指向一个结构到另一个？ （不是永久性的）

Question

我正在努力让一个结构指向另一个依赖于传入命令行的参数，问题是，结构我看起来正在指向所需的初始化结构但是，当我在函数调用后打印它们的地址时，在main中，如果播放器是A，则它看起来没有改变（在输出之后）：

Before initialise: 0x7f8a88403990, 0x7f8a884039f0, 0x7f8a88403a50, 0x7f8a88403ab0, 0x7f8a88403b10 
After initialise: 0x7f8a884039f0, 0x7f8a884039f0, 0x7f8a88403a50, 0x7f8a88403ab0, 0x7f8a88403b10 
After parse args: 0x7f8a88403990, 0x7f8a884039f0, 0x7f8a88403a50, 0x7f8a88403ab0, 0x7f8a88403b10 

int main (int argc, char *argv[]) { 

Player *me = NULL, *playerA = NULL; 
Player *playerB = NULL, *playerC = NULL, *playerD = NULL; 

me = malloc(sizeof(*me)); 
playerA = malloc(sizeof(*playerA)); 
playerB = malloc(sizeof(*playerB)); 
playerC = malloc(sizeof(*playerC)); 
playerD = malloc(sizeof(*playerD)); 

parse_args(me, playerA, playerB, playerC, playerD, argv); 

//should be pointing to the same memory location 
printf("After parse args: %p, %p, %p, %p, %p\n", me, playerA, playerB, playerC, playerD); 

} 

void parse_args(Player *me, Player *a, Player *b, Player *c, Player *d, 
      char *argv[]) { 
initialise_game(*tempChar, tempNum, me, a, b, c, d); 

} 

void initialise_game(char playerID, int numPlayers, Player *me, Player *a, 
       Player *b, Player *c, Player *d) { 
printf("Before initialise: %p, %p, %p, %p, %p\n", me, a, b, c, d); 

switch((int)playerID) { 
    case 'A': 
     me = a; 
     break; 
    case 'B': 
     me = b; 
     break; 
    case 'C': 
     if (numPlayers < 3) { 
      exit_prog(EXIT_PLAYERID); 
     } 
     me = c; 
     break; 
    case 'D': 
     if (numPlayers < 4) { 
      exit_prog(EXIT_PLAYERID); 
     } 
     me = d; 
     break; 
} 

printf("After initialise: %p, %p, %p, %p, %p\n", me, a, b, c, d); 

} 

Answer 1

指针通过值传递。这意味着你不能在函数体内修改它们（）。看到这个StackOverflow的question/answer

我想你想做的事是这样的：

void parse_args(Player **me, Player *a, Player *b, Player *c, Player *d, char *argv[]) { 
    initialise_game(*tempChar, tempNum, me, a, b, c, d); 
} 

void initialise_game(char playerID, int numPlayers, Player **me, Player *a, 
      Player *b, Player *c, Player *d) 
{ 
    printf("Before initialise: %p, %p, %p, %p, %p\n", me, a, b, c, d); 
    switch((int)playerID) { 
    case 'A': 
     *me = a; 
     break; 
    case 'B': 
     *me = b; 
    break; 
    case 'C': 
     if (numPlayers < 3) { 
      exit_prog(EXIT_PLAYERID); 
     } 
     *me = c; 
     break; 
    case 'D': 
     if (numPlayers < 4) { 
      exit_prog(EXIT_PLAYERID); 
     } 
     *me = d; 
     break; 
} 

您对parse_args通话将需要一个指针传递给一个指针对我来说：

parse_args(&me, playerA, playerB, playerC, playerD, argv); 

Answer 2

您需要使用指向函数参数结构的指针（例如，Player **x）。使用指向结构的指针只能修改结构对象，而不能修改指针对象。记住所有的函数参数都是通过C中的值传递的，如果你修改函数中的指针参数，你只是修改指针的一个副本。