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Check Pointer Validity

Check Pointer Validity

Null represents a pointer that does not identify an object.

A school lookup can return no student, which must be handled before dereferencing the result.

Before this lesson, review Point to an Existing Object when needed.

What you’ll learn

  • Use null and dangling pointers to check Pointer Validity.
  • Explain how the complete example produces No mark.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

A school lookup can return no student, which must be handled before dereferencing the result.

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>

int main()  {
  int* mark = nullptr;
  if (mark == nullptr)  {
    std::cout << "No mark" << '\n';
  }
  return 0;
}

Expected output

No mark
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

int main()  {
  int* mark = nullptr;
  if (mark == nullptr)  {
    std::cout << "No mark" << '\n';
  }
  return 0;
}

Expected output

No mark

Null represents a pointer that does not identify an object. A school lookup can return no student, which must be handled before dereferencing the result.

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 No mark. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

A non-null pointer can still dangle after an object expires. A null check alone does not prove valid lifetime or ownership.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  int* mark = nullptr;
  if (mark == nullptr)  {
    std::cout << "No mark" << '\n';
  }
  return 0;
}

Your task: Point mark at a live initialized int instead; the null branch is then skipped.. 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: Point mark at a live initialized int instead; the null branch is then skipped.

Null represents a pointer that does not identify an object. A school lookup can return no student, which must be handled before dereferencing the result.

A non-null pointer can still dangle after an object expires. A null check alone does not prove valid lifetime or ownership.

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>

int main()  {
  int value = 72; int* mark = &value;
  if (mark == nullptr)  {
    std::cout << "No mark" << '\n';
  }
  return 0;
}

Expected output (no printed lines)

Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  int* mark = nullptr;
  if (mark == nullptr)  {
    std::cout << "No mark" << '\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

No mark

Null represents a pointer that does not identify an object. A school lookup can return no student, which must be handled before dereferencing the result. A non-null pointer can still dangle after an object expires. A null check alone does not prove valid lifetime or ownership.

What to remember

Null represents a pointer that does not identify an object. A non-null pointer can still dangle after an object expires. A null check alone does not prove valid lifetime or ownership.

Useful connections: Point to an Existing Object, 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: Understand Manual Allocation →