User Defined Conversions

Implementing Custom Type Conversion

How do you implement a custom type conversion that converts an object to a built-in type?

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Implementing a custom type conversion in C++ that converts an object to a built-in type involves overloading a typecast operator within your class.

This operator defines how the conversion should be performed. Here’s a step-by-step guide using an example:

Example: Converting a Complex Number to double

Let’s implement a custom type conversion for a Complex number class that converts the object to a double representing its magnitude.

#include <iostream>
#include <cmath>

class Complex {
  double real, imag;

  Complex(double r, double i)
    : real(r), imag(i) {}

  // Overload the double typecast operator
  operator double() const {
    return std::sqrt(real * real + imag * imag);

int main() {
  Complex num(3.0, 4.0);

  // Convert Complex to double
  double magnitude = num; 
  std::cout << "Magnitude: " << magnitude;
Magnitude: 5

Steps to Implement Custom Type Conversion

  1. Define the Class: Create the class and define its members.
  2. Implement the Constructor: Initialize the class members through a constructor.
  3. Overload the Typecast Operator: Define the typecast operator function inside the class. This function should not take any parameters and should return the type you want to convert to.


  • Class Definition: The Complex class has two members, real and imag, representing the real and imaginary parts of the complex number.
  • Constructor: The constructor initializes these members.
  • Typecast Operator: The operator double() function calculates the magnitude of the complex number using the formula real2+imag2\sqrt{real^2 + imag^2} and returns it as a double.

Using the Conversion

In the main() function, we create a Complex object and then convert it to a double by simply assigning it to a double variable. The compiler calls the operator double() function to perform the conversion.


  • Readability: Custom conversions make code more intuitive and readable.
  • Convenience: They allow for seamless integration of custom types with built-in types and functions.


  • Implicit Conversions: Overloaded typecast operators can lead to implicit conversions that may cause unexpected behavior. Use the explicit keyword if you want to prevent implicit conversions.

By following these steps, you can implement custom type conversions that integrate your custom types smoothly with C++'s built-in types, enhancing the usability and expressiveness of your code.

Answers to questions are automatically generated and may not have been reviewed.

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