不透明指针

在程序设计中，不透明指针（Opaque pointer）是不透明数据类型（英语：Opaque data type）的一种特殊情况，这种資料類型被声明为指向某种未指定类型的记录或数据结构的指针。不透明指针存在于艾达、C语言、C++、D語言和Modula-2 等多种编程语言中

不透明指针是一种向普通客户端隐藏接口實現细节的方法。这对程序员也有好处，因为可以创建一个简单的接口，而大多数细节可以隐藏在另一个文件中^[1]。

例子

Ada

package Library_Interface is

   type Handle is limited private;

   -- Operations...

private
   type Hidden_Implementation;    -- Defined in the package body
   type Handle is access Hidden_Implementation;
end Library_Interface;

package body Library_Interface is

   type Hidden_Implementation is record
      ...    -- The actual implementation can be anything
   end record;

   -- Definition of the operations...

end Library_Interface;

^[2]

C

/* obj.h */

struct obj;

/*
 * The compiler considers struct obj an incomplete type. Incomplete types
 * can be used in declarations.
 */

size_t obj_size(void);

void obj_setid(struct obj *, int);

int obj_getid(struct obj *);

/* obj.c */

#include "obj.h"

struct obj {
    int id;
};

/*
 * The caller will handle allocation.
 * Provide the required information only
 */

size_t obj_size(void) {
    return sizeof(struct obj);
}

void obj_setid(struct obj *o, int i) {
    o->id = i;
}

int obj_getid(struct obj *o) {
    return o->id;
}

C++

/* PublicClass.h */

#include <memory>

class PublicClass {
 public:
  PublicClass();                               // Constructor
  PublicClass(const PublicClass&);             // Copy constructor
  PublicClass(PublicClass&&);                  // Move constructor
  PublicClass& operator=(const PublicClass&);  // Copy assignment operator
  PublicClass& operator=(PublicClass&&);       // Move assignment operator
  ~PublicClass();                              // Destructor

  // Other operations...

 private:
  struct CheshireCat;                   // Not defined here
  std::unique_ptr<CheshireCat> d_ptr_;  // Opaque pointer
};

/* PublicClass.cpp */

#include "PublicClass.h"

struct PublicClass::CheshireCat {
  int a;
  int b;
};

PublicClass::PublicClass()
    : d_ptr_(std::make_unique<CheshireCat>()) {
  // Do nothing.
}

PublicClass::PublicClass(const PublicClass& other)
    : d_ptr_(std::make_unique<CheshireCat>(*other.d_ptr_)) {
  // Do nothing.
}

PublicClass::PublicClass(PublicClass&& other) = default;

PublicClass& PublicClass::operator=(const PublicClass &other) {
  *d_ptr_ = *other.d_ptr_;
  return *this;
}

PublicClass& PublicClass::operator=(PublicClass&&) = default;

PublicClass::~PublicClass() = default;