Explicit Copy constructor call syntax

Question

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;
}

The compiler says:

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

If I change it to

Base b(a);

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?

They are not the same. One is direct-initialization, the other is copy-initialization. `explicit` constructors cannot be picked by the latter. — Andy Prowl, Jan 18 '15 at 13:53
It was originally, but the compiler error (above) was showing non-const. So just to eliminate that as a possibility I matched what the compiler was showing. — Syed H, Jan 18 '15 at 14:01
@LightnessRacesinOrbit I thought it was very likely for this to be a duplicate, but did not have the time (or rather, did not bother - my bad) to look for an existing answer. I left a comment, hoping to be helpful enough. — Andy Prowl, Jan 18 '15 at 15:41
@AndyProwl: Writing answers in comments encourages help vampirism and ruins the integrity of the Q&A format. SO is not a forum or chatroom!!!! — Lightness Races in Orbit, Jan 18 '15 at 15:43

quantdev · Accepted Answer · 2015-01-18T15:16:37.130

3

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

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

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

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

edited Jan 18 '15 at 15:16

answered Jan 18 '15 at 13:53

quantdev

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So for non-built in types, is it same to assume that () always offers better performance than = since the latter tries to do an implicit conversion? – Syed H Jan 18 '15 at 14:12
1

@SyedH I invite you to read [this answer](http://stackoverflow.com/a/1051468/3510483) for a complete overview of the differences – quantdev Jan 18 '15 at 14:19
Ugh, there's a horrid editorial mistake in this standard text, present in C++11 and C++14 and the latest draft. – Lightness Races in Orbit Jan 18 '15 at 15:08
[I have reported it to `std-discussion`](https://isocpp.org/forums/iso-c-standard-discussion?place=topic%2Fstd-discussion%2F_SBNczqk-NM%2Fdiscussion). – Lightness Races in Orbit Jan 18 '15 at 15:13

Explicit Copy constructor call syntax

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