Can a struct have a constructor in C++?

Question

Can a struct have a constructor in C++?

I have been trying to solve this problem but I am not getting the syntax.

Clerissa

https://mcmap.net/q/23175/-what-are-the-differences-between-struct-and-class-in-c/5740428 pretty much answers the question. – Sherlocke

Answer 1

In C++ the only difference between a class and a struct is that members and base classes are private by default in classes, whereas they are public by default in structs.

So structs can have constructors, and the syntax is the same as for classes.

Educated

And that structures will default to public when deriving from :) – Nesbitt

@Educated Your right on the difference between struct and class, however I think he's having a compile issue. The issue might be because of a union that is using the struct. You can't have non-trivial constructors in the type you have in a union. – Crossley

@Chap: If he has concrete problems where the general solution doesn't work, it would probably be the best idea to post some code that shows the problem and the compiler errors that are generated. But as general as the question is asked I don't think one can really infer too much about the concrete problem the OP is trying to solve... – Educated

@GMan: Right idea, wrong wording. A struct inherits its base classes publicly by default; there is no change to classes deriving from the struct. – Journalism

@BenVoigt: Whoa. How'd you find this old comment. :) Yeesh wish I could edit it...even I'm confused at what I wrote. I think I omitted the word "bases" from the end but even that sucks. – Nesbitt

Does Structure can also have destructors ? – Laundress

@AbhishekGupta Yes it can have destructors as well. Like they are saying the only difference is related to inheritance. – Ungovernable

See my answer below, there is an additional difference that affects how you can do initialization of structure arrays -- a subtle point perhaps -- but used all over the place in WinSDK code... – Whether

It seems that there should always be an implicit copy constructor with zero args for which one have to provide the default values for all arguments in the original constructor. Otherwise, I am getting an error like candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 0 were provided. myConstructor(int a, int b) : a(a), b(b) {}; // incorrect one vs myConstructor(int a=0, int b=3) : a(a), b(b) {}; // correct one – Dawna

For some reason I thought that the constructor will be created automatically, like in Swift, lol – Wittenburg

Answer 2

struct TestStruct {
        int id;
        TestStruct() : id(42)
        {
        }
};

Adulteration

What is the : id(42) part called? – Lixivium

@user13107: "initializer list" is the word you're looking for. – Adrenocorticotropic

That won't work if you inherit from another class and the variable is decleared in the parent class. – Desinence

@user152949: No one said it would. We could all comment on all code excerpts saying 'This won't work if [some totally different scenario]', but what's the point? – Rothman

Hi, i'm a beginner with this, what is the need of '{ }' after id(42) ? (Why is it there) and is it always left empty? – Sato

@Sato It's the body of the constructor function. There's no code we want to run in the constructor in this case, so it's empty. – Adulteration

The moment constructor is called id will be initialized with the value of 42. – Messing

Answer 3

All the above answers technically answer the asker's question, but just thought I'd point out a case where you might encounter problems.

If you declare your struct like this:

typedef struct{
int x;
foo(){};
} foo;

You will have problems trying to declare a constructor. This is of course because you haven't actually declared a struct named "foo", you've created an anonymous struct and assigned it the alias "foo". This also means you will not be able to use "foo" with a scoping operator in a cpp file:

foo.h:

typedef struct{
int x;
void myFunc(int y);
} foo;

foo.cpp:

//<-- This will not work because the struct "foo" was never declared.
void foo::myFunc(int y)
{
  //do something...
}

To fix this, you must either do this:

struct foo{
int x;
foo(){};
};

or this:

typedef struct foo{
int x;
foo(){};
} foo;

Where the latter creates a struct called "foo" and gives it the alias "foo" so you don't have to use the struct keyword when referencing it.

Brittne

what about when: typedef struct foo{ int x; foo(int x){}; }foo; foo::foo(int x){...}; this does not work... – Saddleback

alternatively you can define: struct foo{ int x; foo(){}; }; and then: typedef struct foo foo; – Tavish

Answer 4

As the other answers mention, a struct is basically treated as a class in C++. This allows you to have a constructor which can be used to initialize the struct with default values. Below, the constructor takes sz and b as arguments, and initializes the other variables to some default values.

struct blocknode
{
    unsigned int bsize;
    bool free;
    unsigned char *bptr;
    blocknode *next;
    blocknode *prev;

    blocknode(unsigned int sz, unsigned char *b, bool f = true,
              blocknode *p = 0, blocknode *n = 0) :
              bsize(sz), free(f), bptr(b), prev(p), next(n) {}
};

Usage:

unsigned char *bptr = new unsigned char[1024];
blocknode *fblock = new blocknode(1024, btpr);

Noncombatant

Not just "basically treated as" ... the keyword struct literally creates a class. Period. – Gownsman

Answer 5

Class, Structure and Union is described in below table in short.

Addition

Answer 6

Yes, but if you have your structure in a union then you cannot. It is the same as a class.

struct Example
{
   unsigned int mTest;
   Example()
   {
   }
};

Unions will not allow constructors in the structs. You can make a constructor on the union though. This question relates to non-trivial constructors in unions.

Crossley

Answer 7

In c++ struct and c++ class have only one difference by default struct members are public and class members are private.

/*Here, C++ program constructor in struct*/ 
#include <iostream>
using namespace std;

struct hello
    {
    public:     //by default also it is public
        hello();    
        ~hello();
    };

hello::hello()
    {
    cout<<"calling constructor...!"<<endl;
    }

hello::~hello()
    {
    cout<<"calling destructor...!"<<endl;
    }

int main()
{
hello obj;      //creating a hello obj, calling hello constructor and destructor 

return 0;
}

Bovid

Answer 8

Syntax is as same as of class in C++. If you aware of creating constructor in c++ then it is same in struct.

struct Date
{
    int day;

    Date(int d)
    {
        day = d;
    }

    void printDay()
    {
        cout << "day " << day << endl;
    }
};

Struct can have all things as class in c++. As earlier said difference is only that by default C++ member have private access but in struct it is public.But as per programming consideration Use the struct keyword for data-only structures. Use the class keyword for objects that have both data and functions.

Skeens

Answer 9

One more example but using this keyword when setting value in constructor:

#include <iostream>

using namespace std;

struct Node {
    int value;

    Node(int value) {
        this->value = value;
    }

    void print()
    {
        cout << this->value << endl;
    }
};

int main() {
    Node n = Node(10);
    n.print();

    return 0;
}

Compiled with GCC 8.1.0.

Niklaus

You should consider using an "Initializer List" instead of this-> assignment – Rolland

@CameronTacklind Care to explain why we should do that? – Ovotestis

Answer 10

Note that there is one interesting difference (at least with the MS C++ compiler):

---

If you have a plain vanilla struct like this

struct MyStruct {
   int id;
   double x;
   double y;
} MYSTRUCT;

then somewhere else you might initialize an array of such objects like this:

MYSTRUCT _pointList[] = { 
   { 1, 1.0, 1.0 }, 
   { 2, 1.0, 2.0 }, 
   { 3, 2.0, 1.0 }
};

however, as soon as you add a user-defined constructor to MyStruct such as the ones discussed above, you'd get an error like this:

    'MyStruct' : Types with user defined constructors are not aggregate
     <file and line> : error C2552: '_pointList' : non-aggregates cannot 
     be initialized with initializer list.

So that's at least one other difference between a struct and a class. This kind of initialization may not be good OO practice, but it appears all over the place in the legacy WinSDK c++ code that I support. Just so you know...

Whether

This seems like incorrect behavior from the compiler (assuming the class version declares its members as public). MS says "Visual C++ does not allow data types in an aggregate that contains constructors", but doesn't indicate why that wouldn't apply to classes as well. And it seems to work in VS 2015. – Francois

works fine with the latest bits in VS 2017 Preview 4 as well. API version 141 – Fieldwork

So Aluan, are you saying that VS2017 now allows the initializer list for structs with constructors? I haven't tried the preview yet... Thanks! – Whether

But what is MyStruct and MYSTRUCT? Why it has two names? – Corunna

Hm, got an error: error: 'MYSTRUCT' does not name a type. it does not work for C++ in Arduino... =( – Corunna

Kirby, see the item "Named Structs" in discussed here w3schools.com/cpp/cpp_structs.asp. MyStruct is the name of the struct (it's class name), and MYSTRUCT is supposed to be an instance of the struct (an object). With the Microsoft C++ compiler, however, I was using MYSTRUCT as an alias for MyStruct, which probably isn't the same as for your Arduino version of C++. Hope that helps. – Whether

Answer 11

Yes. A structure is just like a class, but defaults to public:, in the class definition and when inheriting:

struct Foo
{
    int bar;

    Foo(void) :
    bar(0)
    {
    }
}

Considering your other question, I would suggest you read through some tutorials. They will answer your questions faster and more complete than we will.

Nesbitt

Answer 12

struct HaveSome
{
   int fun;
   HaveSome()
   {
      fun = 69;
   }
};

I'd rather initialize inside the constructor so I don't need to keep the order.

Chesnut

Members are always initialized in the order in which they appear in the class/struct body. Creating assignments in the constructor body is just that...assignments. – Kuopio

Answer 13

Yes structures and classes in C++ are the same except that structures members are public by default whereas classes members are private by default. Anything you can do in a class you should be able to do in a structure.

struct Foo
{
  Foo()
  {
    // Initialize Foo
  }
};

Leer

The kind of thing created by the keyword struct is a class. – Gownsman

Answer 14

In C++ both struct & class are equal except struct'sdefault member access specifier is public & class has private.

The reason for having struct in C++ is C++ is a superset of C and must have backward compatible with legacy C types.

For example if the language user tries to include some C header file legacy-c.h in his C++ code & it contains struct Test {int x,y};. Members of struct Test should be accessible as like C.

Magnificence

Answer 15

Yes it possible to have constructor in structure here is one example:

#include<iostream.h> 
struct a {
  int x;
  a(){x=100;}
};

int main() {
  struct a a1;
  getch();
}

Nympha

Answer 16

In C++, we can declare/define the structure just like class and have the constructors/destructors for the Structures and have variables/functions defined in it. The only difference is the default scope of the variables/functions defined. Other than the above difference, mostly you should be able to imitate the functionality of class using structs.

Exemplificative

Answer 17

If you want, instead of using a constructor, you can use a struct initializer. There are a few ways to do that...

See: How to initialize a struct in accordance with C programming language standards

That is old school "C style", i.e. not new school "C++ style", but it works in C++ too of course.

Goral