Struct Constructor in C++?

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.

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

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

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.

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.

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.

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

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

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
  }
};

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.