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Let Members Own Resources

Let Members Own Resources

Composition builds an object from member objects.

Standard strings and vectors already manage their own resources, so a class holding them can often use generated copy and destruction operations safely.

Before this lesson, review Keep Custom Operations Predictable when needed.

What you’ll learn

  • Use composition and rule of zero to let Members Own Resources.
  • Explain how the complete example produces 70 → 90.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

Standard strings and vectors already manage their own resources, so a class holding them can often use generated copy and destruction operations safely.

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 <vector>

struct ClassReport  {
  std::vector<int> marks;
}
;
int main()  {
  ClassReport report {
    {
      70, 80
    }
  }
  ;
  ClassReport copy = report;
  copy.marks.at(0) = 90;
  std::cout << report.marks.at(0) << '\n' << copy.marks.at(0) << '\n';
  return 0;
}

Expected output

70
90
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <vector>

struct ClassReport  {
  std::vector<int> marks;
}
;
int main()  {
  ClassReport report {
    {
      70, 80
    }
  }
  ;
  ClassReport copy = report;
  copy.marks.at(0) = 90;
  std::cout << report.marks.at(0) << '\n' << copy.marks.at(0) << '\n';
  return 0;
}

Expected output

70
90

Composition builds an object from member objects. Standard strings and vectors already manage their own resources, so a class holding them can often use generated copy and destruction operations safely.

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

Important mistakes and boundaries

A class owning a raw resource may need deliberate copy/move/destructor design. Prefer standard owners before implementing the Rule of Five yourself.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <vector>

struct ClassReport  {
  std::vector<int> marks;
}
;
int main()  {
  ClassReport report {
    {
      70, 80
    }
  }
  ;
  ClassReport copy = report;
  copy.marks.at(0) = 90;
  std::cout << report.marks.at(0) << '\n' << copy.marks.at(0) << '\n';
  return 0;
}

Your task: Set the copy’s first mark to 95. 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: Set the copy’s first mark to 95. The original remains 70 and the copy becomes 95.

Composition builds an object from member objects. Standard strings and vectors already manage their own resources, so a class holding them can often use generated copy and destruction operations safely.

A class owning a raw resource may need deliberate copy/move/destructor design. Prefer standard owners before implementing the Rule of Five yourself.

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 <vector>

struct ClassReport  {
  std::vector<int> marks;
}
;
int main()  {
  ClassReport report {
    {
      70, 80
    }
  }
  ;
  ClassReport copy = report;
  copy.marks.at(0) = 95;
  std::cout << report.marks.at(0) << '\n' << copy.marks.at(0) << '\n';
  return 0;
}

Expected output

70
95
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <vector>

struct ClassReport  {
  std::vector<int> marks;
}
;
int main()  {
  ClassReport report {
    {
      70, 80
    }
  }
  ;
  ClassReport copy = report;
  copy.marks.at(0) = 90;
  std::cout << report.marks.at(0) << '\n' << copy.marks.at(0) << '\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

70 90

Composition builds an object from member objects. Standard strings and vectors already manage their own resources, so a class holding them can often use generated copy and destruction operations safely. A class owning a raw resource may need deliberate copy/move/destructor design. Prefer standard owners before implementing the Rule of Five yourself.

What to remember

Composition builds an object from member objects. A class owning a raw resource may need deliberate copy/move/destructor design. Prefer standard owners before implementing the Rule of Five yourself.

Useful connections: Keep Custom Operations Predictable, 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: Reuse a Typed Calculation →