C Cpp programming

Generic pair in C99

  typedef struct pairX{
  void* x;
  void* y;
  } pairX;

  pairX* pt = (pairX*)malloc(sizeof(pairX));
  int a = 1;
  int b = 4;
  pt -> x = (int*)&a;
  pt -> y = (int*)&b;

  printf("x=%d\n", *(int*)(pt -> x));
  printf("y=%d\n", *(int*)(pt -> y));
  
  free(pt);

Assign an array to void* x

  typedef struct pairX{
  void* x;
  void* y;
  } pairX;

  const int len = 4;
  pairX* pt = (pairX*)malloc(sizeof(pairX));
  int arr[len] = {1, 2, 3, 4};   // DO NOT need to free this
  int b = 10;
  pt -> x = (int*)arr;
  pt -> y = (int*)&b;
  print_array_int(pt -> x, len, len);
  printf("pt -> y = %d\n", *(int*)(pt -> y));
  
  free(pt);

return a struct from a function

  typedef struct pair{
	int x;
	int y;
  } pair;

  pair fun(int a, int b){
	pair p;
	p.x = a;
	p.y = b;
	return p;
  }

const int* pt and int* const pt

  const int* pt;
  pt = (int*) malloc(sizeof(int) * 4);
  pt[0] = 10;  // ERROR
  
  int arr[4] = {10, 20, 30, 40};
  pt = arr;    // OK

  void fun(const int* pt){
  }

  int arr[4];
  fun(arr);  // OK

  int* const pt;
  pt += 1;   // ERROR

char, unsigned char, signed char in C

char type in C is 8 bits, integer type, it has minimum range from 0 to 127 char can be unsigned or signed unsigned char range: 0000 0000 → 1111 1111 → 0x00 0xFF → 0 to 2^8 ⇒ 0 to 255 signed char range: 0000 0000 → 1xxx xxxx → 0xxx xxxx → [-2⁷ → -1] [0 → 2⁷ - 1]

Conver integer, float to string

  const int size = 100;
  char buf[size];
  int num = 10;
  float pi = 3.1415;
  snprintf(buf, size, "%d-%f", num, pi);

C struct

  struct my_stuff_{
        int n;
        char name[100];
  } my_stuff_;
  typedef struct my_stuff_ my_stuff;


  my_stuff* pt = (my_stuff*)malloc(sizeof(my_stuff));
  pt -> n = 10;
  strcpy(pt -> name, "abcde fghij klmno pqrst uwwxy z");
		
  free(pt);

Pass C function as argument to a function


int myfun(int(*funpt)(int, int)){
    int sum = 0;
    for(int i = 0; i < 3; i++){
        sum += (*funpt)(i, i);
    }
    return sum;
}

int add(int a, int b){
    return a + b;
}

int num = myfun(add);

// how to pass paramter from outside to add
// int a=1, b=2;  add(a, b)

  
funpt is function pointer, (*funpt) is a function which have two parameters: int, int and return type is int. myfun only accept function pointer has the same signature, otherwise there is compiling error.

Pass function to a function pointer

In order to assign a function to other function pointer. *funpt and add both have to have the same signature.


int(*funpt)(int, int);

int addMe(int a, int b){
    return a + b;
}

funpt = &addMe;
printf("num=%d\n", (*funpt)(1, 2));
printf("funpt=x%x\n", funpt);
printf("&add=x%x\n", &addMe);

C++ unique_ptr, C++ Smart pointer

  template<typename T1, typename T2>
    class MyTuple{
    public:
      T1 x;
      T2 y;
    public:
    MyTuple<T1, T2>(){
    }
    MyTuple<T1, T2>(T1 t1, T2 t2){
      x = t1;
      y = t2;
    }
  };

  unique_ptr<MyTuple<int, int>> tuplept(new MyTuple<int, int>(1, 2));
  cout<<"x="<<tuplept -> x<<endl;
  cout<<"y="<<tuplept -> y<<endl;