c ++如何写一个构造函数？

问题描述

我不习惯c ++，我在写一个构造函数时遇到问题。

看到这个例子，是我正在处理的代码的一个简短版本：

I'm not used to c++ and I'm having a problem writing a constructor.
See this example, is a short version of the code I'm working on:

class B {
public:
  B(int x);
}

class A {
public:
  B b;
  A(){
    // here I have to initialize b
  }
}

这会抛出编译器错误，因为我需要在A的构造函数中初始化b，因为B没有默认构造函数。

That throws a compiler error since I need to initialize b in A's constructor because B does not have a default constructor.

在初始化列表中做，但是B（int x）参数是一个值，我必须用一些算法计算，所以我不知道这应该如何正确完成，或者如果我缺少的东西或做它错误。

I think I have do it in the initialization list, but the B(int x) argument is a value I have to calculate with some algorithm, so I don't know how this should be properly done, or if I'm missing something or doing it wrong.

在其他语言如java中，我将引用B并在A的构造函数中初始化它，然后在其他代码之后，我需要获取初始化的值。

In other language like java I would have a reference to B and initialize it inside the A's constructor after the other code I need to get the value for the initialization.

在这种情况下，初始化b的正确方法是什么？

What would be the right way to initialize b in this case?

推荐答案

您可以在构造函数初始化列表中调用函数

You can invoke functions in your constructor initializer list

class B {
public:
  B(int x);
}; // note semicolon

class A {
public:
  B b;

  A()
  :b(calculateValue()) {
    // here I have to initialize b
  }

  static int calculateValue() {
    /* ... */
  }
}; // note semicolon

请注意，在初始化器列表中，类被认为是完全定义的，看到稍后宣布的成员。还最好不要在构造函数初始化器列表中使用非静态函数，因为并不是所有的成员都在这一点上被初始化。静态成员函数调用很好。

Note that in the initializer list, the class is considered completely defined, so you can see members declared later on too. Also better not use non-static functions in the constructor initializer list, since not all members have yet been initialized at that point. A static member function call is fine.

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