C++ 中的基于成员的习语

发布于 2024-10-14 06:24:51 字数 706 浏览 8 评论 0原文

以下代码来自此处：

#include <streambuf>  // for std::streambuf
#include <ostream>    // for std::ostream

class fdoutbuf
    : public std::streambuf
{
public:
    explicit fdoutbuf( int fd );
    //...
};

class fdostream
    : public std::ostream
{
protected:
    fdoutbuf buf;
public:
    explicit fdostream( int fd ) 
        : buf( fd ), std::ostream( &buf ) // This is not allowed. 
                                          // buf can't be initialized before std::ostream.
        {}
    //...
};

我没有真的很理解评论。为什么“buf 不能在 std::ostream 之前初始化”？我可以使用一些帮助来理解这一点吗？

原文

The following code is from here:

#include <streambuf>  // for std::streambuf
#include <ostream>    // for std::ostream

class fdoutbuf
    : public std::streambuf
{
public:
    explicit fdoutbuf( int fd );
    //...
};

class fdostream
    : public std::ostream
{
protected:
    fdoutbuf buf;
public:
    explicit fdostream( int fd ) 
        : buf( fd ), std::ostream( &buf ) // This is not allowed. 
                                          // buf can't be initialized before std::ostream.
        {}
    //...
};

I didn't really understand the comment. Why "buf can't be initialized before std::ostream"? Can I use some help understanding this?

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只为一人 2024-10-21 06:24:51

初始化的顺序由声明类成员的顺序决定，继承的类位于所有这些之前。举一个简单的例子来说明基本问题，而不涉及继承：

class C
{
  int a, b;
public:
  C() : b(1), a(b) {} // a is initialized before b!
};

代码没有按照你的想法做！首先a被初始化，然后b被初始化为1。因此，它取决于声明的顺序而不是初始化列表中的顺序：

int a, b;

现在，相同的想法适用于在派生类成员之前初始化的基类。为了解决这个问题，您创建一个您固有的类，其中包含您想要从基类初始化的成员。当然，该辅助类必须位于您实际派生的辅助类之前。

The order of initialization is determined by the order of declaring your class members, and inherited classes come before all of that. Take a simple example that illustrate the basic problem without referring to inheritance :

class C
{
  int a, b;
public:
  C() : b(1), a(b) {} // a is initialized before b!
};

The code doesn't do what you think! a is initialized first, then b is initialized to one. So it depends on the order of the declaration not the order in the initialization list:

int a, b;

Now, the same idea applies to base classes which are initialized before the derived class members. To solve this problem, you create a class that you inherent which contains the member you want to initialize from a base class. Of course, that helper class must come before the one you are actually deriving from.

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