Interview questions - C

char str[] = "ABCDE"; // sizeof(str): 6 char *p = str; // sizeof(p): 4 char q = *str; // sizeof(q): 1 int n = 10; // sizeof(n): 4 int *ptr = &n; // sizeof(ptr): 4 ; sizeof(*ptr): 4

char的pointer之size是4 (因為存的是位置)

char本身是1 (存的是資料)

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int arr[] = {6, 7, 8, 9, 10}; int *ptr = arr; *ptr++ += 123; *++ptr += 123; int *p = arr; int *q = arr + sizeof(arr)/sizeof(*arr); while( p != q ) cout << *p++ << " "; //129 7 131 9 10

*ptr++

等價於

*(ptr++)

compound assignment operator:

將 right-hand operand 加到左邊, 再將結果存入 left-hand operand
(assignment是最後做!)

*ptr++ += 123;

左邊取值6 (位置仍在6), 將123 + 6 = 129 assign到 6 的位置, 完成後 ptr 前進到 7 的位置

*++ptr += 123;

左邊先前進到8的位置, 取值8, 將123 + 8 = 131 assign到 8 的位置

注意這兩個意義不一樣:

*ptr++ += 123;

*ptr++ = *ptr++ + 123;

後者是 Undefined behavior

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int* arr1[8]; int (*arr2)[8]; int *(arr3[8]); printf("size of arr1 = %d\n", sizeof(arr1)); // 32 printf("size of arr2 = %d\n", sizeof(arr2)); // 4 printf("size of arr3 = %d\n", sizeof(arr3)); // 32

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union{ int num; unsigned char n[4]; }q; q.num = 10; for( int i = 0 ; i< 4 ; i++) printf("%x ", q.n[i]); // a 0 0 0

此片段可用來判斷機器是little-endian

10 => 0 0 0 a 

n      ->a

n+1  ->0

n+2  ->0

n+3  ->0

(由尾端開始放入memory, 記得前題都是記憶體位置 低到高/小到大/index 0到n)

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#include <iostream> using namespace std; class Animal{ public: Animal() { cout << "Animal" << endl; } ~Animal() { cout << "~Animal" << endl; } void move() { cout << "Animal move" << endl; } virtual void eat() { cout << "Animal eat" << endl; } }; class Dog: public Animal{ public: Dog() { cout << "Dog" << endl; } ~Dog() { cout << "~Dog" << endl; } void move() { cout << "Dog move" << endl; } void eat() { cout << "Dog eat" << endl; } }; int main( ){ { Dog dog; // Animal // Dog Animal *pAnimal = &dog; Dog *pDog = &dog; pAnimal->eat(); // Dog eat pAnimal->move(); // Animal move pDog->eat(); // Dog eat pDog->move(); // Dog move } // ~Dog // ~Animal { Dog *pDog = new Dog(); // Animal // Dog Animal *pAnimal = pDog; pAnimal->eat(); // Dog eat pAnimal->move(); // Animal move pDog->eat(); // Dog eat pDog->move(); // Dog move delete pAnimal; // ~Animal } }

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cout << sizeof(short int) << endl; // 2 cout << sizeof(unsigned short int) << endl; // 2 cout << sizeof(unsigned int) << endl; // 4 cout << sizeof(int) << endl; // 4 cout << sizeof(long int) << endl; // 4 cout << sizeof(signed char) << endl; // 1 cout << sizeof(unsigned char) << endl; // 1 cout << sizeof(float) << endl; // 4 cout << sizeof(double) << endl; // 8 cout << sizeof(long double) << endl; // 12

short int = short

long int = long