工厂模式：typedef类*（createClassFunction）（void） [英] Factory Pattern: typedef Class *(createClassFunction)(void)

问题描述

typedef Class *（createClassFunction）（void）（或另一个变体是 typedef Class *（__stdcall * CreateClassFunction） / code>）代表？
是什么意思？我应该怎么解释呢？
特别是在工厂模式的上下文中...

What does typedef Class *(createClassFunction)(void) (or another variation is typedef Class* (__stdcall *CreateClassFunction)(void))stand for? What does it mean? How am I supposed to explain it? Especially in the context of Factory Patterns...

推荐答案

读取C类型表达式

createClassFunction 是没有参数并返回 Class * 的函数的typedef。

createClassFunction is a typedef for a function taking no arguments and returning a Class *.

声明，指向这样的函数的指针显然可以作为 Class 的 factory 。用法可能如下：

With that declaration, a pointer to such a funciton can obviously act as factory for a Class. Usage might be as follows:

// the class factory
Class * MostTrivialFactoryKnownToMan()
{
 return new Class;
}

// another class factory
Class * CreateClassWithLog()
{
   ClassWithLog * p = new ClassWithLog; // derived from Class
   p->SetLogFile("log.txt");
   return p;
}

// code consuming the class factory
void PopulateStars(createClassFunction * factory)
{
   // creates many instances of `Class` through `factory`
   Sky * sky = GetSky();
   for(int i=0; i<sky->GetStarCount(); ++i)
   {
      Class * inhabitant = (*factory)();
      sky->GetStar(i)->SetInhabitant(inhabitant);
   }
}

// code deciding which factory to use
const bool todayWeWriteALog = (rand() %2) != 0;
createClassFunction * todaysFactory = todayWeWriteALog ?
   &MostTrivialFactoryKnownToMan : 
   &CreateClassWithLog);

PopulateStars(factory);

__ stdcall 调用约定（参数和返回值如何在调用者和实现之间传递）。这对于二进制兼容性通常是重要的。当一个Pascal程序需要调用一个在C或C ++中实现的函数。

__stdcallis a compiler specific attribute changin the calling convention (how parameters and return value are passed between caller and implementation). This is often important for binary compatibility - e.g. when a Pascal program needs to call a funciton imlemented in C or C++.

问题：

工厂函数返回原始指针。在工厂和消费者之间必须有一个隐式的契约，如何释放该指针（例如通过 delete in或example）。
使用智能指针（例如shared_ptr）作为返回类型将允许工厂确定删除策略。

The factory function returns a raw pointer. There must be an implicit contract between the factory and the consumer how to free that pointer (e.g. through delete in or example). Using a smart pointer (e.g. shared_ptr) for the return type would allow the factory to determine the deletion policy.

工厂作为函数指针，可能不保存状态（例如日志文件名，需要在函数中硬编码，或全局可用）。使用可调用对象将允许实现可配置工厂。

The factory, as a function pointer, may not hold state (such as the log file name, it needs to be hard coded in the function, or available globally). using a callable object instead would allow to implement configurable factories.

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