[英]How to use two functions that return different types as a function pointer in C?
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。 这个对吗? 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。 为了澄清,我想将populateMonsters
或populateClassesTaken
传递给create
并能够在create
内调用它们。
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);
}
首先,不用说您不能编写像ClassesTaken *c = NULL; c->classID = idC;
这样的代码ClassesTaken *c = NULL; c->classID = idC;
ClassesTaken *c = NULL; c->classID = idC;
。 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,如下所述:将数据复制/扫描/读取到未初始化的指针时发生崩溃或“分段错误”
至于不同的功能-它们是不同的功能类型这一事实清楚地表明此设计没有意义。 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);
其中fp是函数指针,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parenthis ()
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。)
更换:
node* create(node* next, void *fp)
{
与:
node* create(node* next, void* (*fp)())
{
和
create(tmp, *fp);
与
create(tmp, (void*)fp); //[updated]
您的populateMonsters()导致分段错误。 我将其代码替换为fputs("In populateMonsters\\n", stderr);
它似乎起作用:
$ ./每年
在populateMonsters中
populateMonsters()
和populateClassesTaken()
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
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