如何编写一个方法来在运行时确定 typedef 类型? [英] How can I write a method to determine the typedef type at runtime?

问题描述

我有一个二叉搜索树，我想用不同的类型实现它.二叉搜索树是模板化的，并使用以下语句确定:

I have a binary search tree that I want to implement with different types. The binary search tree is templated and is determined using the following statement:

typedef desiredType TreeItemType; // desired type of tree items i.e. string, int, Person

通常我只会将字符串树的所需类型更改为字符串，但是如果我还想创建整数树怎么办?我想我需要制定一种在运行时确定 typdef 类型的方法，但我不知道如何开始，因为 desiredType 可以是各种可变对象类型.有什么想法吗?

Normally I would just change the desiredType to string for a tree of strings but what if I want to also make a tree of integers? I'm thinking I need to make a method for determining the typdef type at runtime but I don't know to begin because desiredType can be a variety of variable object types. Any ideas?

推荐答案

很难从一行说出来，但看起来您可以使用 C 预处理器命令喜欢

It is hard to tell from one line, but it looks like you could use a C Preprocessor command like

#define saveDesiredType desiredType    // save previous setting
#define desiredType char*                 // change to type   

... <code>

#define  desiredType saveDesiredType  // restore previous setting

但是，我认为您只能在模块(目标文件 .o)中定义一次特定的 typedef.

however, I think you may only define a particular typedef once in a module (object file .o).

我知道在 C 中创建可行的变量类型树结构的唯一方法是转到全指针操作模型，或者将类型作为额外参数添加到树函数中，并对不同的指针进行所有的数学运算类型的大小.
稍微以对象为中心的方法是将数据封装在 tree_node 结构中带有类型数据.

The ONLY way I know to create workable tree structures of variable types in C is to go to an all pointer manipulation model, or add the type as an extra parameter to the tree functions, and do ALL the pointer math for the different sizes of the types.
A slightly more object centric approach woul be to encapsulate your data in a tree_node struct with type data.

typedef enum D_Type { ANINT , AFLOAT, ADOUBLE, ACHAR, ASTRING, OTHER} DATATYPE; 



typedef struct t_node{
DATATYPE dtype;
union {   // union is ONE of the following types, and Only one.
       int i;    // size of the union is the size of the largest type. 
       float f;
       double d;
       char c;
       char* string;
      }   // this union is unnamed , but could have a name.
} Tree_Node;

typedef Tree_Node* TreeItem;  //pass by reference

在您的代码中，您必须打开 node->dtype 并仅使用该类型的变量.

In your code you must switch on node->dtype and work only with the variable of that type.

void tree_add (Tree T, TreeItem item)
{
  int i;
  float f;
  double d;
  char c;
  char* s;

  switch (item->dtype){
    case ANINT:
      i = item->i;
      break;
    case AFLOAT:
      f = item->f;
      break;
    case ADFLOAT:
      d = item->d;
      break;
    ...
   }//switch
    ...<code>
}//tree_add

double Pi = 3.141592653589793238;
TreeItem ti = ( TreeItem ) malloc (sizeof(Tree_Node) ); // struct Must be allocated
ti->dtype = ADOUBLE;
ti->d = Pi; 
ti->s = Pi; /* OOPS! error. (might go through without being detected -- depending on compiler) */ 
tree_add( atree , ti);

ti->s = "Now is the time for all good programmers to come the aid of their computers."; 
 /* OOPS! error. undefined behavior what about the double d? 
 (this might go through without being detected -- depending on compiler ) 
  but ti->dtype was not changed either so code will break. */

(看起来像工作，不是吗?)

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