CODEWITHSUNDEEP
c++constructorcopy-constructor
Nishikant
Nishikant

Reputation: 95

Why is there a difference in call of constructors when passed by value to a function and pass by value to another constructor?

I was trying the following program :

#include <iostream>
using namespace std;
class Type{
    int i;
    public: 
    Type() {cout << "type constructor "<<endl;}
    Type (const Type &) { cout << "type copy constructor called" << endl;}
};

class MyClass {
    Type variable;
public:
    MyClass(Type a) {   
    cout << "inside MyClass constructor "<<endl;
    variable = a;
    }
};
void fun (Type){
    return;
}

int main (){
    Type t;
    cout <<"t created"<<endl;
    MyClass tmp = MyClass(t);
    cout<<"calling fun"<<endl;
    fun(t);
}

The output of this is : 

type constructor 
t created
type copy constructor called
type constructor 
inside MyClass constructor 
calling fun
type copy constructor called

I am wondering why default constructor is called when I pass it to MyClass constructor and why copy constructor is called when I pass it to fun()?
BTW same happens when I use initializer list.

Upvotes: 2

Views: 57

Answers (2)

songyuanyao
songyuanyao

Reputation: 172884

I am wondering why default constructor is called when I pass it to MyClass constructor

It has nothing to do with passing argument here. As a member variable, variable will be default constructed at first.

class MyClass {
    Type variable;  
public:
    MyClass(Type a) {   // variable will be default constructed at first, since it's not initialized via member initializer list
    cout << "inside MyClass constructor "<<endl;
    variable = a;       // then variable is assgined via assignment operator
    }
};

You can specify how variable would be initialized by member intializer list, like

class MyClass {
    Type variable;  
public:
    MyClass(Type a) : variable(a) {   // variable will be direct initialized via copy constructor
    cout << "inside MyClass constructor "<<endl;
    // variable = a;                  // no need for assignment
    }
};

The default constructor won't be called for this case.

Upvotes: 2

skypjack
skypjack

Reputation: 50540

Copy constructor is invoked for both the parameters (the one for MyClass constructor and the one for fun) and the logs you posted report that.
In fact, you have two times the following line:

type copy constructor called

Default constructor is invoked for data member variable, for it is not explicitly initialized.
The same happens if you use an initializer list like this:

MyClass(Type a): variable{} {
    //...
}

Actually, variable is default initialized if you don't have any initializer list, so there is no reason to expect a different log.

Upvotes: 0

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