Re: conversion operator problem

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Thu, 26 Jul 2007 15:25:02 +0200

Message-ID:

<13ah85i9krdcd10@corp.supernews.com>

* kowochen@gmail.com:

i encounter a problem as following:

#include <iostream>
#include <string>
using namespace std;

class X
{
public:
    X(const string& s, int i):s_(s),i_(i)
    {}

    operator string ()
    {
        return s_;
    }

    operator int ()
    {
        return i_;
    }

private:
    string s_;
    int i_;
};

int main()
{
    string s("abc");
    int i(10);

    X x("abc", 10);

    if (x == i)
    {
        cout<<"int ok"<<endl;
    }

    return 0;
}

================================
x can be compared with the integer i and the output is "int ok",
but when i wanna compare with string s, it is a compile error:
if (x == s)
{
    ...
}

If convert x to string explicitly, it is ok:
if ((string)x == s)
{
    ...
}

Why couldn't it do a implicit type conversion to string like build in
type?

Comparision with built-in type int uses the built-in "==". This invokes
X::operator int().

Comparision with std::string needs some custom "==". No such is found.

--
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