dynamic_array.cpp

#include <iostream>
#include <algorithm>
using namespace std;

template<typename T>
class dynamic_array{
private:
    T *arr;
    size_t sz, cap;
    void set_capacity(size_t new_cap){
        T *new_arr = new T[new_cap];
        for(int i=0; i<min(sz, new_cap); i++) new_arr[i] = arr[i];
        delete[] arr;
        arr = new_arr;
        cap = new_cap;
    }
public:
    dynamic_array(){
        sz = 0;
        set_capacity(0);
    }
    void push_back(const T x){
        if(sz == cap) set_capacity(cap == 0 ? 1 : cap * 2);
        arr[sz++] = x;
    }
    void pop_back(){
        sz--;
        if(sz == 0 || sz * 4 <= cap) set_capacity(cap / 2);
    }
    void resize(size_t new_sz){
        if(new_sz > sz){
            if(new_sz > cap) set_capacity(max(new_sz, cap*2));
            while(sz < new_sz) arr[sz++] = T();
        }
        else if(new_sz < sz){
            if(new_sz * 4 <= cap) set_capacity(min(new_sz*2, cap/2));
            sz = new_sz;
        }
    }
    size_t size() const { return sz; }
    size_t capacity() const { return cap; }
    T& operator [] (const size_t index) { return arr[index]; }
    const T& operator [] (const size_t index) const { return arr[index]; }
    T& front() { return arr[0]; }
    const T& front() const { return arr[0]; }
    T& back() { return arr[sz-1]; }
    const T& back() const { return arr[sz-1]; }
    T* begin() { return arr; }
    const T* begin() const { return arr; }
    T* end() { return arr + sz; }
    const T* end() const { return arr + sz; }
};

template<typename T>
void print_array(const dynamic_array<T> &v){
    for(auto i : v) cout << i << " ";
    cout << "\n";
}

void vectorTest(){
    dynamic_array<int> vec;
    for(int i=0; i<10; i++) vec.push_back(i);
    cout << "size / capacity : " << vec.size() << " / " << vec.capacity() << "\n";
    cout << "front / back : " << vec.front() << " / " << vec.back() << "\n";
    vec.pop_back();
    cout << "back(after pop_back) : " << vec.back() << "\n";

    vec.resize(5);
    cout << "resize(5) : ";
    print_array(vec);

    vec.resize(25);
    cout << "resize(25) : ";
    print_array(vec);

    for(int i=0; i<20; i++) vec.pop_back();

    cout << "size / capacity(after pop_back*20) : " << vec.size() << " " << vec.capacity() << "\n";

    vec[0] = 10; vec[1] = 5; vec[2] = 9; vec[3] = 1; vec[4] = 7;

    cout << "before sort : ";
    print_array(vec);

    sort(vec.begin(), vec.end());
    cout << "after sort : ";
    print_array(vec);
}

int main(){
    vectorTest();
}