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Grow a Class List

Grow a Class List

Std::vector owns a growable contiguous sequence.

It handles element storage and cleanup, making it a practical default for a changing class list.

Before this lesson, review Use a Fixed Collection when needed.

What you’ll learn

  • Use vectors to grow a Class List.
  • Explain how the complete example produces 3 → 90.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

It handles element storage and cleanup, making it a practical default for a changing class list.

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>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  marks.push_back(90);
  std::cout << marks.size() << '\n' << marks.back() << '\n';
  return 0;
}

Expected output

3
90
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>
#include <vector>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  marks.push_back(90);
  std::cout << marks.size() << '\n' << marks.back() << '\n';
  return 0;
}

Expected output

3
90

Std::vector owns a growable contiguous sequence. It handles element storage and cleanup, making it a practical default for a changing class list.

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

Important mistakes and boundaries

Growing a vector can invalidate references, pointers and iterators into its storage. Size and reserved capacity are different values.

Change the school example

C++17 · edit this complete example

#include <iostream>
#include <vector>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  marks.push_back(90);
  std::cout << marks.size() << '\n' << marks.back() << '\n';
  return 0;
}

Your task: Append 95 instead of 90. 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: Append 95 instead of 90. The size stays 3 and the last mark is 95.

Std::vector owns a growable contiguous sequence. It handles element storage and cleanup, making it a practical default for a changing class list.

Growing a vector can invalidate references, pointers and iterators into its storage. Size and reserved capacity are different values.

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>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  marks.push_back(95);
  std::cout << marks.size() << '\n' << marks.back() << '\n';
  return 0;
}

Expected output

3
95
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>
#include <vector>

int main()  {
  std::vector<int> marks {
    70, 80
  }
  ;
  marks.push_back(90);
  std::cout << marks.size() << '\n' << marks.back() << '\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

3 90

Std::vector owns a growable contiguous sequence. It handles element storage and cleanup, making it a practical default for a changing class list. Growing a vector can invalidate references, pointers and iterators into its storage. Size and reserved capacity are different values.

Check a common mistake

Can a vector growth operation invalidate a reference to one of its elements?
Choose one answer. You can try again.

Answer and explanation

Yes, when storage reallocates.

Reallocation can move elements regardless of whether they are integers or strings; do not keep using invalidated references.

What to remember

Std::vector owns a growable contiguous sequence. Growing a vector can invalidate references, pointers and iterators into its storage. Size and reserved capacity are different values.

Useful connections: Use a Fixed Collection, 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: Visit Each Element →