Re: Compiler chooses conv ctor - why?

"Cezary H. Noweta" <chncc@noemail.noemail>
Mon, 14 Apr 2008 09:26:24 +0200
Alex Blekhman wrote:

"Cezary H. Noweta" wrote;

It seems that according to 13.3 of ISO 14882 none of the
sequences is better then other. Thus ambiguity should occur
causing error.

It seems like a compiler bug. I tried the folowing code and it
fails to compile with Comeau C++ as expected:

struct B;

struct A {
    operator B() const;

struct B {
    B() {}
    B(const A&) {}

A::operator B() const
{ return *((B*)0); }

void foo(B) {}

int main()
    A a;
    B b = a; // Bug! Should not compile.

    foo(a); // OK. Fails to compile.

    return 0;

OK - thank you. I've tested above example with various compilers, which
produced several different effects:

MSC12.0, MSC14.0: #1: conversion constructor called; #2: error (like in
your comments)
Watcom 1.7.1: #1, #2: conversion function called
Intel 7.1, Gnu 3.4.4: #1, #2: conversion constructor called

Certainly, it seems to be a bug. Hopefully I'm not blind ;)

Removing of const qualifier from A::operator B() will drop away an
ambiguity, as according to[3], a qualification conversion of an
implicit object parameter (B& to const B&) makes the constructor worse
conversion sequence and the conversion function should be chosen. Is
this right? If so, then there is another (or the same) bug in MSC, which
chooses a conversion constructor in such case.

-- best regards

Cezary Noweta

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