C/C++ new and declare

發表於
文章字數： 2k 所需閱讀時間 ≈ 4 分鐘

C/C++ new and declare

  • new int 的用法
  • new int 一維陣列
  • new int 二維陣列
  • new struct 範例
  • new struct 一維陣列範例

new

  • 以下是動態記憶體配置一個 int 的範例
    1
    2
    3
    int *p = new int;
    int *p = new int();
    int *p = new int(5);
  • 使用 new 就要配合 delete
    • delete p;

declare v.s. definition

宣告(Declaration)

  • 說明變數的存在和其型態。
  • 特性:
    • 一個變量可能有一次以上的宣告。
  • 範例 :
    • 包含extern，且未進行初始化的變量。
    • 非函式主體的函式宣告。
    • 類別宣告內的static成員，無法在類別宣告內定義及初始化，而需在類別外。
      1
      2
      3
      4
      5
      6
      7
      8
      9
      10
      11
      12
      13
      14
      15
      16
      17
      18
      19
      20
      21
      22
      #include <string>

      int f(int i); // forward declaration

      int main()
      {
          const double pi = 3.14; //OK
          int i = f(2); //OK. f is forward-declared
          std::string str; // OK std::string is declared in <string> header
          C obj; // error! C not yet declared.
          j = 0; // error! No type specified.
          auto k = 0; // OK. type inferred as int by compiler.
      }

      int f(int i)
      {
          return i + 42;
      }

      namespace N {
         class C{/*...*/};
      }

      定義(Definition)

  • 分配Memory。
  • 特性:
    • 定義同時也會進行宣告。
    • 一個變量只能有一次定義。
  • 範例 :
    • 不包含extern的變量。
    • 初始化的變量。
    • 函式主體。
      1
      2
      3
      4
      5
      int i;
      extern int j = 3; 
      void dosomething() {
          cout << "hello world" << endl;
      }

with braces

1
2
3
4
int main() {
  int a = 20;
  cout << "The value of variable a is "<< a << endl;
}
  • The code above shows how curly braces can be used to declare various types of variables and assign values to them.
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    int a{20};
    cout << "The value of variable a is: "<< a << endl;

    string b{"This is a string"};
    cout << "The value of variable b is: "<< b << endl;

    std::vector<int> c{10, 5};
    cout<< "The values of variable c are: " << endl;
    ````
    - Using curly braces to initialize a variable also prevents narrowing.
        - **Narrowing**, or more precisely narrowing conversion, is the implicit conversion of arithmetic values that includes​ a loss of accuracy.
        ```clike
        int myint(3.14); 
        // output: 3
        std::cout << "myint: " << myint << std::endl;

        // complile error
        int myint1{3.14}; 
        std::cout << "myint: " << myint1 << std::endl;

reference