使用memset调零派生结构 [英] zeroing derived struct using memset

问题描述

我想清除派生结构的所有成员。

I want to zero out all members of a derived structure.

有几百个成员和更多的被添加每次一次，所以我觉得初始化它们显式地是容易出错的。

There are hundreds of members and more are added every once in a while so I feel that initializing them explicitly is error-prone.

结构没有虚函数，所有的成员字段都是内置的。

The structures have no virtual functions and all the member fields are built-in. However, they are not POD by virtue of having non-trivial constructors.

除了标准的皱眉外，你看到以下的问题吗？

Apart from the standard frowning on the practice, do you see any issues with the following?

struct Base
{
    // Stuff
};

struct Derived : public Base
{
    // Hundreds of fields of different built-in types
    // including arrays

    Derived()
    {
        ::memset(reinterpret_cast<char*>this + sizeof (Base), 0, sizeof *this - sizeof (Base));
    }
};

谢谢。

推荐答案

这假设 Base 基类子对象位于 Derived 的开头。如果你添加另一个基类，这将不会工作。

This assumes that the Base base class subobject is located at the beginning of Derived. This won't work if you add another base class.

此外，你的代码是错误的：指针运算是根据对象，而不是字节。您需要使用 reinterpret_cast< char *>（this）按字节执行算术。

Further, your code is wrong: pointer arithmetic is performed in terms of objects, not in terms of bytes. You need to use reinterpret_cast<char*>(this) to perform the arithmetic in terms of bytes. In any case, you still shouldn't do this.

使用价值初始化考虑以下非丑陋，符合标准的方法：

Consider the following, non-ugly, standards-conforming approach utilizing value initialization:

struct Derived : public Base
{
    struct DerivedMembers { /* ... */ }

    DerivedMembers data;

    Derived() : data() { }
};

只要 DerivedMembers 没有构造函数，这将会初始化 data 的每个数据成员，这看起来像是你想要的行为。

As long as DerivedMembers has no constructor, this will value initialize each of the data members of data, which look like it's exactly the behavior you want.

或者，如果您希望在不使用数据成员变量的情况下访问成员，请考虑使用另一个基类：

Or, if you want the members to be accessible without using a "data" member variable, consider using another base class:

struct DerivedMembers { /* ... */ }

struct Derived : Base, DerivedMembers
{
    Derived() : DerivedMembers() { }
};

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