Re: Initialization of reference from implicitly-converted unrelated type

From:

Ulrich Eckhardt <doomster@knuut.de>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 17 Oct 2011 23:48:10 -0700 (PDT)

Message-ID:

<9g4hl0Fd9mU1@mid.uni-berlin.de>

nikkoara wrote:

I am inclined to believe the following is a compiler error. Any
opinions?

$ g++ --version | head -n 1; cat -n t.cpp; g++ -c t.cpp
g++ (GCC) 4.6.1
     1 struct B { };
     2 struct D : B { } d;
     3
     4 struct X { };
     5 struct Y : X {
     6 Y (D const&);
     7 };
     8
     9 X const& ref = d;
t.cpp:9:16: error: invalid initialization of reference of type ???const
X&??? from expression of type ???D???

I wouldn't consider that a fault. The question, looking at the last line,
is why should the compiler consider the side-trip through a 'Y' for
initialisation of 'ref'? There could be a limitless number of classes that
derive from 'X' and the code doesn't explicitly or implicitly specify one.

This slightly changed version compiles:

$ g++ --version | head -n 1; cat -n t2.cpp; g++ -c t2.cpp
g++ (GCC) 4.6.1
     1 struct X { };
     2 struct Y : X { };
     3
     4 struct B { };
     5 struct D : B {
     6 operator Y();
     7 } d;
     8
     9 X const& ref = d;

Here, the number of conversions is limited to the bases of 'd' and the
defined conversion operators, so no code external to this could change the
meaning of this code.

Uli

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