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Handle a Recoverable Failure

Handle a Recoverable Failure

Throw raises an exception and a matching catch handles it.

A school mark validator can reject an out-of-range value without pretending it is a valid score.

Before this lesson, review Represent Missing or Alternative Data when needed.

What you’ll learn

  • Use exceptions to handle a Recoverable Failure.
  • Explain how the complete example produces Invalid mark.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

A school mark validator can reject an out-of-range value without pretending it is a valid score.

Read the declarations above main before tracing its statements. Each declared name supplies the value, type or operation used later. The std:: prefix selects a name from the standard library, and cout << sends a value to the output stream.

C++17 · main.cpp · complete program

#include <iostream>
#include <stdexcept>

void validateMark(int mark)  {
  if (mark < 0 || mark > 100) throw std::invalid_argument("Invalid mark");
}
int main()  {
  try  {
    validateMark(101);
  }
  catch (const std::invalid_argument& error)  {
    std::cout << error.what() << '\n';
  }
  return 0;
}

Expected output

Invalid mark
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <stdexcept>

void validateMark(int mark)  {
  if (mark < 0 || mark > 100) throw std::invalid_argument("Invalid mark");
}
int main()  {
  try  {
    validateMark(101);
  }
  catch (const std::invalid_argument& error)  {
    std::cout << error.what() << '\n';
  }
  return 0;
}

Expected output

Invalid mark

Throw raises an exception and a matching catch handles it. A school mark validator can reject an out-of-range value without pretending it is a valid score.

Trace the result: Read the complete program below from its starting values to the output operations. Each std::cout statement displays the value supplied at that point; \n begins the next line. The resulting lines, in order, are Invalid mark. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

Catching does not automatically undo earlier state changes. Catch exceptions by appropriate reference types and use resource owners for cleanup.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <stdexcept>

void validateMark(int mark)  {
  if (mark < 0 || mark > 100) throw std::invalid_argument("Invalid mark");
}
int main()  {
  try  {
    validateMark(101);
  }
  catch (const std::invalid_argument& error)  {
    std::cout << error.what() << '\n';
  }
  return 0;
}

Your task: Validate 80 instead. Before running your edited program, predict the new output and explain which stored value, condition or object changes. Keep the rest of the example intact.

Use the complete source repeated here; this task does not depend on remembering code from another section.

Worked answer and reasoning

Worked variation: Validate 80 instead. No exception is raised and the catch message does not print.

Throw raises an exception and a matching catch handles it. A school mark validator can reject an out-of-range value without pretending it is a valid score.

Catching does not automatically undo earlier state changes. Catch exceptions by appropriate reference types and use resource owners for cleanup.

For a baseline run, save this source as main.cpp and use clang++ -std=c++17 -Wall -Wextra -pedantic main.cpp -o school, or equivalent g++ flags. Recompile after editing so that you run the updated executable.

The source and output below belong to the edited program. Compare them with the original example to see the consequence of this single change.

C++17 · worked variation · complete program

#include <iostream>
#include <stdexcept>

void validateMark(int mark)  {
  if (mark < 0 || mark > 100) throw std::invalid_argument("Invalid mark");
}
int main()  {
  try  {
    validateMark(80);
  }
  catch (const std::invalid_argument& error)  {
    std::cout << error.what() << '\n';
  }
  return 0;
}

Expected output (no printed lines)

Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <stdexcept>

void validateMark(int mark)  {
  if (mark < 0 || mark > 100) throw std::invalid_argument("Invalid mark");
}
int main()  {
  try  {
    validateMark(101);
  }
  catch (const std::invalid_argument& error)  {
    std::cout << error.what() << '\n';
  }
  return 0;
}
Before making the practice edit, what does the original C++17 program print?
Choose one answer. You can try again.

Answer and explanation

Invalid mark

Throw raises an exception and a matching catch handles it. A school mark validator can reject an out-of-range value without pretending it is a valid score. Catching does not automatically undo earlier state changes. Catch exceptions by appropriate reference types and use resource owners for cleanup.

What to remember

Throw raises an exception and a matching catch handles it. Catching does not automatically undo earlier state changes. Catch exceptions by appropriate reference types and use resource owners for cleanup.

Useful connections: Represent Missing or Alternative Data, Name and Update Values.

Language and design guidance: C++ Core Guidelines and C++ working draft. The draft describes an evolving language; these examples deliberately target C++17.

Reaching the recap and completing the quizzes are separate milestones.

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