Re: resolving of return types

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 03 Dec 2009 08:11:16 -0500

Message-ID:

<hf8dev$fr9$1@news.datemas.de>

kmw wrote:

Hello,

consider the following code snippet. I do not understand why the
compiler is not able to choose the correct variation of the method get
():
test.cpp: In function ?int main()?:
test.cpp:31: error: conversion from ?B? to non-scalar type ?C?
requested

What is the difference to the std::vector example? Isn't begin ()
defined as below?

iterator begin();
const_iterator begin() const;()

Well, yes, without the last parentheses, of course. :-) But what is
'iterator' and 'const_iterator', and what's the relationship between them?

Any comments are appreciated.
Best regards,
Kay

#include <vector>
class B
{
};
class C
{
};
class A
{
    public:
        B get ( )
        {
            B ret;
            return ret;
        }
        C get ( ) const
        {
            C ret;
            return ret;
        }
};
int main ( )
{
    // this works
    std::vector<int> test_vector;
    std::vector<int>::const_iterator it = test_vector.begin ();
    std::vector<int>::iterator it2 = test_vector.begin ();
    // but this not
    A test_a;
    B test_b = test_a.get ();
    C test_c = test_a.get ();
    return 0;
}

The answer is simple: the 'const_iterator' implemented in 'std::vector'
can be initialized from a regular 'iterator'. The first time you call
'begin()' (when initializing 'it'), it returns a regular iterator, and
calls the regular 'begin()', not a const one. Then 'it' is initialized
from the regular iterator. Add a c-tor in 'C' to initialize it from a
'B', and you're going to be OK:

class C { public: C(); C(B); };

V
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