如何在C中通过结构使用函数指针？

Question

我正在尝试使用函数指针和结构打印时间。它不会产生任何错误。它首先工作，但后来"Test.exe停止运行！“。

我的文件是: Random.c Random.h，Randomness.c Randomness.h，Test.c

Random.h

struct RANDOM {
    char* date;
    char* (*Date) (struct RANDOM*);
    void (*Write) (struct RANDOM*);
};
typedef struct RANDOM* Random;

Random CreateRandom();
char* DateOfNow(const Random);
void WriteDate(const Random);

Random.c

char* BringTime(){
    char* buff = malloc(sizeof(char)*100);
    time_t now = time(0);
    strftime(buff, 100, "%Y-%m-%d %H:%M",localtime(&now));

    return buff;
}

Random CreateRandom(){
    Random this;
    this = (Random) malloc(sizeof(struct RANDOM));  
    this->date = BringTime();

    return this;
}

char* DateOfNow(const Random this){
     return this->date;
}

void WriteDate(const Random this){
    printf("\n\n Date is: %s", this->date);
}

Randomness.h

struct RANDOMNESS{
    Random super;
};

typedef struct RANDOMNESS* Randomness;

Randomness CreateRandomness();

Randomness.c

Randomness CreateRandomness(){
    Randomness this;
    this = (Randomness)malloc(sizeof(struct RANDOMNESS));
    this->super = CreateRandom();

    return this;
}

Test.c

int main() {

    Randomness rnd = CreateRandomness();
    printf("works till here");
    rnd->super->Write(rnd->super);
}

输出为: works is here

在该输出之后，它将停止运行"Test.exe stops“。

我试过printf("%p", rnd->super)，它给了我地址。因此，Write(rnd->super)函数可能存在问题。

Answer 1

您必须将函数指针分配给结构中的成员字段：

Random CreateRandom(){
    Random this;
    this = (Random) malloc(sizeof(struct RANDOM));  
    this->date = BringTime();
    // assign function pointer to actual functions
    this->Date = &DateOfNow; 
    this->Write = &WriteDate;

    return this;
}

可以肯定的是，DateOfNow和WriteDate的原型应该在CreateRandom定义之前可用。

注意:您可以编写this->Date = DateOfNow; (没有&，因为带有函数标识符的&是一个剩余)。

Answer 2

您的create函数不完整：

Random CreateRandom(){
    Random this;
    this = (Random) malloc(sizeof(struct RANDOM));  
    // Content of the memory is undefined!
    this->date = BringTime();
    // What about Write() and Date()? <<<======= ERROR IS HERE
    return this;
}

...

Randomness CreateRandomness(){
    Randomness this;
    this = (Randomness)malloc(sizeof(struct RANDOMNESS));
    this->super = CreateRandom();

    return this;
}

...

int main() {

    Randomness rnd = CreateRandomness();
    printf("works till here");
    rnd->super->Write(rnd->super); // << using unspecified values is undefined behaviour.
}

您为date赋值，但不为Date和Write赋值。这意味着您在rnd->super中有一些有用的值，但是rnd->super->Write的内容是未定义的。

如果要将Create*函数用作一种构造函数，还必须正确设置函数指针。