How to reduce the space complexity of C++ programs?

In order to reduce the space complexity of C++ programs, you can take the following methods: delete unnecessary variables and release them. Use references and pointers to access variables without copying the contents. Use dynamic memory allocation to allocate the required amount of memory at runtime. Use smart pointers to automatically manage dynamically allocated memory.

Reducing the space complexity of a C++ program

Space complexity measures how much memory a program consumes during runtime. In C++, the space complexity of the program can be reduced by the following methods:

1. Delete unnecessary variables
Determine which variables are necessary and release them as early as possible Unnecessary variable. For example:

int main() {
  int x = 5; // 必需
  {
    int y = 10; // 不必需
    // 使用 y
    y = 0;
  }  // y 退出作用域，释放内存
  // 使用 x
  x = 10;
  return 0;
}

2. Using references and pointers
References and pointers provide a way to access a variable without copying its contents. This can save a lot of memory, especially for large objects. For example:

void swap(int& a, int& b) {
  int temp = a;
  a = b;
  b = temp;
}

int main() {
  int x = 5;
  int y = 10;
  swap(x, y);  // 交换 x 和 y 的内容，不复制
  return 0;
}

3. Use dynamic memory allocation
When you don't know the exact size of the data, you can use dynamic memory allocation (using the new operator ). This allows you to allocate the required amount of memory at runtime. For example:

int* arr = new int[100];  // 分配 100 个整数的数组
// 使用 arr
delete[] arr;  // 释放数组的内存

4. Use smart pointers
Smart pointers automatically manage dynamically allocated memory, and there is no need to manually call delete when releasing memory. For example:

std::unique_ptr<int> ptr = std::make_unique<int>(5);  // 创建智能指针
// 使用 *ptr
ptr.reset();  // 智能指针自动释放内存

Practical example:

Consider an application that tracks strings entered by the user. To save memory, you can use std::map แทわりに std::vector. std::map is an associative container that stores data using key-value pairs, where keys can be used to find and retrieve values ​​efficiently.

std::map<std::string, int> str_counts;  // 存储字符串和它们出现次数
while (true) {
  std::string str;
  std::cin >> str;
  if (!str.empty()) {
    ++str_counts[str];
  } else {
    break;
  }
}

By using std::map, the application only stores a unique copy of the string and keeps track of the number of occurrences of each string. This saves memory over using std::vector to store multiple copies of a string.

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