeclaring:

class Foo;
struct Bar;

(although some compilers will emit a warning when you do this, on the assumption that you probably didn't intend to do something so confusing and that you should therefore be prompted to double-check your code.)

and the following expressions both evaluate to true:

std::is_class<Foo>::value
std::is_class<Bar>::value
Ferruccio
5#
Ferruccio Reply to 2008-09-10 19:18:58Z

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()); 
Sumurai8
6#
Sumurai8 Reply to 2015-09-10 09:19:07Z

From the C++ FAQ Lite:

The members and base classes of a struct are public by default, while in class, they default to private. Note: you should make your base classes explicitly public, private, or protected, rather than relying on the defaults.

struct and class are otherwise functionally equivalent.

OK, enough of that squeaky clean techno talk. Emotionally, most developers make a strong distinction between a class and a struct. A struct simply feels like an open pile of bits with very little in the way of encapsulation or functionality. A class feels like a living and responsible member of society with intelligent services, a strong encapsulation barrier, and a well defined interface. Since that's the connotation most people already have, you should probably use the struct keyword if you have a class that has very few methods and has public data (such things do exist in well designed systems!), but otherwise you should probably use the class keyword.

start2learn
7#
start2learn Reply to 2016-12-12 11:15:02Z

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.

Adisak
8#
Adisak Reply to 2011-09-20 21:53:42Z

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.

Deduplicator
9#
Deduplicator Reply to 2016-01-31 11:51:44Z

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...

ogoid
10#
ogoid Reply to 2009-07-28 16:13:06Z

As every one says, the only real difference is the default access. But I particularly use struct when I don't want any sort of encapsulation with a simple data class, even if I implement some helper methods. For instance, when I need something like this:

struct myvec {
    int x;
    int y;
    int z;

    int length() {return x+y+z;}
};
Andy
11#
Andy Reply to 2008-09-10 16:36:00Z

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.

Alan Hinchcliffe
12#
Alan Hinchcliffe Reply to 2008-09-10 16:50:18Z

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.

enigmatic
13#
enigmatic Reply to 2008-09-10 16:36:17Z

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.

Javier
14#
Javier Reply to 2009-07-28 03:49:00Z

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.

anio
15#
anio Reply to 2009-07-28 03:51:02Z

Structs by default have public access and classes by default have private access.

Personally I use structs for Data Transfer Objects or as Value Objects. When used as such I declare all members as const to prevent modification by other code.

AraK
16#
AraK Reply to 2009-07-28 20:40:22Z

When would you choose to use struct and when to use class in C++?

I use struct when I define functors and POD. Otherwise I use class.

// '()' is public by default!
struct mycompare : public std::binary_function<int, int, bool>
{
    bool operator()(int first, int second)
    { return first < second; }
};

class mycompare : public std::binary_function<int, int, bool>
{
public:
    bool operator()(int first, int second)
    { return first < second; }
};
sehe
17#
sehe Reply to 2017-11-12 00:09:18Z

Class.

Class members are private by default.

class test_one {
    int main_one();
};

Is equivalent to

class test_one {
  private:
    int main_one();
};

So if you try

int two = one.main_one();

We will get an error: main_one is private because its not accessible. We can solve it by initializing it by specifying its a public ie

class test_one {
  public:
    int main_one();
};

Struct.

A struct is a class where members are public by default.

struct test_one {
    int main_one;
};

Means main_one is private ie

class test_one {
  public:
    int main_one;
};

I use structs for data structures where the members can take any value, it's easier that way.

harik
18#
harik Reply to 2009-07-28 04:29:10Z

I use struct only when I need to hold some data without any member functions associated to it (to operate on the member data) and to access the data variables directly.

Eg: Reading/Writing data from files and socket streams etc. Passing function arguments in a structure where the function arguments are too many and function syntax looks too lengthy.

Technically there is no big difference between class and struture except default accessibility. More over it depends on programming style how you use it.

Maciek
19#
Maciek Reply to 2009-07-28 04:42:17Z

Technically both are the same in C++ - for instance it's possible for a struct to have overloaded operators etc.

However :

I use structs when I wish to pass information of multiple types simultaneously I use classes when the I'm dealing with a "functional" object.

Hope it helps.

#include <string>
#include <map>
using namespace std;

struct student
{
    int age;
    string name;
    map<string, int> grades
};

class ClassRoom
{
    typedef map<string, student> student_map;
  public :
    student getStudentByName(string name) const 
    { student_map::const_iterator m_it = students.find(name); return m_it->second; }
  private :
    student_map students;
};

For instance, I'm returning a struct student in the get...() methods over here - enjoy.

ivan.ukr
20#
ivan.ukr Reply to 2016-09-15 09:30:25Z

I use structs when I need to create POD type or functor.

Vorac
21#
Vorac Reply to 2017-12-10 09:49:59Z

An advantage of struct over class is that it save one line of code, if adhering to "first public members, then private". In this light, I find the keyword class useless.

Here is another reason for using only struct and never class. Some code style guidelines for C++ suggest using small letters for function macros, the rationale being that when the macro is converted to an inline function, the name shouldn't need to be changed. Same here. You have your nice C-style struct and one day, you find out you need to add a constructor, or some convenience method. Do you change it to a class? Everywhere?

Distinguishing between structs and classes is just too much hassle, getting into the way of doing what we should be doing - programming. Like so many of C++'s problems, it arises out of the strong desire for backwards compatability.

GaiusSensei
22#
GaiusSensei Reply to 2009-07-28 04:07:59Z

I thought that Structs was intended as a Data Structure (like a multi-data type array of information) and classes was inteded for Code Packaging (like collections of subroutines & functions)..

:(

Baltimark
23#
Baltimark Reply to 2008-09-10 18:02:25Z

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
 };
Eitan Myron
24#
Eitan Myron Reply to 2014-02-21 18:58:36Z

Access functions aren't the only difference between structs and classes. A struct is better for POD (Plain Old Data) types, since it is easier to manage for the compiler and the programmer. For beginners who don't use object-oriented programming, using a struct is fine.

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