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Point to an Existing Object

Point to an Existing Object

A pointer stores an address that may identify an object.

Dereferencing a valid pointer reads or updates that object, allowing an explicit indirect mark correction.

Before this lesson, review Choose Processing Order when needed.

What you’ll learn

  • Use pointers to point to an Existing Object.
  • Explain how the complete example produces 72.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Dereferencing a valid pointer reads or updates that object, allowing an explicit indirect mark correction.

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 marks = 70;
  int* pointer = &marks;
  *pointer = 72;
  std::cout << marks << '\n';
  return 0;
}

Expected output

72
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

int main()  {
  int marks = 70;
  int* pointer = &marks;
  *pointer = 72;
  std::cout << marks << '\n';
  return 0;
}

Expected output

72

A pointer stores an address that may identify an object. Dereferencing a valid pointer reads or updates that object, allowing an explicit indirect mark correction.

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

Point to an Existing Object: cause and effect

  1. Operation

    A pointer stores an address that may identify an object.

  2. Effect on school data

    Dereferencing a valid pointer reads or updates that object, allowing an explicit indirect mark correction.

  3. Observed result

    The program displays these lines after the shown operations.

    72
Conceptual relationship for the actual program above; this visual does not execute C++.

Important mistakes and boundaries

A raw pointer does not say who owns the object. Dereferencing null, an expired object or an invalid address is not safe.

Change the school example

C++17 · edit this complete example

#include <iostream>

int main()  {
  int marks = 70;
  int* pointer = &marks;
  *pointer = 72;
  std::cout << marks << '\n';
  return 0;
}

Your task: Assign 80 through the pointer. 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: Assign 80 through the pointer. The original mark becomes 80.

A pointer stores an address that may identify an object. Dereferencing a valid pointer reads or updates that object, allowing an explicit indirect mark correction.

A raw pointer does not say who owns the object. Dereferencing null, an expired object or an invalid address is not safe.

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 marks = 70;
  int* pointer = &marks;
  *pointer = 80;
  std::cout << marks << '\n';
  return 0;
}

Expected output

80
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

int main()  {
  int marks = 70;
  int* pointer = &marks;
  *pointer = 72;
  std::cout << marks << '\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

72

A pointer stores an address that may identify an object. Dereferencing a valid pointer reads or updates that object, allowing an explicit indirect mark correction. A raw pointer does not say who owns the object. Dereferencing null, an expired object or an invalid address is not safe.

What to remember

A pointer stores an address that may identify an object. A raw pointer does not say who owns the object. Dereferencing null, an expired object or an invalid address is not safe.

Useful connections: Choose Processing Order, 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.

    Next lesson: Check Pointer Validity →