SmallVector is a static length-optimized dynamic buffer

smallvec.cpp

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <new>
using namespace std;

template <typename T, size_t SmallSizeThreshold>
class SmallVector {
public:
    SmallVector() = default;

    template <typename... Args>
    T& emplace_back(Args&&... args) {
        if (size() < SmallSizeThreshold)
            return *new (&stack[length++]) T(std::forward<Args>(args)...);
        
        if (!dyArray) {
            capacity = SmallSizeThreshold;
            dyArray = new std::aligned_storage_t<sizeof(T), alignof(T)>[capacity];
            
            for (size_t i = 0; i < length; i++)
                *std::launder(reinterpret_cast<T*>(&dyArray[i])) = std::move(*std::launder(reinterpret_cast<T*>(&stack[i])));
        }
        
        if (size() >= capacity) {
            capacity *= 2;
            auto temp = new std::aligned_storage_t<sizeof(T), alignof(T)>[capacity];
            
            for (size_t i = 0; i < length; i++)
                *std::launder(reinterpret_cast<T*>(&temp[i])) = std::move(*std::launder(reinterpret_cast<T*>(&dyArray[i])));
            
            delete[] dyArray;
            dyArray = temp;
        }
        
        return *new (&dyArray[length++]) T(std::forward<Args>(args)...);
    }

    void pop_back() {
        if (dyArray)
            std::destroy_at(std::launder(reinterpret_cast<T*>(&dyArray[--length])));
        else
            std::destroy_at(std::launder(reinterpret_cast<T*>(&stack[--length])));
    }

    T& operator[](size_t idx) {
        if (dyArray)
            return *std::launder(reinterpret_cast<T*>(&dyArray[idx]));
        return *std::launder(reinterpret_cast<T*>(&stack[idx]));
    }

    size_t size() const {
        return length;
    }
    
private:
    std::aligned_storage_t<sizeof(T), alignof(T)> stack[SmallSizeThreshold];
    std::aligned_storage_t<sizeof(T), alignof(T)>* dyArray = nullptr;
    size_t length = 0;
    size_t capacity = SmallSizeThreshold;
};