C++ friend class

有些時候我們會利用 C++ 中的 friend 關鍵字來設定類別的好友.
但可惜一個 friend 宣告只會對一個類別產生作用:

class Texture {
public:
 friend class IResourceLoader;
 uint width() const { return mWidth; }
 uint height() const { return mHeight; }
 private:
 uint mWidth;
 uint mHeight;
};

class IResourceLoader {};

class JpegLoader : public IResourceLoader {
 void load() {
  // ...
  mTexture.mWidth = 128;  // Compiler error!!
 }
};

以上的 Texture 類別有成員 mWidth 和 mHeight, 它們都只應由 IResourceLoader 的實作類別所能更改. 幸好有 Inner class 可以幫到我們:

// Texture.h
class Texture {  
public:  
 friend class IResourceLoader;  
 uint width() const { return mWidth; }  
 uint height() const { return mHeight; }  

 // We declare a templated inner class here (but not defined yet)
 template class PrivateAccessor;

 private:  
 uint mWidth;  
 uint mHeight;  
};

class IResourceLoader {};  

// JpegLoader.cpp
// Define Texture::PrivateAccessor here, access what ever you want
template<>
class Texture::PrivateAccessor {
public:
 static size_t& width(Texture& texture) {
  return texture.mWidth;
 }
 static size_t& height(Texture& texture) {
  return texture.mHeight;
 }
};
typedef Texture::PrivateAccessor Accessor;

void JpegLoader::load() {
 // ...  
 Accessor::width(mTexture) = 128; // Ok! no problem
 Accessor::height(mTexture) = 128;
}

你可能會認為這樣做等同把所有成員變為公開 (public), 事實上這個方案的精神在於迫使程序員清楚自己正在做什麼, 而不是錯誤地更改了變數.