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

C++

#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:

C++

#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:

1. Extended initializer lists are available in C++11 and on.
2. We can directly try assigning a extended initializer list to a object when it is being created.
3. An implicit class-type conversion feature of a constructor doesn’t affect its normal behavior.
4. Using the explicit function-specifier for a constructor removes the implicit conversions using that constructor

Usage of conversion constructor

1. 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. 

2. 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().

Credits: Bharath Vignesh J K