Void pointers

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本文介绍了如何通过使用C++的void指针来创建通用的接口调用，允许函数调用在不同的数据类型之间进行转换。通过示例代码演示了如何在不同的函数中灵活地使用void指针，实现对不同类型参数的处理，并在讨论结构体部分进一步展示其应用。这一技巧展示了void指针在避免类型特定依赖性和提高代码复用性方面的强大能力。

At first glance, a void pointer seems to be of limited, if any, use. However, when combinedwith the ability to cast such a pointer to anothertype, they turn out to be quite useful and flexible.

Consider the example of the previous section, where weconstructed a function pointer to a function oftypevoid and argumentint. Such a functionpointer in this form could not be used for avoid functionwith a different type of argument (for example,float).This can be done, however, through the use ofvoidpointers, as the following example illustrates.

#include <stdio.h>
void use_int(void *);
void use_float(void *);
void greeting(void (*)(void *), void *);

int main(void) {
  char ans;
  int i_age = 22;
  float f_age = 22.0;
  void *p;
  printf("Use int (i) or float (f)? ");
  scanf("%c", &ans);
  if (ans == 'i') {
    p = &i_age;
    greeting(use_int, p);
  }
  else {
    p = &f_age;
    greeting(use_float, p);
  }
  return 0;
}
void greeting(void (*fp)(void *), void *q) {
  fp(q);
}
void use_int(void *r) {
  int a;
  a = * (int *) r;
  printf("As an integer, you are %d years old.\n", a);
}
void use_float(void *s) {
  float *b;
  b = (float *) s;
  printf("As a float, you are %f years old.\n", *b);
}

Although this requires us to cast the void pointer intothe appropriate type in the relevant subroutine( use_int or use_float), the flexibilityhere appears in the greeting routine, which cannow handle in principle a function with any type ofargument. This will especially become apparent whenwe discuss structures in the next section.

http://theoryx5.uwinnipeg.ca/programming/node87.html

http://www.learncpp.com/cpp-tutorial/613-void-pointers/

http://stackoverflow.com/questions/2589214/problem-using-void-pointer-as-a-function-argument

http://bytes.com/topic/c/answers/351539-what-use-void-pointer