C ++：创建未初始化的占位符变量而不是默认对象 [英] C++: Creating an uninitialized placeholder variable rather than a default object

问题描述

我现在从Java迁移到C ++，每当Java中常用的概念不直接映射到C ++时，我遇到了一些困难。例如，在Java中我会做一些类似的事情：

I'm moving from Java to C++ right now and I'm having some difficulties whenever a commonly used concept in Java doesn't map directly into C++. For instance, in Java I would do something like:

Fruit GetFruit(String fruitName) {
    Fruit fruit;
    if(fruitName == "apple") fruit = new Fruit("apple");
    else if(fruitName == "banana") fruit = new Fruit("banana");
    else fruit = new Fruit("kumquat"); //'cause who really wants to eat a kumquat?

    return fruit;
}

当然，在C ++中 / code>语句实际上创建一个水果。这是否意味着我必须有一个默认的构造函数？这似乎不安全！如果我的默认水果逃脱了怎么办？

Of course, in C++ the Fruit fruit; statement actually creates a fruit. Does this mean I have to have a default constructor? This seems unsafe! What if my default fruit escaped?

推荐答案

C ++给你创造水果更令人头痛。根据您的需要，您可以选择以下选项之一：

C++ gives you much more headache when it comes to creating fruits. Depending on your needs, you can choose one of the following options:

1）在堆栈上创建一个水果并返回一个副本（你需要一个副本构造函数）然后和必须提供一些默认的水果，以防名称不匹配：

1) create a Fruit on a stack and return a copy (you need a copy constructor) then and must provide some default fruit in case the name does not match:

Fruit GetFruit(const std::string &name)
{
   if ( name == "banana" ) return Fruit("banana");
   if ( name == "apple" )  return Fruit("apple");
   return Fruit("default");
}

2）在堆上创建一个Fruit并且小心，空指针返回，还记得删除这个水果在某个地方，并注意它是只删除一次，只有它的所有者（并注意没有一个指针指向删除的水果）：

2) create a Fruit on a heap and take care, that there could be null-pointer returned and also remember to delete this fruit somewhere and take care that it is deleted only once and only by its owner (and take care that noone holds a pointer to the deleted fruit):

Fruit* GetFruit(const std::string &name)
{
   if ( name == "banana" ) return new Fruit("banana");
   if ( name == "apple" )  return new Fruit("apple");
   return NULL;
}

3）最后，使用智能指针避免许多可能的指针问题但是处理空指针）。此选项最接近您的Java编程体验：

3) and finally, use a smart pointer to avoid many possible pointer problems (but take care of null pointers). This option is the closest to your Java programming experience:

typedef boost::shared_ptr<Fruit> FruitRef;

FruitRef GetFruit(const std::string &name)
{
   if ( name == "banana" ) return new Fruit("banana");
   if ( name == "apple" )  return new Fruit("apple");
   return FruitRef();
}

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