这个C ++结构初始化技巧是否安全？ [英] Is this C++ structure initialization trick safe?
问题描述
不必记住初始化一个简单的C结构，我可以从它派生并在构造函数中将其归零，如下所示：
Instead of having to remember to initialize a simple 'C' structure, I might derive from it and zero it in the constructor like this:
struct MY_STRUCT
{
int n1;
int n2;
};
class CMyStruct : public MY_STRUCT
{
public:
CMyStruct()
{
memset(this, 0, sizeof(MY_STRUCT));
}
};
这个技巧通常用于初始化Win32结构，有时可以设置无处不在的 cbSize member。
This trick is often used to initialize Win32 structures and can sometimes set the ubiquitous cbSize member.
现在，只要没有一个虚函数表用于memset调用destroy，这是一个安全的做法吗？
Now, as long as there isn't a virtual function table for the memset call to destroy, is this a safe practice?
推荐答案
PREAMBLE：
虽然我的答案还是，我发现litb's answer 相当优于我，因为：
PREAMBLE:
While my answer is still Ok, I find litb's answer quite superior to mine because:
- 它教我一个我不知道的骗子（litb的答案通常有这个效果，但这是我第一次写下来）
- 它回答的是正确的问题（也就是将初始结构的部分初始化为零）
litb的答案在我之前。
So please, consider litb's answer before mine. In fact, I suggest the question's author to consider litb's answer as the right one.
投放一个真正的对象（即std :: string）等会破坏，因为真正的对象将在memset之前被初始化，然后被零覆盖。
Putting a true object (i.e. std::string) etc. inside will break, because the true object will be initialized before the memset, and then, overwritten by zeroes.
初始化列表不工作的g ++（我很惊讶...）。在CMyStruct构造函数体中初始化它。它将是C ++友好的：
Using the initialization list doesn't work for g++ (I'm surprised...). Initialize it instead in the CMyStruct constructor body. It will be C++ friendly:
class CMyStruct : public MY_STRUCT
{
public:
CMyStruct() { n1 = 0 ; n2 = 0 ; }
};
PS：我假设你对MY_STRUCT有没有 。使用控件，您将直接在MY_STRUCT中添加构造函数，并忘记关于继承。注意，你可以将非虚拟方法添加到类似C的结构，并且仍然具有结构的作用。
P.S.: I assumed you did have no control over MY_STRUCT, of course. With control, you would have added the constructor directly inside MY_STRUCT and forgotten about inheritance. Note that you can add non-virtual methods to a C-like struct, and still have it behave as a struct.
编辑：添加缺少括号，在Lou Franco的评论。谢谢！
Added missing parenthesis, after Lou Franco's comment. Thanks!
编辑2：我试过g ++上的代码，并且由于某种原因，使用初始化列表不工作。我使用正文构造函数更正了代码。
EDIT 2 : I tried the code on g++, and for some reason, using the initialization list does not work. I corrected the code using the body constructor. The solution is still valid, though.
请重新评估我的帖子，因为原始代码已更改（有关更多信息，请参阅changelog）。
Please reevaluate my post, as the original code was changed (see changelog for more info).
编辑3：阅读Rob的评论后，我想他有一个值得讨论的问题：同意，但这可能是一个巨大的Win32结构，可能会随着新的SDK改变，所以memset是未来的证明。
EDIT 3 : After reading Rob's comment, I guess he has a point worthy of discussion: "Agreed, but this could be an enormous Win32 structure which may change with a new SDK, so a memset is future proof."
我不同意：知道微软，它不会改变，因为他们需要完美的向后兼容性。他们将创建一个扩展的MY_STRUCT Ex 结构，它与MY_STRUCT具有相同的初始布局，在末尾使用了addnal成员，通过一个size成员变量，如用于RegisterWindow的结构体IIRC可以识别。
I disagree: Knowing Microsoft, it won't change because of their need for perfect backward compatibility. They will create instead an extended MY_STRUCTEx struct with the same initial layout as MY_STRUCT, with additionnal members at the end, and recognizable through a "size" member variable like the struct used for a RegisterWindow, IIRC.
因此，Rob的评论中剩下的唯一有效点是巨大的结构。在这种情况下，memset可能更方便，但是你必须让MY_STRUCT成为CMyStruct的可变成员，而不是继承它。
So the only valid point remaining from Rob's comment is the "enormous" struct. In this case, perhaps a memset is more convenient, but you will have to make MY_STRUCT a variable member of CMyStruct instead of inheriting from it.
我看到另一个黑客，我猜这会破坏，因为可能的结构对齐问题。
I see another hack, but I guess this would break because of possible struct alignment problem.
编辑4：请看看弗兰克·克鲁格的解决方案。我不能承诺它是可移植的（我猜是），但它仍然是有趣的从技术角度来看，因为它显示了一个案例，在C + +，这个指针地址从其基类移动到其继承类。
EDIT 4: Please take a look at Frank Krueger's solution. I can't promise it's portable (I guess it is), but it is still interesting from a technical viewpoint because it shows one case where, in C++, the "this" pointer "address" moves from its base class to its inherited class.
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