什么样的特征/概念可以保证对象的良好定义? [英] What trait / concept can guarantee memsetting an object is well defined?

问题描述

假设我已经定义了zero_initialize()函数:

Let's say I have defined a zero_initialize() function:

template<class T>
T zero_initialize()
{
    T result;
    std::memset(&result, 0, sizeof(result));
    return result;
}

// usage: auto data = zero_initialize<Data>();

为某些类型调用zero_initialize()会导致未定义的行为 ^{1， 2} .我目前正在强制T验证 std::is_pod .随着C ++ 20中不赞成使用该特性以及概念的出现，我很好奇zero_initialize()应该如何发展.

Calling zero_initialize() for some types would lead to undefined behavior^{1, 2}. I'm currently enforcing T to verify std::is_pod. With that trait being deprecated in C++20 and the coming of concepts, I'm curious how zero_initialize() should evolve.

1. 什么(最小)特征/概念可以确保对对象进行精确定义?
2. 我应该使用 std::uninitialized_fill 而不是std::memset吗?为什么?
3. 该函数是否由于一种类型子集的C ++初始化语法之一而过时?还是将来的C ++版本即将出现?

1. What (minimal) trait / concept can guarantee memsetting an object is well defined?
2. Should I use std::uninitialized_fill instead of std::memset? And why?
3. Is this function made obsolete by one of C++ initialization syntaxes for a subset of types? Or will it be with the upcoming of future C++ versions?

---

¹⁾ 擦除类的所有成员.
²⁾ 在库类(std :: string)上使用memset会导致未定义行为"的原因是什么? [关闭]

---

¹⁾ Erase all members of a class.
²⁾ What would be reason for "undefined behaviors" upon using memset on library class(std::string)? [closed]

推荐答案

从技术上讲，C ++中没有对象属性，该属性指定用户代码可以合法地memset C ++对象.其中包括POD，因此，如果您想成为技术专家，则代码永远是不正确的.甚至TriviallyCopyable都是关于在现有对象之间进行字节复制的属性(有时通过中间字节缓冲区)；它没有说明发明数据并将其推入对象的位.

There is technically no object property in C++ which specifies that user code can legally memset a C++ object. And that includes POD, so if you want to be technical, your code was never correct. Even TriviallyCopyable is a property about doing byte-wise copies between existing objects (sometimes through an intermediary byte buffer); it says nothing about inventing data and shoving it into the object's bits.

话虽如此，如果您测试is_trivially_copyable 和 is_trivially_default_constructible，您可以合理地确定这将起作用.最后一个很重要，因为某些TriviallyCopyable类型仍然希望能够控制其内容.例如，这种类型可以具有始终为5的私有int变量，该变量在其默认构造函数中初始化.只要没有可访问该变量的代码对其进行更改，它将始终为5.C++对象模型对此进行了保证.

That being said, you can be reasonably sure this will work if you test is_trivially_copyable and is_trivially_default_constructible. That last one is important, because some TriviallyCopyable types still want to be able to control their contents. For example, such a type could have a private int variable that is always 5, initialized in its default constructor. So long as no code with access to the variable changes it, it will always be 5. The C++ object model guarantees this.

因此，您不能memset这样的对象，仍然会从对象模型中获得明确定义的行为.

So you can't memset such an object and still get well-defined behavior from the object model.

这篇关于什么样的特征/概念可以保证对象的良好定义?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！