Re: Ambiguous constructor call

From:
=?ISO-8859-1?Q?Erik_Wikstr=F6m?= <Erik-wikstrom@telia.com>
Newsgroups:
comp.lang.c++
Date:
Sun, 02 Sep 2007 18:52:39 GMT
Message-ID:
<XDDCi.7655$ZA.3994@newsb.telia.net>
On 2007-09-02 19:58, xtrigger303@gmail.com wrote:

Hi to all,
I'm working on a smart pointer implementation and I'm trying to get
automatic type conversion between different pointer types. I stumbled
upon something weird (at least for me) I summarized it in the code
below. I was expecting both things at the end to work or not work at
all....
Any insight?
Thanks in advance,
Francesco

#include <iostream>

class A;

//

class B
{
public:

    B() { std::cout << "B()\n"; }

    B( B const & ) { std::cout << "B( B const & )\n"; }

    ~B() { std::cout << "~B()\n"; }

    B & operator=( B const & ) { std::cout << "B & operator=( B const & )
\n"; return *this; }

    template< typename T >
    operator T() const;
};

//

class A
{
public:

    A() { std::cout << "A()\n"; }

    explicit A( int ) { std::cout << "A( int )\n"; }

    A( A const & ) { std::cout << "A( A const & )\n"; }

    ~A() { std::cout << "~A()\n"; }

    A & operator=( A const & ) { std::cout << "A & operator=( A const & )
\n"; return *this; }
};

//

template< typename T >
B::operator T() const { std::cout << "B::operator T() const\n";
return T(); }

//

int main( )
{
    B obj001;
    A obj002 = obj001; // this works
    //A obj003( obj001 ); // this is ambiguous
}


Since B can be converted to any type you like, it can be converted to
either A or int. Both of which are types for which A has a constructor,
and the compiler have no idea which of them you would like to use. Using
explicit on a constructor only prevents it from being used like this:

   A a = 1;

You have to do

   A a(1);

--
Erik Wikstr?m

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