C++ - lesson 2 | HackIndia Skills

C++ - lesson 2 | HackIndia Skills | HackIndia

Example

Safe function inputs

This pattern fits many hackathon projects: read a list, compute something, and optionally print extra details.

#include <iostream>
#include <string>
#include <vector>
using namespace std;

struct Expense {
    string label;
    int amount;
};

int totalAmount(const vector<Expense>& expenses) {
    int total = 0;
    for (const Expense& e : expenses) {
        total += e.amount;
    }
    return total;
}

void addExpense(vector<Expense>& expenses, const string& label, int amount) {
    expenses.push_back({label, amount});
}

void printFirst(const vector<Expense>* expenses) {
    if (expenses == nullptr || expenses->empty()) {
        cout << "No expenses\n";
        return;
    }
    cout << expenses->at(0).label << " " << expenses->at(0).amount << "\n";
}

int main() {
    vector<Expense> trip;
    addExpense(trip, "chai", 20);
    addExpense(trip, "metro", 60);

    cout << totalAmount(trip) << "\n";
    printFirst(&trip);
    printFirst(nullptr);
}

Common mistake

Dangling addresses

Do not return a reference or pointer to a local variable. The local dies when the function ends.

Also be careful with addresses of vector elements. push_back can move the vector storage, so old pointers or references to elements may become invalid.

int& badRef() {
    int marks = 95;
    return marks; // bad: marks dies here
}

Expense* badPtr() {
    Expense e{"idli", 40};
    return &e; // bad: e dies here
}

Tip

Use AI carefully

If you ask ChatGPT or Gemini to refactor C++ code, ask it to explain every pointer and reference it uses.

Check three things in the answer: can this be nullptr, who owns this object, and can this address outlive the object? If the answer is unclear, prefer a reference or a normal value.

Prompt:
Refactor this C++ function to avoid unsafe raw pointers where possible.
For every reference or pointer you use, explain whether it can be null and who owns the object.

Use references and pointers safely

One rule

Why it matters

Choose the parameter

Safe function inputs

Dangling addresses

Use AI carefully

Do this now

Quick check