Question

我有一个create函数，我想传递一个函数指针，但是我希望该函数指针能够调用以下我遇到的问题之一，即每个函数都返回不同的类型。但是，当我传递函数指针时，我必须指定函数指针的返回类型。它是否正确？我认为在中使用空指针将解决返回类型问题，但是现在我无法调用函数指针。为了明确起见，我想将populateMonsters或populateClassesTaken传递到create中，并能够在create中调用它们。

typedef struct node{
   void * data;
   struct node* next;
} node;

typedef struct MonsterAttacks{
    unsigned int attackID;
    char monsterName[41];
    char attackLocation[41];
    unsigned int numOfVictims;
}MonsterAttacks;

typedef struct ClassesTaken{
    unsigned int classID;
    char semester[41];
    unsigned int year;
    unsigned int numberOfUnits;
    char grade[10];
}ClassesTaken;


  unsigned int idM = 1;

  MonsterAttacks * populateMonsters(){

    MonsterAttacks *m = NULL;

    m->attackID = idM;
    printf("Enter the name for the Monster \n");
    scanf("%40s",m->monsterName);
    puts("What is his/her attack location?");
    scanf("%40s",m->attackLocation);
    puts("What are the number of victims this monster has demolished?");
    scanf("%ud", &m->numOfVictims);      
    //attackID is assigned and id when node is created
    idM++;
    return m;
}

  unsigned int idC = 1;

  ClassesTaken* populateClassesTaken(){

    ClassesTaken *c = NULL;
    c->classID = idC;
    scanf("What semester was the class taken in? %40s",c->semester);
    scanf("In what year? %ud", &c->year);  
    scanf("How many units was the class? %ud", &c->numberOfUnits);
    scanf("What was your grade in the class? %9s", c->grade);     


    idC++;
    return c;
}

node* create(node* next, void *fp)
{
   node* new_node = (node*)malloc(sizeof(node));
   if(new_node == NULL)
   {
       printf("Error creating a new node.\n");
       exit(0);
   }
    void *data = (*fp)();
}



int main(void)
{

   node* tmp = NULL;
   MonsterAttacks* (*fp)() = &populateMonsters;

   create(tmp, *fp);
}

Answer 1

首先，毋庸置疑，您不能编写像ClassesTaken *c = NULL; c->classID = idC;这样的代码。如果您不明白为什么不能这样做，则需要重新学习指针的基本知识，如此处所述：Crash or "segmentation fault" when data is copied/scanned/read to an uninitialized pointer

对于不同的功能-它们是不同的功能类型这一事实清楚表明，这种设计没有意义。如果希望使用通用函数指针，则需要定义统一的函数格式，例如void* func (void)。

您不能将void*用作通用函数指针。它仅定义为用于指向对象的指针，而不用于函数。诸如create(tmp, *fp);之类的代码，其中fp是函数指针，该函数期望void*不是有效的标准C，并且不应编译，除非您使用的是非标准编译器扩展。

值得注意的是，在C语言中，绝对不要声明带空括号()的函数。这是一种过时的语言功能。（与C ++不同，这很好并且值得鼓励。）

Answer 2

替换：

BestModel.set_weights(Model.get_weights())

具有：

node* create(node* next, void *fp)
{

和

node* create(node* next, void* (*fp)())
{

使用

create(tmp, *fp);

您的populateMonsters（）导致分段错误。我将其代码替换为create(tmp, (void*)fp); //[updated]

它似乎起作用：

$ ./a.out

在populateMonsters中

Answer 3

populateMonsters()和populateClassesTaken()中仍然存在问题。您想用用户输入来填充结构，但您永远不要分配结构（既不在堆上也不在堆栈上）。因此，您会遇到细分错误。这是我的没有typedef的代码，包括...

typedef void*(*myFunctionInterface)(void);

MonsterAttacks * populateMonsters (){

  MonsterAttacks *m = (MonsterAttacks*)malloc(sizeof(MonsterAttacks));
  if (m == NULL){
      printf("Error creating MonsterAttacks");
      exit (-1);
  }

  m->attackID = idM;
  printf ("Enter the name for the Monster \n");
  scanf ("%40s", m->monsterName);
  puts ("What is his/her attack location?");
  scanf ("%40s", m->attackLocation);
  puts ("What are the number of victims this monster has demolished?");
  scanf ("%ud", &m->numOfVictims);
  //attackID is assigned and id when node is created
  idM++;
  return m;
}

unsigned int idC = 1;

ClassesTaken * populateClassesTaken (){

  ClassesTaken *c = (ClassesTaken*)malloc(sizeof(ClassesTaken));
  if (c == NULL){
      printf("Error creating ClassesTaken");
      exit(-2);
  }
  c->classID = idC;
  scanf ("What semester was the class taken in? %40s", c->semester);
  scanf ("In what year? %ud", &c->year);
  scanf ("How many units was the class? %ud", &c->numberOfUnits);
  scanf ("What was your grade in the class? %9s", c->grade);


  idC++;
  return c;
}

node * create (node * next, void *(*fp) ()){
  node *new_node = (node *) malloc (sizeof (node));
  if (new_node == NULL)
    {
      printf ("Error creating a new node.\n");
      exit (-3);
    }
  void *data = fp();
  new_node->data = data;
  return new_node;
}



int main (void) {    
  node *tmp = NULL;
  MonsterAttacks *(*fp_M) () = &populateMonsters;
  ClassesTaken *(*fp_C) () = &populateClassesTaken;
  create (tmp, (myFunctionInterface)fp_M);
}

如果出现错误，则返回0也不是一个好的样式。 0通常表示一切正常。因此，我将退出值更改为一些不同的负数以区分错误

如何在C中使用两个返回不同类型的函数作为函数指针？

3 个答案: