用c++实现使用模板和动态内存分配实现可变长数组

您好！根据您的要求，我为您编写了以下的可变长数组类模板代码：

#include <iostream>
#include <algorithm>

template <typename T>
class DynamicArray {
public:
    DynamicArray() : size_(0), capacity_(kInitCapacity), data_(new T[kInitCapacity]) {}
    ~DynamicArray() { delete[] data_; }

    void push_back(const T& value) {
        if (size_ == capacity_) {
            reserve(capacity_ * 2);
        }
        data_[size_++] = value;
    }

    void pop_back() {
        if (size_ > 0) {
            size_--;
        }
    }

    T& operator[](const int index) { return data_[index]; }

    const T& operator[](const int index) const { return data_[index]; }

    int size() const { return size_; }

    int capacity() const { return capacity_; }

    void reserve(const int new_capacity) {
        if (new_capacity > capacity_) {
            capacity_ = new_capacity;
            T* new_data = new T[capacity_];
            std::copy(data_, data_ + size_, new_data);
            delete[] data_;
            data_ = new_data;
        }
    }

private:
    static const int kInitCapacity = 8;
    int size_;
    int capacity_;
    T* data_;
};

int main() {
    DynamicArray<int> arr;
    for (int i = 0; i < 10; ++i) {
        arr.push_back(i);
    }
    std::cout << "size: " << arr.size() << ", capacity: " << arr.capacity() << std::endl;
    for (int i = 0; i < arr.size(); ++i) {
        std::cout << arr[i] << " ";
    }
    std::cout << std::endl;
    arr.pop_back();
    std::cout << "size: " << arr.size() << ", capacity: " << arr.capacity() << std::endl;
    for (int i = 0; i < arr.size(); ++i) {
        std::cout << arr[i] << " ";
    }
    std::cout << std::endl;
    arr.reserve(20);
    std::cout << "size: " << arr.size() << ", capacity: " << arr.capacity() << std::endl;
    for (int i = 0; i < arr.size(); ++i) {
        std::cout << arr[i] << " ";
    }
    std::cout << std::endl;

    return 0;
}

这个可变长数组类模板中，我们使用了动态内存分配和模板技术，实现了能够动态扩容的可变长数组类。在实现中，我们使用了 `new` 和 `delete` 运算符来实现动态内存分配和释放，使用了 `std::copy()` 函数来实现数组元素的拷贝，使用了 `reserve()` 函数来实现动态扩容。同时，我们还重载了 `[]` 运算符，实现了数组元素的访问。