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 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
  }
};

Answer 2

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.

Answer 3

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 initialise 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);

Answer 4

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.