显式复制构造函数调用语法 [英] Explicit Copy constructor call syntax

问题描述

当我将副本构造函数声明为显式时，使用=而不是()调用它不会编译.这是我的代码:

When I declare my copy constructor as explicit, calling it using = instead of () doesn't compile. Here's my code:

class Base
{
    public:
        explicit Base(){cout<<__PRETTY_FUNCTION__<<endl;}
        explicit Base(Base& b){cout <<__PRETTY_FUNCTION__<<endl;}
};

int main()
{
    Base a;
    Base b=a;
}

编译器说:

错误:没有匹配的函数可以调用"Base :: Base(Base&)"

error: no matching function for call to ‘Base::Base(Base&)’

如果我将其更改为

Base b(a);

它可以编译.我以为C ++认为这两种实例化样式是相同的.如果我删除了显式关键字，它确实可以双向工作.我猜我使用=时会发生一些隐式转换.那我在这里想念什么?

It compiles fine. I thought C++ considers these two styles of instantiations the same. If I remove the explicit keyword it does works both ways. I'm guessing there is some implicit conversion going on when I use =. So what am I missing here?

推荐答案

不，它们不一样.C ++标准部分第12.3.1节[class.conv.ctor]

No, they are not the same. C++ Standard section § 12.3.1 [class.conv.ctor]

一个显式构造函数像非显式一样构造对象构造函数，但仅在直接初始化语法的情况下才这样做(8.5)或明确使用强制转换(5.2.9，5.4)的地方

An explicit constructor constructs objects just like non-explicit constructors, but does so only where the direct-initialization syntax (8.5) or where casts (5.2.9, 5.4) are explicitly used

---

Base b(a); // Direct initialization
Base b=a;  // Copy initialization

复制初始化(使用 = )不会考虑显式构造函数，但直接初始化(使用())会考虑.

Copy initialization (using =) doesn't consider explicit constructors, but direct initialization (using ()) does.

如果要使用复制初始化，则必须使用强制转换或使构造函数不明确.

You'll have to use a cast or make your constructor non explicit if you want to use copy initialization.

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