Struct 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.

Answer 1

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.

Answer 2

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.

Answer 3

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

Answer 4

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();
}

Answer 5

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...

Answer 6

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.