C++ STL Patterns
Container Selection
| Need |
Container |
Notes |
| Dynamic array |
std::vector |
Cache-friendly, O(1) amortized push_back |
| Sorted unique keys |
std::set |
O(log n), ordered |
| Fast lookup |
std::unordered_map |
O(1) avg, needs hashable key |
| FIFO |
std::queue |
deque-backed |
| Priority queue |
std::priority_queue |
max-heap by default |
| Fixed-size |
std::array<T,N> |
Stack-allocated, zero overhead |
Algorithms
#include <algorithm>
#include <numeric>
#include <ranges>
std::vector<int> v = {5, 3, 1, 4, 2};
std::ranges::sort(v);
std::ranges::sort(v, std::greater{}); // descending
auto it = std::ranges::find(v, 3);
bool found = std::ranges::binary_search(v, 3);
int sum = std::reduce(v.begin(), v.end(), 0);
// C++20 erase_if
std::erase_if(v, [](int x) { return x % 2 == 0; });
C++20 Ranges / Views (Lazy)
auto result = nums
| std::views::filter([](int x) { return x % 2 == 0; })
| std::views::transform([](int x) { return x * x; })
| std::views::take(3);
auto reversed = nums | std::views::reverse;
auto iota = std::views::iota(1, 11); // 1..10
auto keys = mymap | std::views::keys;
String Patterns
// string_view for read-only args (zero copy)
void process(std::string_view sv) {
auto pos = sv.find('.');
auto ext = sv.substr(pos + 1);
}
// C++20 formatting
#include <format>
auto msg = std::format("User {} at {}", name, ts);
Custom Hash for Unordered Containers
struct Point { int x, y; };
struct PointHash {
std::size_t operator()(const Point& p) const {
return std::hash<int>{}(p.x) ^ (std::hash<int>{}(p.y) << 1);
}
};
std::unordered_map<Point, int, PointHash> grid;
Performance Tips
v.reserve(1000); // avoid reallocations
v.emplace_back(42); // construct in-place
// find before [] to avoid unintended insert
if (auto it = m.find(key); it != m.end()) use(it->second);
// Sort by member projection (C++20)
std::ranges::sort(data, {}, &Pair::first);
Mattakushi432