No default constructor exists for class error

Question

Some simple code:

class Thing {
public:
    int num;
    Thing(int num) { 
        this->num = num; 
    }
};

class Stuff {
public:
    Thing thing;  // an instance of thing is declared here but it cannot construct it
    Stuff(Thing thing) {
        this->thing = thing;
    }
};

int main() {
    Thing thing = Thing(5);
    Stuff stuff = Stuff(thing);
}

So here, I'm trying to work out how I should be taking a new instance of Thing in the constructor of Stuff without pointing to it as I want Stuff to hold its own copy. Of course, I can't declare thing like I have above because it's trying to initialise it.

How can I get around this problem of assigning a new object copy to a class' variable through its constructor?

Exact error is:

In constructor 'Stuff::Stuff(Thing)':
error: no matching function for call to 'Thing::Thing()'
  Stuff(Thing thing){ this->thing = thing; }

candidate expects 1 argument, 0 provided

Answer 1

The problem is here:

Stuff(Thing thing) {
    this->thing = thing;
}

By the time you enter the constructor's body, the compiler will have already initialized your object's data members. But it can't initialize thing because it does not have a default constructor.

The solution is to tell the compiler how to initialize it by using an initlizer list.

Stuff(Thing thing) : thing(thing) {
    // Nothing left to do.
}

This is less typing, cleaner code and more efficient. (More efficient, because if the variable is going to be initialized anyway, then why initialize it with an unwanted value first just to assign another one as quickly as you can? Of course, since your current code doesn't even compile, “more efficient” is a somewhat dubious statement, here.)

Answer 2

Initialize thing member in Stuff with initializer list:

class Stuff {
public:
    Thing thing;  // an instance of thing is declared here but it cannot construct it
    Stuff(Thing thing): thing(thing) { }
};