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C++ references and the STL

& and const references, vector/map/set/unordered_map, algorithms that take lambdas, and the copy, invalidation and auto-insert traps.

This lesson is for C++.
C has neither references nor containers, so it uses pointers and arrays you manage yourself.
Python, Java, C# and JavaScript have their own collections (see the list and dictionary lessons), and a list you pass is always shared, so they need no &.

In C++, passing a value copies it, while a reference & works on the original.
The example below doubles a vector through a reference, uses STL algorithms such as sort, find and sum, and then counts words with a std::map.

#include <algorithm>#include <iostream>#include <map>#include <numeric>#include <set>#include <string>#include <vector> void doubleAll(std::vector<int>& v) {          // & : 원본을 바꿔요    for (auto& x : v) x *= 2;} int total(const std::vector<int>& v) {         // const& : 복사 없이 읽기만    return std::accumulate(v.begin(), v.end(), 0);} void print(const std::vector<int>& v) {    for (size_t i = 0; i < v.size(); i++) std::cout << (i ? " " : "") << v[i];    std::cout << '\n';} int main() {    int n;    std::cin >> n;    std::vector<int> v(n);    for (auto& x : v) std::cin >> x;     for (auto x : v) x = 0;                     // 복사본만 바뀌어요    print(v);    doubleAll(v);    print(v);    std::cout << total(v) << '\n';     std::sort(v.begin(), v.end(), [](int a, int b) { return a > b; });    print(v);    auto it = std::find(v.begin(), v.end(), 8);    std::cout << (it - v.begin()) << ' ' << std::set<int>(v.begin(), v.end()).size() << '\n';     std::map<std::string, int> count;    std::string w;    while (std::cin >> w) count[w]++;    for (const auto& [word, c] : count) std::cout << word << ' ' << c << '\n';    std::cout << count.contains("kiwi") << ' ' << count.size() << '\n';    int k = count["kiwi"];                      // 없으면 0으로 새로 넣어요    std::cout << k << ' ' << count.size() << '\n';}

5
3 1 4 1 5
apple banana apple cherry banana apple

3 1 4 1 5
6 2 8 2 10
28
10 8 6 2 2
1 4
apple 3
banana 2
cherry 1
0 3
0 4

Key points

• Passing by value copies every element of a vector, so changes inside the function leave the original untouched.
• A reference int& or std::vector<int>& is another name for the same variable.
  The example's doubleAll(v) doubles the original.
  A reference must be bound when it is made, cannot be rebound later, and, unlike a pointer, cannot be empty (null).
• If you only read, take const std::vector<int>&.
  Nothing is copied, and changing it by mistake is a compile error.
• STL containers: std::vector (a growable array), std::map (a dictionary sorted by key, O(log n) lookup), std::set (a sorted collection without duplicates), std::unordered_map (a hash dictionary, O(1) on average, no fixed order).
• Algorithms (<algorithm>, <numeric>) take an iterator range v.begin(), v.end().
  std::sort with the lambda [](int a, int b) { return a > b; } sorts largest first, and std::find returns an iterator to the match (or v.end()), so it - v.begin() is its index.
• std::accumulate(v.begin(), v.end(), 0) is the sum, and the type of that last 0 is the type of the sum (0 gives int; for big sums use 0LL).

How each language writes it

Common mistakes

for (auto x : v) x = 0;

int& first = v[0];
v.push_back(9);
std::cout << first;

int k = count["kiwi"];

Practice