Conversion operators: different behaviour for built-in types

[Paul]

struct Cref {
char c;
operator const char&() const { return c; }
};

struct Sref {
std::string s;
operator const std::string&() const { return s; }
};

int main()
{
Cref cr;
if (cr == 'a') return 1;

Sref sr;
if (sr == "a") return 2; // same if "a" is replced with std::string("a") or
const std::string&
}

The above code works for cr but not for sr and I wonder why and how to make
it work. I am trying to use a reference type in a way similar to how
Stroustrup does this for strings in his example (p. 296) and just stumbled
upon this.

Thank you.

Paul

[Igor Tandetnik]

Post by Paul
struct Cref {
char c;
operator const char&() const { return c; }
};
struct Sref {
std::string s;
operator const std::string&() const { return s; }
};
int main()
{
Cref cr;
if (cr == 'a') return 1;
Sref sr;
if (sr == "a") return 2; // same if "a" is replced with std::string("a") or
const std::string&
}
The above code works for cr but not for sr and I wonder why

There is a whole bunch of templated operator== implementations in scope. When deducing template parameters from arguments, user-defined conversions are not considered, and so none of them match.

On the other hand, a built-in == operator is treated essentially as a non-template function for the purposes of overloading resolution, and can accept user-defined conversions.

Post by Paul
and how to make
it work.

Why do you want to make it work? What are you trying to achieve? Is there any practical purpose for this?

For this particular case, instead of a conversion operator, you could define three overloads of operator==, taking (Sref, string), (string, Sref) and (Sref, Sref).

[Paul]

Post by Igor Tandetnik
There is a whole bunch of templated operator== implementations in scope. When deducing template parameters from arguments, user-defined conversions are not considered, and so none of them match.
On the other hand, a built-in == operator is treated essentially as a non-template function for the purposes of overloading resolution, and can accept user-defined conversions.

I forgot that

...bool operator ==(const ...string<...>&, const ...string<...>&)
...bool operator ==(const charT*, const ...string<...>&)
...bool operator ==(const ...string<...>&, const charT*)

are all templates.

Post by Igor Tandetnik
Why do you want to make it work? What are you trying to achieve? Is there any practical purpose for this?
For this particular case, instead of a conversion operator, you could define three overloads of operator==, taking (Sref, string), (string, Sref) and (Sref, Sref).

I had this implementation in mind:

class Message {
typedef std::map<int, std::string> Fields;
public:
class Fref;

//...

Fref operator [](int tag);
const std::string& operator [](int tag) const;
private:
std::string m_empty;
Fields m_fields;
};

class Message::Fref {
friend class Message;

Message& message;
int tag;

Fref(Message& m, int t) : message(m), tag(t) {}
Fref(const Fref& r) : message(r.message), tag(r.tag) {}

void operator =(const Fref&);
public:
operator const std::string&() const { return const_cast<const
Message&>(message)[tag]; }
void operator =(const std::string& v) { if (!v.empty())
message[tag] = v; }
};

inline Message::Fref Message::operator [](int tag)
{
return Fref(*this, tag);
}

inline const std::string& Message::operator [](int tag) const
{
Fields::const_iterator p = m_fields.find(tag);
return p != m_fields.end() ? p->second : m_empty;
}

and the idea, as in Stroustrup, was to use different semantics to read and
to write. Reading would just return a const std::string& to field data whilst
writing would check the value to write was not empty and would perform some
calculations (not shown) on the field being added or modified.

Thank you, Igor.

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[Paul]

Forgot to mention: this is the error message from the compiler:

1>.\....cpp(558) : error C2678: binary '==' : no operator found which takes
a left-hand operand of type 'Sref' (or there is no acceptable conversion)
1> could be 'built-in C++ operator==(const char [2], const char [2])'
1> C:\Program Files\Microsoft
SDKs\Windows\v6.0A\include\guiddef.h(192): or 'int operator ==(const
GUID &,const GUID &)'
1> while trying to match the argument list '(Sref, const char [2])'