确保派生类构造函数必须调用特定的基类方法 [英] Ensure derived class constructor must call specific base class method
问题描述
在C ++(03)类中，我有一个成员变量，在对象构造过程中必须为该变量赋一个值.但是，只有派生类可以计算所需的值.如本博文所述确实C ++要求您从其派生类中初始化基类成员吗?，我知道派生类不能初始化基类成员，但是分配对我来说已经足够.
In a C++(03) class, I have a member variable, which must be assigned a value during object construction. However, only the derived class can compute the required value. As discussed in this post Does C++ require you to initialize base class members from its derived class?, I understand that the derived class cannot initialize a base class member, but an assignment is sufficient for me.
因此，我在基类中提供了一种分配值的方法，但是我无法弄清楚如何强制派生类对其进行调用.下面的示例代码说明了这一点.
Hence, I provide a method in the base class to assign a value, but I cannot figure out how I can force the derived class to call it. The below sample code illustrates this point.
class Base {
public:
Base() {}
setFoo(unsigned inFoo) { foo = inFoo; }
private:
unsigned foo;
};
class Derived : public Base {
Derived() : Base() {
unsigned desired = ... // do calculations to get the desired value
setFoo(desired); // --> how to ensure that derived class calls this?
}
};
foo然后由其他几种基类方法使用.即使我在setFoo()中设置了标志"，也没有可以检查它的后构造器"方法.基类中还有其他使用foo的方法.我可以在那里检查它，但是这样做太乏味，容易出错并且效率低下.
The foo is then used by several other Base class methods. Even if I set a "flag" in the setFoo(), there's no "post-constructor" method where I can check it. There are several other methods in the base class where foo is used. I could check it there, but it is too tedious, error-prone and inefficient to do so.
我很抱歉，如果以前有人问过这个问题.在通过询问确保派生类构造函数必须调用特定的基类方法"和强制派生类分配基类成员"进行询问之前，我确实尝试在此处找到它.我遇到了一些有用的帖子强制派生类使用基类的构造函数，但不幸的是，它们没有解决我的问题.
My apologies if this has been asked before. I did try to find it here before asking by searching for "Ensure derived class constructor must call specific base class method" and "Force derived class to assign base class member". I came across some useful posts How can I initialize a const variable of a base class in a derived class' constructor in C++? and Force a Derived Class to use the Constructor of the Base Class , but they unfortunately do not address my question.
您能否建议我采用一种合适的方法来解决这种情况?需要一种编译时检查解决方案，但如果不可能，则运行时检查也将有所帮助.尽管我不能使用C ++ 11解决我的问题，但我也很高兴学习任何C ++ 11解决方案.感谢您的考虑.
Could you please suggest me a suitable approach to address this situation? A compile-time checking solution would be desirable, but if that is not possible, a runtime check would also help. Although I cannot use C++11 for my problem, I would be glad to learn any C++11 solutions as well. Thanks for your consideration.
编辑当然，也可以将其记录为一个要求，但是我想了解是否有程序化的解决方案.
Edit Of course, documenting this as a requirement is also an option, but I would like to learn if there's a programmatic solution.
Edit2 我同意答案，即应该在构造函数参数中传递该值，如果我必须从头开始编写Base，那么我就不会要求这种方法了.问题是，由于遗留原因，Base已经具有多个构造函数参数(9)，并且我希望通过将它们卸载"到构造函数主体来避免添加更多的(3).从长远来看，必须对代码进行重构.
Edit2 I agree with the answers saying that the value should be passed in constructor parameter, and if I had to write Base from scratch, I wouldn't have asked for this approach. The problem is that due to legacy reasons, Base already has several constructor parameters (9), and I was hoping to avoid adding more (3) by "offloading" them to the constructor body. In the long term though, the code has to be refactored.
Edit3 很抱歉，进行了如此多的修改.我可以随意修改Base类，但是Derived类在客户端代码中，我无法控制.
Edit3 Sorry for so many edits. I can modify the Base class as I please, but the Derived class is in client code, which I cannot control.
推荐答案
如果派生类构造函数必须调用setFoo()，那么我认为这是一个设计问题.最好设计基类，以使其构造函数将所需值作为参数，并让派生类将计算值传递为:
If the derived class constructor must call setFoo(), then I think that is a design problem. It is better to design the base class, such that it's constructor takes the desired value as argument, and let the derived class pass the computed value as:
class Base {
public:
Base(unsigned int foo) : m_foo (foo) {} //define this constructor
private:
unsigned int m_foo; //member data
};
class Derived : public Base {
Derived() : Base(computeValue())
{
}
private:
static unsigned int computeValue() //it may take argument(s)
{
unsigned int desired = ... //do calculations to get the desired value
return desired;
}
};
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