What is conversion constructor in C++?
Pre-requisite: Type Conversion in C++ and Use of explicit keyword in C++
In C++, if a class has a constructor which can be called with a single argument, then this constructor becomes conversion constructor because such a constructor allows automatic conversion to the class being constructed.
Example
CPP
#include <iostream>class MyClass { int a, b;public: MyClass(int i) { a = i; b = i; } void display() { std::cout << " a = " << a << " b = " << b << "\n"; }};int main(){ MyClass object(10); object.display(); // Single parameter conversion constructor is invoked. object = 20; object.display(); return 0;} |
Output:
a = 10 b = 10 a = 20 b = 20
Conversion Constructors: There are constructors that convert types of its parameter into a type of the class. The compiler uses these constructors to perform implicit class-type conversions. These conversions are made by invoking the corresponding constructor with matches the list of values/objects that are assigned to the object.
This previous example only deals with one parameter, to extend it to several parameters i.e., extended initializer lists or braced-init-lists. That is, we enclose the parameters to be passed to it inside a pair of curly braces ({}).
Example with Multiple Parameters:
CPP
#include <iostream>class MyClass { int a, b;public: MyClass(int i, int y) { a = i; b = y; } void display() { std::cout << " a = " << a << " b = " << b << "\n"; }};int main(){ MyClass object(10, 20); object.display(); // Multiple parameterized conversion constructor is invoked. object = { 30, 40 }; object.display(); return 0;} |
Output:
a = 10 b = 20 a = 30 b = 40
Note:
- Extended intializer lists are available in C++11 and on.
- We can directly try assigning a extended initializer list to a object when it is being created.
- An implicit class-type conversion feature of a constructor doesn’t affect its normal behaviour.
- Using the explicit function-specifier for a constructor removes the implicit conversions using that constructor
Usage of conversion constructor
- In return value of a function:
When the return type of a function is a class, instead of returning a object, we can return a braced-init-list, now since the return type is a class instance, a object of that class is created with the braced-init-list, given that the class has a corresponding conversion constructor.
Example:
MyClass create_object(int x, int y)
{
return {x, y};
}
This function create_object will return a MyClass object with a and b values as the x and y passed to the function.
- As a parameter to a function:
When a function’s parameter type is of a class, instead of passing a object to the function, we can pass a braced-init-list to the function as the actual parameter, given that the class has a corresponding conversion constructor.
An example :
void display_object(MyClass obj)
{
obj.display();
}
// This function is invoked in the main function with a braced-init-list with two integers as the parameter.
// e.g. :
// display_object({10, 20});
- Note: This function display_object creates a new class instance of MyClass called obj and will call its member function display().
