C++ for kids: when it is the right start

// hello.cpp (ours)
#include <iostream>
#include <string>

int main() {
    std::string name;
    std::cout << "What is your name? ";
    std::cin >> name;
    std::cout << "Hello, " << name << ". Welcome to C++.\n";
}

There is no return statement, and that is allowed: cppreference says that reaching the end of main without one has the effect of return 0. std::cin reads from the keyboard, the standard input.

// guess.cpp: a number guessing game (ours)
#include <iostream>
#include <random>

int main() {
    std::random_device rd;          // a seed source
    std::mt19937 gen(rd());         // the random engine
    std::uniform_int_distribution<int> pick(1, 50);

    int secret = pick(gen);         // 1 to 50, both ends included
    int guess = 0;
    int tries = 0;

    while (guess != secret) {
        std::cout << "Guess a number from 1 to 50: ";
        std::cin >> guess;
        tries = tries + 1;
        if (guess < secret) std::cout << "Higher\n";
        else if (guess > secret) std::cout << "Lower\n";
    }
    std::cout << "Got it in " << tries << " tries.\n";
}

uniform_int_distribution produces whole numbers on the closed interval from a to b, so 1 and 50 can both be the secret. Our arithmetic: halving the range each time finds any number from 1 to 50 in at most 6 guesses, because 2 to the power 6 is 64, which is at least 50.

// scores.cpp: why .at() is the safer way in (ours)
#include <iostream>
#include <stdexcept>
#include <vector>

int main() {
    std::vector<int> scores = {7, 9, 4};
    scores.push_back(10);           // positions 0, 1, 2 and 3

    std::cout << "First score: " << scores.at(0) << "\n";
    std::cout << "How many: " << scores.size() << "\n";

    try {
        // there is no position 4
        std::cout << scores.at(4) << "\n";
    } catch (const std::out_of_range& e) {
        std::cout << "Caught it: no score at position 4\n";
    }
    // scores[4] also compiles, but reading it is undefined behavior
}

vector::at checks the position and throws std::out_of_range when it is outside the vector. The square-bracket version does no check, so the program in the last comment would compile and then behave unpredictably.