复制和直接初始化背后的动机有什么不同？ [英] What's the motivation behind having copy and direct initialization behave differently?

问题描述

与有关的原因为什么是调用复制构造函数而不是转换构造函数？

初始化，直接和复制初始化有两种语法：

There are two syntaxes for initialization, direct- and copy-initialization:

A a(b);
A a = b;

我想知道他们有不同定义行为的动机。对于副本初始化，涉及额外的副本，我不能想到该副本的任何目的。因为它是一个来自temp的副本，它可以并且可能会被优化，所以用户不能依赖它发生 - ergo额外的副本本身不足以为不同的行为。那么......为什么？

I want to know the motivation for them having different defined behavior. For copy initialization, an extra copy is involved, and I can't think of any purpose for that copy. Since it's a copy from a temp, it can and probably will be optimized out, so the user can't rely on it happening - ergo the extra copy itself isn't reason enough for the different behavior. So... why?

推荐答案

只是一个猜测，但恐怕很难更确定没有Bjarne Stroustrup确认它是如何是真的：

Only a speculation, but I am afraid it will be hard to be more certain without Bjarne Stroustrup confirming how it really was:

它是这样设计的，因为它被认为这样的行为将被程序员期望，他期望复制完成时=符号，而不是直接的初始化语法。

It was designed this way because it was assumed such behaviour will be expected by the programmer, that he will expect the copy to be done when = sign is used, and not done with the direct initializer syntax.

我认为可能的复制elision只添加在标准的更高版本，但我不知道 - 这是某人可能能够通过检查标准历史来确定。

I think the possible copy elision was only added in later versions of the standard, but I am not sure - this is something somebody may be able to tell certainly by checking the standard history.

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