Automatic Type Conversions in C++

You can define functions to perform automatic conversion from an object to a primitive type value and vice versa.

C++ can perform certain type conversions automatically. You can define functions to enable conversions from a Rational object to a primitive type value or vice versa.

1. Converting to a Primitive Data Type

You can add an int value with a double value such as

4 + 5.5

In this case, C++ performs automatic type conversion to convert an int value 4 to a double value 4.0.

Can you add a rational number with an int or a double value? Yes. You have to define a function operator to convert an object into int or double. Here is the implementation of the function to convert a Rational object to a double value.

Rational::operator doublet)


return doub1eVa1ue(); // doub1eVa1ue() already in Rational.h


Don’t forget that you have to add the member function header in the Rational.h header file.

operator double();

This is a special syntax for defining conversion functions to a primitive type in C++. There conversion function syntax is no return type like a constructor. The function name is the type that you want the object to be converted to.

So, the following code

1 Rational r1(1, 4);
double d = r1 + 5.1;
3 cout <<
“r1 + 5.1 is ” << d << endl;


The statement in line 2 adds a rational number r1 with a double value 5.1. Since the con­version function is defined to convert a rational number to a double, r1 is converted to a double value 0.25, which is then added with 5.1.

2. Converting to an Object Type

A Rational object can be automatically converted to a numeric value. Can a numeric value be automatically converted to a Rational object? Yes, it can.

To achieve this, define the following constructor in the header file:

Rational(int numerator);

and implement it in the implementation file as follows:

Rationa1::Rationa1(int numerator)


this->numerator = numerator;

this->denominator = 1;


Provided that the + operator is also overloaded (see Section 14.3), the following code

Rational r1(2, 3);

Rational r = r1 + 4; // Automatically converting 4 to Rational

cout << r << endl;


When C++ sees r1 + 4, it first checks to see if the + operator has been overloaded to add a Rational with an integer. Since no such function is defined, the system next searches for the + operator to add a Rational with another Rational. Since 4 is an integer, C++ uses the constructor that constructs a Rational object from an integer argument. In other words, C++ performs an automatic conversion to convert an integer to a Rational object. This automatic conversion is possible because the suitable constructor is available. Now two Rational objects are added using the overloaded + operator to return a new Rational object (14 / 3).

A class can define the conversion function to convert an object to a primitive type value or define a conversion constructor to convert a primitive type value to an object, but not both simultaneously in the class. If both are defined, the compiler will report an ambiguity error.

Source: Liang Y. Daniel (2013), Introduction to programming with C++, Pearson; 3rd edition.

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