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C++ composition and RAII

Let owned members manage their resources automatically.

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Before you start

Classes, constructors, and vector.

Let owned members manage their resources automatically.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

RAII ties resource lifetime to object lifetime. A vector member manages its own allocation and releases it when its containing object is destroyed.

GradeBook has a collection of scores. This composition avoids exposing raw allocation or requiring a manual cleanup method.

Code example

C++

#include <iostream>
#include <vector>
class GradeBook {
    std::vector<int> scores_;
public:
    void add(int score) { scores_.push_back(score); }
    int total() const {
        int sum = 0;
        for (int score : scores_) sum += score;
        return sum;
    }
};
int main() {
    GradeBook book;
    book.add(80);
    book.add(90);
    std::cout << book.total() << '\n';
}

Output

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1#include <iostream>

2#include <vector>

3class GradeBook {

4 std::vector<int> scores_;

5public:

6 void add(int score) { scores_.push_back(score); }

7 int total() const {

8 int sum = 0;

9 for (int score : scores_) sum += score;

10 return sum;

11 }

12};

13int main() {

14 GradeBook book;

15 book.add(80);

16 book.add(90);

17 std::cout << book.total() << '\n';

18}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Construct an empty vector member.	`scores_ = []`	—
Append the first value.	`scores_ = [80]`	—
Append the second.	`scores_ = [80, 90]`	—
Accumulate 80 then 90.	`sum: 0 -> 80 -> 170`	—
Print the total. When main ends, the vector cleans up its storage.	`sum: 0 -> 80 -> 170`	170

Behind the scenes

How C++ processes this code

The C++ compiler produces a native executable. Execution enters main, where objects and function calls perform the work.

Cost and common mistakes

Amortized O(1) append, O(n) total calculation, O(n) storage.

Do not manually delete storage managed by vector.
Scores and totals need validation and wider types for large production datasets.

Try it yourself

Create an empty GradeBook and call total.

Reveal answer and reasoning

It returns 0 because the loop has no iterations.