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Check Behavior at Boundaries

Check Behavior at Boundaries

A test compares a result with a stated expectation.

Assertions can expose a wrong internal assumption in development, while required input validation must remain active in normal builds.

Before this lesson, review Validate a Complete Record when needed.

What you’ll learn

  • Use testing and assertions to check Behavior at Boundaries.
  • Explain how the complete example produces Checks passed.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Assertions can expose a wrong internal assumption in development, while required input validation must remain active in normal builds.

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 <cassert>

bool passed(int marks)  {
  return marks >= 50;
}
int main()  {
  assert(passed(50));
  assert(!passed(49));
  std::cout << "Checks passed" << '\n';
  return 0;
}

Expected output

Checks passed
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <cassert>

bool passed(int marks)  {
  return marks >= 50;
}
int main()  {
  assert(passed(50));
  assert(!passed(49));
  std::cout << "Checks passed" << '\n';
  return 0;
}

Expected output

Checks passed

A test compares a result with a stated expectation. Assertions can expose a wrong internal assumption in development, while required input validation must remain active in normal builds.

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 Checks passed. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

Defining NDEBUG disables ordinary assert checks. Tests should include boundary cases and meaningful variations rather than repeating the implementation.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <cassert>

bool passed(int marks)  {
  return marks >= 50;
}
int main()  {
  assert(passed(50));
  assert(!passed(49));
  std::cout << "Checks passed" << '\n';
  return 0;
}

Your task: Add assert(passed(80)). 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: Add assert(passed(80)). The checks still pass and print the same success line.

A test compares a result with a stated expectation. Assertions can expose a wrong internal assumption in development, while required input validation must remain active in normal builds.

Defining NDEBUG disables ordinary assert checks. Tests should include boundary cases and meaningful variations rather than repeating the implementation.

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 <cassert>

bool passed(int marks)  {
  return marks >= 50;
}
int main()  {
  assert(passed(50));
  assert(!passed(49));
  assert(passed(80)); std::cout << "Checks passed" << '\n';
  return 0;
}

Expected output

Checks passed
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <cassert>

bool passed(int marks)  {
  return marks >= 50;
}
int main()  {
  assert(passed(50));
  assert(!passed(49));
  std::cout << "Checks passed" << '\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

Checks passed

A test compares a result with a stated expectation. Assertions can expose a wrong internal assumption in development, while required input validation must remain active in normal builds. Defining NDEBUG disables ordinary assert checks. Tests should include boundary cases and meaningful variations rather than repeating the implementation.

What to remember

A test compares a result with a stated expectation. Defining NDEBUG disables ordinary assert checks. Tests should include boundary cases and meaningful variations rather than repeating the implementation.

Useful connections: Validate a Complete Record, Give a Task a Name.

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.

    Next lesson: Separate Declarations and Definitions →