这是一个复制构造函数吗？ [英] Is this a copy constructor?

问题描述

class A {};
class B { public: B (A a) {} };

A a;
B b=a;

从技术上讲，这是一个复制构造函数，应用于创建 b

Technically speaking, is a copy constructor being applied here on the creation of b ?

推荐答案

从技术上讲，关于b的创建？

Technically speaking, is a copy constructor being applied here on the creation of b ?

是...但可能不是你的想法。 在创建b时调用的复制构造函数，以便执行参数传递值 A a 作为B构造函数的参数。

Yes...but probably not how you think. A's copy constructor is being invoked on the creation of b, in order to do the pass-by-value of the parameter A a as a parameter to the B constructor.

然而，它不是在创建时运行B的复制构造函数b。

However, it is not running B's copy constructor in the creation of b.

EDIT：每天都会学到新的东西。显然，甚至更技术上来说，如@CharlesBailey指出的...如果你使用 B b = a; 语法（复制初始化）而不是 B b（a）; 语法（直接初始化），需要创建类型B 的临时值 。在这一点上，B的拷贝构造函数将被调用。

One learns something new every day. Apparently even more-technically-speaking, as @CharlesBailey pointed out...if you use the B b = a; syntax ("copy initialization") instead of B b (a); syntax ("direct initialization"), a temporary value of type B might need to be created. At this point B's copy constructor would wind up being called.

这是一个很难研究的现象，但Charles指出，gcc有一个 -fno-elide-constructors 选项（也：复制Elision上的维基百科） ）@ JesseGood的链接有一个详尽的解释和一些演示代码：

It's a little hard to study the phenomenon, but Charles points out that gcc has a -fno-elide-constructors option (also: Wikipedia on Copy Elision) @JesseGood's link has an exhaustive explanation and some demonstration code:

C ++在复制初始化和直接初始化之间有区别吗？

Is there a difference in C++ between copy initialization and direct initialization?

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