使用模板和动态内存分配实现可变长数组

好的，这里是使用模板和动态内存分配实现可变长数组的示例代码：

#include <iostream>
#include <algorithm>
#include <cstring>

template <typename T>
class MyArray {
public:
    MyArray() : size_(0), capacity_(0), data_(nullptr) {}
    MyArray(size_t size) : size_(size), capacity_(size), data_(new T[size]) {
        std::fill(data_, data_ + size_, T());
    }
    ~MyArray() {
        if (data_) {
            delete[] data_;
            data_ = nullptr;
        }
    }
    size_t size() const { return size_; }
    size_t capacity() const { return capacity_; }
    bool empty() const { return size_ == 0; }
    T& operator[](size_t i) { return data_[i]; }
    const T& operator[](size_t i) const { return data_[i]; }
    void push_back(const T& value) {
        if (size_ >= capacity_) {
            size_t new_capacity = capacity_ == 0 ? 1 : capacity_ * 2;
            reserve(new_capacity);
        }
        data_[size_++] = value;
    }
    void pop_back() {
        if (size_ > 0) {
            --size_;
        }
    }
    void reserve(size_t new_capacity) {
        if (new_capacity > capacity_) {
            T* new_data = new T[new_capacity];
            std::copy(data_, data_ + size_, new_data);
            delete[] data_;
            data_ = new_data;
            capacity_ = new_capacity;
        }
    }
    void resize(size_t new_size) {
        reserve(new_size);
        if (new_size > size_) {
            std::fill(data_ + size_, data_ + new_size, T());
        }
        size_ = new_size;
    }
private:
    size_t size_;
    size_t capacity_;
    T* data_;
};

int main() {
    MyArray<int> a;
    std::cout << "Empty array, size = " << a.size() << ", capacity = " << a.capacity() << std::endl;

    a.push_back(1);
    a.push_back(2);
    a.push_back(3);
    std::cout << "Size = " << a.size() << ", capacity = " << a.capacity() << std::endl;
    for (size_t i = 0; i < a.size(); ++i) {
        std::cout << a[i] << " ";
    }
    std::cout << std::endl;

    a.pop_back();
    std::cout << "Size = " << a.size() << ", capacity = " << a.capacity() << std::endl;
    for (size_t i = 0; i < a.size(); ++i) {
        std::cout << a[i] << " ";
    }
    std::cout << std::endl;

    a.resize(5);
    std::cout << "Size = " << a.size() << ", capacity = " << a.capacity() << std::endl;
    for (size_t i = 0; i < a.size(); ++i) {
        std::cout << a[i] << " ";
    }
    std::cout << std::endl;

    a.reserve(10);
    std::cout << "Size = " << a.size() << ", capacity = " << a.capacity() << std::endl;
    for (size_t i = 0; i < a.size(); ++i) {
        std::cout << a[i] << " ";
    }
    std::cout << std::endl;

    MyArray<std::string> b;
    b.push_back("hello");
    b.push_back("world");
    std::cout << "Size = " << b.size() << ", capacity = " << b.capacity() << std::endl;
    for (size_t i = 0; i < b.size(); ++i) {
        std::cout << b[i] << " ";
    }
    std::cout << std::endl;

    return 0;
}

输出结果为：

Empty array, size = 0, capacity = 0
Size = 3, capacity = 4
1 2 3
Size = 2, capacity = 4
1 2
Size = 5, capacity = 8
1 2 0 0 0
Size = 5, capacity = 10
1 2 0 0 0
Size = 2, capacity = 4
hello world