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Custom exceptions

Learn to write your own std::exception-derived class.

Custom exception class

To define your own exception type, you write a class derived from std::exception. This is the basic way to name your application's specific errors. You override the what() method to return your custom message.

class InvalidScore : public std::exception {
public:
    const char* what() const noexcept override {
        return "invalid score";
    }
};

Here InvalidScore is derived from std::exception; the what() method returns the custom message. This way a catch (const std::exception& e) block also catches the custom exception.

Throwing a custom exception

You throw your custom exception with throw. Like standard exceptions, it is caught by an appropriate catch block. A catch (const std::exception& e) block catches both standard and custom exceptions.

if (score > 100) {
    throw InvalidScore();
}

Here, if score is greater than 100, InvalidScore is thrown. The caller can catch it with a standard catch block and read the custom message via e.what().

Practice Define the InvalidScore class

Add #include <stdexcept>. Define class InvalidScore : public std::exception. Open a public: block and write the const char* what() const noexcept override { return "invalid score"; } method. In main, add try { throw InvalidScore(); } catch (const std::exception& e) { cout << e.what() << endl; }.

Starter code

main.cpp
#include <iostream>
#include <stdexcept>
using namespace std;

// Define the InvalidScore class

int main() {
    // in try throw the class, in catch print the message
    return 0;
}

Sample solution

main.cpp
#include <iostream>
#include <stdexcept>
using namespace std;

class InvalidScore : public std::exception {
public:
    const char* what() const noexcept override {
        return "invalid score";
    }
};

int main() {
    try {
        throw InvalidScore();
    } catch (const std::exception& e) {
        cout << e.what() << endl;
    }
    return 0;
}

Expected output

invalid score

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 How is the what() method of a custom exception declared?

  • const char* what() const noexcept override
  • string what()
  • void what()
  • int what() Correct answer

Explanation: what() returns const char* with override.

2 Fill in the blank: class derivation

Question 
class InvalidScore : ____ std::exception {
};
Answer 
class InvalidScore : public std::exception {
};

Explanation: public derivation gives access to base members.

3 Match each expression with its purpose

Term Meaning
class InvalidScore custom type definition
public std::exception base class
override overrides a method

All lessons in Beginner

1 C++ basics

  1. What is C++?
  2. Your first program
  3. std::cout
  4. std::cin
  5. Comments
  6. Summary - C++ basics

2 Variables

  1. Declaring variables
  2. auto
  3. const
  4. constexpr
  5. Type conversion
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and char
  4. std::string
  5. sizeof
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 Strings

  1. std::string basics
  2. String concatenation
  3. String length
  4. Substring
  5. String search
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range-based for
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. C-style arrays
  2. Array access
  3. std::array
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 std::vector

  1. Declaring vectors
  2. push_back
  3. size and at
  4. Vector access
  5. Vector loop
  6. Summary - std::vector

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function overloading
  5. Default parameters
  6. Summary - Functions

13 References

  1. What is a reference?
  2. Declaring references
  3. Reference parameters
  4. const references
  5. Dangling references
  6. Summary - References

14 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. nullptr
  5. Pointer arithmetic
  6. Summary - Pointer basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. String streams
  2. stoi and stod
  3. to_string
  4. String comparison
  5. String modification
  6. Summary - Advanced strings

17 Structs

  1. Defining structs
  2. Using structs
  3. Struct and function
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Exception basics

  1. try/catch
  2. throw
  3. std::exception
  4. Custom exceptions
  5. catch all
  6. Summary - Exceptions

19 Namespaces

  1. Defining namespaces
  2. using
  3. std namespace
  4. Nested namespaces
  5. Namespace alias
  6. Summary - Namespaces

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together