在班级内部初始化班级 [英] Initializing class inside class

问题描述

我是C ++的新手，我的问题也许很简单，但是我找不到解决方案。

I am new to C++, and my question might be trivial, but I wasn't able to find a solution.

我有两个类， S 和 L 。 S 看起来像这样：

I have two classes, S and L. S looks like this:

class S
{
private:
    int m_x;

public:
    S(int x) : m_x(x)
    {
    }

    int m_y = 2*m_x;   // some random action in S
};

现在我有第二类 L ，我要在其中初始化 S 对象的地方：

Now I have a second class L, where I want to initialize an S-object:

class L
{
private:
    S s(10);   // 10 is just some random value
    int m_y;
public:
    L(int y): m_y(y)
    {
    }

// ignore the rest for now.
};

这会产生错误错误：数字常量之前的预期标识符在 s（10）的初始化行。

This produces an error error: expected identifier before numeric constant at the line of initialization of s(10).

我不明白为什么我不能这样做。我该如何解决？如果我想初始化对象 S s（m_y）怎么办？

I don't understand why I can't do that. How could I fix this? What if I wanted to initialize the object S s(m_y) instead?

推荐答案

您可以使用成员初始值设定项列表，就像对 m_y ：

L(int y): s(10), m_y(y)
{
}

或使用 C ++ 11中的默认初始值设定项列表，但请注意，仅支持大括号或等于初始值设定项，不支持

Or use default initializer list from C++11, but note it's only supported for brace or equals initializer, not including the parenthesis one.

class L
{
private:
    S s{10};   // or S s = S(10);
    int m_y;
public:
    L(int y): m_y(y) // s is initialized via default initializer list
                     // m_y is initialized via member initializer list
    {
    }
};

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