'What are the differences between struct and class in C++?

This question was already asked in the context of C#/.Net.

Now I'd like to learn the differences between a struct and a class in C++. Please discuss the technical differences as well as reasons for choosing one or the other in OO design.

I'll start with an obvious difference:

  • If you don't specify public: or private:, members of a struct are public by default; members of a class are private by default.

I'm sure there are other differences to be found in the obscure corners of the C++ specification.

c++oopclassstructc++-faq


Solution 1:[1]

Quoting The C++ FAQ,

[7.8] What's the difference between the keywords struct and class?

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.

Solution 2:[2]

It's worth remembering C++'s origins in, and compatibility with, C.

C has structs, it has no concept of encapsulation, so everything is public.

Being public by default is generally considered a bad idea when taking an object-oriented approach, so in making a form of C that is natively conducive to OOP (you can do OO in C, but it won't help you) which was the idea in C++ (originally "C With Classes"), it makes sense to make members private by default.

On the other hand, if Stroustrup had changed the semantics of struct so that its members were private by default, it would have broken compatibility (it is no longer as often true as the standards diverged, but all valid C programs were also valid C++ programs, which had a big effect on giving C++ a foothold).

So a new keyword, class was introduced to be exactly like a struct, but private by default.

If C++ had come from scratch, with no history, then it would probably have only one such keyword. It also probably wouldn't have made the impact it made.

In general, people will tend to use struct when they are doing something like how structs are used in C; public members, no constructor (as long as it isn't in a union, you can have constructors in structs, just like with classes, but people tend not to), no virtual methods, etc. Since languages are as much to communicate with people reading the code as to instruct machines (or else we'd stick with assembly and raw VM opcodes) it's a good idea to stick with that.

Solution 3:[3]

Class' members are private by default. Struct's members are public by default. Besides that there are no other differences. Also see this question.

Solution 4:[4]

According to Stroustrup in the C++ Programming Language:

Which style you use depends on circumstances and taste. I usually prefer to use struct for classes that have all data public. I think of such classes as "not quite proper types, just data structures."

Functionally, there is no difference other than the public / private

Solution 5:[5]

  1. Members of a class are private by default and members of struct are public by default.

For example program 1 fails in compilation and program 2 works fine.

// Program 1
#include <stdio.h>
 
class Test {
    int x; // x is private
};

int main()
{
  Test t;
  t.x = 20; // compiler error because x is private
  getchar();
  return 0;
}

// Program 2
#include <stdio.h>
 
struct Test {
    int x; // x is public
};

int main()
{
  Test t;
  t.x = 20; // works fine because x is public
  getchar();
  return 0;
}
  1. When deriving a struct from a class/struct, default access-specifier for a base class/struct is public. And when deriving a class, default access specifier is private.

For example program 3 fails in compilation and program 4 works fine.

// Program 3
#include <stdio.h>
 
class Base {
public:
    int x;
};
 
class Derived : Base { }; // is equivalent to class Derived : private Base {}
 
int main()
{
  Derived d;
  d.x = 20; // compiler error because inheritance is private
  getchar();
  return 0;
}

// Program 4
#include <stdio.h>
 
class Base {
public:
    int x;
};
 
struct Derived : Base { }; // is equivalent to struct Derived : public Base {}
 
int main()
{
  Derived d;
  d.x = 20; // works fine because inheritance is public
  getchar();
  return 0;
}

Solution 6:[6]

STRUCT is a type of Abstract Data Type that divides up a given chunk of memory according to the structure specification. Structs are particularly useful in file serialization/deserialization as the structure can often be written to the file verbatim. (i.e. Obtain a pointer to the struct, use the SIZE macro to compute the number of bytes to copy, then move the data in or out of the struct.)

Classes are a different type of abstract data type that attempt to ensure information hiding. Internally, there can be a variety of machinations, methods, temp variables, state variables. etc. that are all used to present a consistent API to any code which wishes to use the class.

In effect, structs are about data, classes are about code.

However, you do need to understand that these are merely abstractions. It's perfectly possible to create structs that look a lot like classes and classes that look a lot like structs. In fact, the earliest C++ compilers were merely pre-compilers that translates C++ code to C. Thus these abstractions are a benefit to logical thinking, not necessarily an asset to the computer itself.

Beyond the fact that each is a different type of abstraction, Classes provide solutions to the C code naming puzzle. Since you can't have more than one function exposed with the same name, developers used to follow a pattern of _(). e.g. mathlibextreme_max(). By grouping APIs into classes, similar functions (here we call them "methods") can be grouped together and protected from the naming of methods in other classes. This allows the programmer to organize his code better and increase code reuse. In theory, at least.

Solution 7:[7]

The only other difference is the default inheritance of classes and structs, which, unsurprisingly, is private and public respectively.

Solution 8:[8]

The difference between class and struct is a difference between keywords, not between data types. This two

struct foo : foo_base { int x;};
class bar : bar_base { int x; };

both define a class type. The difference of the keywords in this context is the different default access:

  • foo::x is public and foo_base is inherited publicly
  • bar::x is private and bar_base is inherited privately

Solution 9:[9]

  1. The members of a structure are public by default, the members of class are private by default.
  2. Default inheritance for Structure from another structure or class is public.Default inheritance for class from another structure or class is private.
class A{    
public:    
    int i;      
};

class A2:A{    
};

struct A3:A{    
};


struct abc{    
    int i;
};

struct abc2:abc{    
};

class abc3:abc{
};


int _tmain(int argc, _TCHAR* argv[])
{    
    abc2 objabc;
    objabc.i = 10;

    A3 ob;
    ob.i = 10;

    //A2 obja; //privately inherited
    //obja.i = 10;

    //abc3 obss;
    //obss.i = 10;
}

This is on VS2005.

Solution 10:[10]

Not in the specification, no. The main difference is in programmer expectations when they read your code in 2 years. structs are often assumed to be POD. Structs are also used in template metaprogramming when you're defining a type for purposes other than defining objects.

Solution 11:[11]

One other thing to note, if you updated a legacy app that had structs to use classes you might run into the following issue:

Old code has structs, code was cleaned up and these changed to classes. A virtual function or two was then added to the new updated class.

When virtual functions are in classes then internally the compiler will add extra pointer to the class data to point to the functions.

How this would break old legacy code is if in the old code somewhere the struct was cleared using memfill to clear it all to zeros, this would stomp the extra pointer data as well.

Solution 12:[12]

Another main difference is when it comes to Templates. As far as I know, you may use a class when you define a template but NOT a struct.

template<class T> // OK
template<struct T> // ERROR, struct not allowed here

Solution 13:[13]

  1. Member of a class defined with the keyword class are private by default. Members of a class defined with the keywords struct (or union) are public by default.

  2. In absence of an access-specifier for a base class, public is assumed when the derived class is declared struct and private is assumed when the class is declared class.

  3. You can declare an enum class but not an enum struct.

  4. You can use template<class T> but not template<struct T>.

Note also that the C++ standard allows you to forward-declare a type as a struct, and then use class when declaring the type and vice-versa. Also, std::is_class<Y>::value is true for Y being a struct and a class, but is false for an enum class.

Solution 14:[14]

Here is a good explanation: http://carcino.gen.nz/tech/cpp/struct_vs_class.php

So, one more time: in C++, a struct is identical to a class except that the members of a struct have public visibility by default, but the members of a class have private visibility by default.

Solution 15:[15]

It's just a convention. Structs can be created to hold simple data but later evolve time with the addition of member functions and constructors. On the other hand it's unusual to see anything other than public: access in a struct.

Solution 16:[16]

ISO IEC 14882-2003

9 Classes

§3

A structure is a class defined with the class-key struct; its members and base classes (clause 10) are public by default (clause 11).

Solution 17:[17]

The other answers have mentioned the private/public defaults, (but note that a struct is a class is a struct; they are not two different items, just two ways of defining the same item).

What might be interesting to note (particularly since the asker is likely to be using MSVC++ since he mentions "unmanaged" C++) is that Visual C++ complains under certain circumstances if a class is declared with class and then defined with struct (or possibly the other way round), although the standard says that is perfectly legal.

Solution 18:[18]

  • . In classes all the members by default are private but in structure members are public by default.

    1. There is no term like constructor and destructor for structs, but for class compiler creates default if you don't provide.

    2. Sizeof empty structure is 0 Bytes wer as Sizeof empty class is 1 Byte The struct default access type is public. A struct should typically be used for grouping data.

    The class default access type is private, and the default mode for inheritance is private. A class should be used for grouping data and methods that operate on that data.

    In short, the convention is to use struct when the purpose is to group data, and use classes when we require data abstraction and, perhaps inheritance.

    In C++ structures and classes are passed by value, unless explicitly de-referenced. In other languages classes and structures may have distinct semantics - ie. objects (instances of classes) may be passed by reference and structures may be passed by value. Note: There are comments associated with this question. See the discussion page to add to the conversation.

Solution 19:[19]

While implied by other answers, it's not explicitly mentioned - that structs are C compatible, depending on usage; classes are not.

This means if you're writing a header that you want to be C compatible then you've no option other than struct (which in the C world can't have functions; but can have function pointers).

Solution 20:[20]

There exists also unwritten rule that tells: If data members of class have no association with itself, use struct. If value of data member depends on another value of data member, use class.

f.e

class Time
{
    int minutes;
    int seconds;
}

struct Sizes
{
    int length;
    int width;
};

Solution 21:[21]

You might consider this for guidelines on when to go for struct or class, https://msdn.microsoft.com/en-us/library/ms229017%28v=vs.110%29.aspx .

? CONSIDER defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.

X AVOID defining a struct unless the type has all of the following characteristics:

It logically represents a single value, similar to primitive types (int, double, etc.).

It has an instance size under 16 bytes.

It is immutable.

It will not have to be boxed frequently.

Solution 22:[22]

Class is only meaningful in the context of software engineering. In the context of data structures and algorithms, class and struct are not that different. There's no any rule restricted that class's member must be referenced.

When developing large project with tons of people without class, you may finally get complicated coupled code because everybody use whatever functions and data they want. class provides permission controls and inherents to enhance decoupling and reusing codes.

If you read some software engineering principles, you'll find most standards can not be implemented easily without class. for example: http://en.wikipedia.org/wiki/SOLID_%28object-oriented_design%29

BTW, When a struct allocates a crunch of memory and includes several variables, value type variables indicates that values are embbeded in where struct is allocated. In contrast, reference type variable's values are external and reference by a pointer which is also embedded in where struct is allocated.

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