Feb 18, 2008

Direct Initialisation and Copy Initialisation

How many ways are there to initialize a variable?

1. MyType type;

Here the variable type is initialised using the default ctor MyType::MyType();

2. MyType type();

This looks like a variable declaration, but it's a function declaration for a function type that takes no parameters and returns a MyType.

3. MyType type(u);

This is direct initialization. The variable type is initialized using MyType::MyType(u);

4. MyType type = u;

This is copy initialization and the variable type is always initialized using MyType's copy ctor. Even though there is an '=' sign, this is a copy initialization and not an assignment.

The form MyType type(u) should be preferred. It always works where 'MyType type = u' works and has other advantages e.g (It can take multiple parameters)

Code

/* Example1.cpp */
#include <iostream>
#include <string>
#include <iomanip.h>
class MyType{
private:
int age;
std::string name;
public:
MyType():age(10),name("pankaj"){
std::cout << "\nCalling Constructor 1\n";
}
MyType(int age, std::string str):age(age), name(str){
std::cout << "\nCalling Constructor 2\n";
}
MyType(MyType &t):age(t.age), name(t.name){
std::cout << "\nCalling Constructor 3\n";
}
void show(){
std::cout << "\nName is = "<<name;
std::cout << "\nAge is = "<<age;
std::cout << endl;
}
};

int main(){
MyType type;
type.show();
MyType t(type);
t.show();
MyType b(50, "Anky");
MyType a = b;
a.show();
}


Output:
$ ./a.exe

Calling Constructor 1

Name is = pankaj
Age is = 10

Calling Constructor 3

Name is = pankaj
Age is = 10

Calling Constructor 2

Calling Constructor 3

Name is = Anky
Age is = 50

Here, MyType a = b; also calls the Constructor 3

Code

/* Example2.cpp */
#include <iostream>
#include <string>
#include <iomanip.h>
class MyType{
private:
int age;
std::string name;
public:
MyType():age(10),name("pankaj"){
std::cout << "\nCalling Constructor 1\n";
}
MyType(int age, std::string str):age(age), name(str){
std::cout << "\nCalling Constructor 2\n";
}
//MyType(MyType &t):age(t.age), name(t.name){
std::cout << "\nCalling Constructor 3\n";
}
void show(){
std::cout << "\nName is = "<<name;
std::cout << "\nAge is = "<<age;
std::cout << endl;
}
};
int main(){
MyType type;
type.show();
//MyType t(type);
//t.show();
MyType b(50, "Anky");
MyType a = b;
a.show();
}


Output:
$ ./a.exe

Calling Constructor 1

Name is = pankaj
Age is = 10

Calling Constructor 2

Name is = Anky
Age is = 50

Here MyType a = b; works even when Constructor 3 is commented out because a default copy constructor is created for you when you don't specify one yourself. In such case, the default copy constructor will simply do a
bitwise copy for primitives (including pointers) and for objects types call their copy constructor. If in the later case, a copy constructor is not explicitly defined then, in turn, a default copy constructor will be implicitly created.

1 comment :

Φërry said...

A definition specifies a variable's type and identifier. A definition may also provide an initial value for the object. An object defined with a specified first value is spoken of as initialized. C++ supports two forms of variable initialization: copy-initialization and direct-initialization. The copy-initialization syntax uses the equal (=) symbol; direct-initialization places the initializer in parentheses:
int ival(1024); // direct-initialization
int ival = 1024; // copy-initialization

In both cases, ival is initialized to 1024.

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