C＃键入Generic类(C# Type to Generic class)

 尽管你可以使用嵌套的条件表达式来做到这一点，但我建议不要这样做：即使只有一个条件表达式，表达式也是不可读的，更不用说两个或三个嵌套的表达式。 
 
 使用私有静态成员函数对于此任务来说是一个不错的选择：函数可以“解析” foo的值，并决定要返回的正确对象： 
 
Child::Child(const std::string & foo)
:   Parent(constructParentArgs(foo)) {
}
private static someClassPtr *constructParentArgs(const std::string& foo) {
    if (foo == "x") return someClassPtr(new someClass());
    if (foo == "y") return someClassPtr(new someOtherClass());
    if (foo == "z") return someSubClassPtr(new yetAnotherClass());
    // provide a default value here
    return someClassPtr(new defaultClass());
}
 
 这种方法对于代码的读者的优点是，他们不必为调用父构造函数的细节而烦恼，除非他们想看看发生了什么。 当他们读取标题时，他们都知道你的代码以某种方式创建了父参数。 如果他们想要了解参数的构造方式，他们总是可以直接进入私有成员函数。 

Although you can definitely do it with nested conditional expressions, I would recommend against it: the expression borders on non-readable even with a single conditional, let alone two or three nested ones.
 
Using a private static member function would be a good choice for this task: the function could "parse" the value of foo, and decide on the proper object to return:
 
Child::Child(const std::string & foo)
:   Parent(constructParentArgs(foo)) {
}
private static someClassPtr *constructParentArgs(const std::string& foo) {
    if (foo == "x") return someClassPtr(new someClass());
    if (foo == "y") return someClassPtr(new someOtherClass());
    if (foo == "z") return someSubClassPtr(new yetAnotherClass());
    // provide a default value here
    return someClassPtr(new defaultClass());
}
 
The advantage of this approach for the readers of your code is that they do not have to bother themselves with the details of calling the parent constructor, unless they want to see what is going on. When they read the header, all they know that your code somehow creates parent argument. If they want to learn how exactly the argument gets constructed, they can always go straight to the private member function.