你如何在C中实现一个类？ [英] How do you implement a class in C?

问题描述

假设我必须使用C（没有C ++或面向对象的编译器），并且我没有动态内存分配，我可以使用一些技术来实现一个类，还是一个类的很好的近似？将类隔离到单独的文件总是一个好主意？假设我们可以通过假设固定数量的实例来预分配内存，或者甚至在编译时间之前将每个对象的引用定义为常量。请自由地假设我需要实现哪些OOP概念（它会有所不同），并为每个建议最好的方法。

限制：

• 我必须使用C而不是OOP
  ，因为我为
  嵌入式系统编写代码，编译器和
  预先存在的代码库在C中。


• 没有动态内存分配
  ，因为我们没有足够的内存
  来合理地假设我们不会运行如果我们开始动态分配
  ，那么
  。


• 我们使用的编译器没有函数指针的问题

解决方案

这取决于你想要的确切的面向对象的功能集。如果你需要重载和/或虚方法，你可能需要在结构中包含函数指针：

  typedef struct {
 float（* computeArea）（const ShapeClass * shape）; 
} ShapeClass; 
 
 float shape_computeArea（const ShapeClass * shape）
 {
 return shape-> computeArea（shape）; 
} 
  

这将允许你实现一个类，通过继承并实现一个合适的函数：

  typedef struct {
 ShapeClass shape; 
 float width，height; 
} RectangleClass; 
 
 static float rectangle_computeArea（const ShapeClass * shape）
 {
 const RectangleClass * rect =（const RectangleClass *）shape; 
 return rect-> width * rect-> height; 
} 
  

这当然需要你实现一个构造函数，指针正确设置。通常你会为实例动态分配内存，但是你也可以让调用者这样做：

  void rectangle_new（RectangleClass * rect）
 {
 rect-> width = rect-> height = 0.f; 
 rect-> shape.computeArea = rectangle_computeArea; 
} 
  

如果你想要几个不同的构造函数，你必须装饰函数名称，您不能有多个 rectangle_new（）函数：

  void rectangle_new_with_lengths（RectangleClass * rect，float width，float height）
 {
 rectangle_new（rect）; 
 rect-> width = width; 
 rect-> height = height; 
} 
  

下面是一个显示用法的基本示例：

  int main（void）
 {
 RectangleClass r1; 
 
 rectangle_new_with_lengths（& r1，4.f，5.f）; 
 printf（rectangle r1's area is％f units square \\\
，shape_computeArea（& r1））; 
 return 0; 
} 
  

我希望这至少给你一些想法。对于C中成功且丰富的面向对象框架，请查看glib的 GObject 库。

另外请注意，上面没有明确的类模型，每个对象都有自己的方法指针，这比在C ++中通常更灵活。此外，它花费记忆。你可以通过在类结构中填充方法指针，并为每个对象实例创建一个引用类的方法。

Assuming I have to use C (no C++ or object oriented compilers) and I don't have dynamic memory allocation, what are some techniques I can use to implement a class, or a good approximation of a class? Is it always a good idea to isolate the "class" to a separate file? Assume that we can preallocate the memory by assuming a fixed number of instances, or even defining the reference to each object as a constant before compile time. Feel free to make assumptions about which OOP concept I will need to implement (it will vary) and suggest the best method for each.

Restrictions:

• I have to use C and not an OOP because I'm writing code for an embedded system, and the compiler and preexisting code base is in C.
• There is no dynamic memory allocation because we don't have enough memory to reasonably assume we won't run out if we start dynamically allocating it.
• The compilers we work with have no problems with function pointers

解决方案

That depends on the exact "object-oriented" feature-set you want to have. If you need stuff like overloading and/or virtual methods, you probably need to include function pointers in structures:

typedef struct {
  float (*computeArea)(const ShapeClass *shape);
} ShapeClass;

float shape_computeArea(const ShapeClass *shape)
{
  return shape->computeArea(shape);
}

This would let you implement a class, by "inheriting" the base class, and implementing a suitable function:

typedef struct {
  ShapeClass shape;
  float width, height;
} RectangleClass;

static float rectangle_computeArea(const ShapeClass *shape)
{
  const RectangleClass *rect = (const RectangleClass *) shape;
  return rect->width * rect->height;
}

This of course requires you to also implement a constructor, that makes sure the function pointer is properly set up. Normally you'd dynamically allocate memory for the instance, but you can let the caller do that, too:

void rectangle_new(RectangleClass *rect)
{
  rect->width = rect->height = 0.f;
  rect->shape.computeArea = rectangle_computeArea;
}

If you want several different constructors, you will have to "decorate" the function names, you can't have more than one rectangle_new() function:

void rectangle_new_with_lengths(RectangleClass *rect, float width, float height)
{
  rectangle_new(rect);
  rect->width = width;
  rect->height = height;
}

Here's a basic example showing usage:

int main(void)
{
  RectangleClass r1;

  rectangle_new_with_lengths(&r1, 4.f, 5.f);
  printf("rectangle r1's area is %f units square\n", shape_computeArea(&r1));
  return 0;
}

I hope this gives you some ideas, at least. For a successful and rich object-oriented framework in C, look into glib's GObject library.

Also note that there's no explicit "class" being modelled above, each object has its own method pointers which is a bit more flexible than you'd typically find in C++. Also, it costs memory. You could get away from that by stuffing the method pointers in a class structure, and invent a way for each object instance to reference a class.

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