Implicit conversion and user defined conversion

Question

When I write code like this:

struct foo {
    operator int() const { return 1; }
};
int main() {
    foo a, b;
    auto tmp = (a < b);
}

it works, but while I write code like this:

struct foo {
    operator string() const { return string("foo"); }
};
int main() {
    foo a, b;
    auto tmp = (a < b);
}

Compiler(clang++) says that error: invalid operands to binary expression ('foo' and 'foo')

I wonder why, as both string type and int type have comparison operators, but when foo has a user defined int conversion, it will implicit convert to int to compare, however when foo only has a user defined string conversion, compiler doesn't do implicit conversion though (string)a<(string)b works well.

I dont know the rule that causes this, but most likely it has something to do with `int` being a built in type — 463035818_is_not_an_ai, Feb 20 '17 at 12:09
related/dupe: http://stackoverflow.com/questions/42252023/implicit-conversion-operator-priority — NathanOliver, Feb 20 '17 at 12:34
@NathanOliver imho question isnt really a dupe, but the answer pefectly explains also this one — 463035818_is_not_an_ai, Feb 20 '17 at 12:36
@tobi303 Yeah, that is why I did not close it myself. Not sure if it should be closed as a dupe or not but the answer does explain the problem. — NathanOliver, Feb 20 '17 at 12:38
Possible duplicate of [Implicit conversion operator priority](http://stackoverflow.com/questions/42252023/implicit-conversion-operator-priority) — Tsumiki Mikan, Feb 20 '17 at 12:45
@Tsumiki I think this one deserves its own answer, I would expect [this](http://ideone.com/TAQlJJ) to compile, or maybe I missed something — 463035818_is_not_an_ai, Feb 20 '17 at 12:53

score 1 · Accepted Answer · answered Feb 20 '17 at 13:50

I think the problem is that string is not a basic type. std::string is a specialization of a template, specifically std::basic_string<char>

So operator < is defined as

template <class CharT, class Traits, class Allocator>
    bool operator< (const std::basic_string<CharT, Traits, Allocator> &_Left,  const std::basic_string<CharT, Traits, Allocator> &_Right);

It will work with:

auto tmp = (static_cast<std::string>(a) < static_cast<std::string>(b));

Then operator < becomes:

bool std::operator< <char, std::char_traits<char>, std::allocator<char>>(const std::string &_Left, const std::string &_Right)

And because user defined conversion are not considered for template function specialization. See this related answer: [http://stackoverflow.com/questions/42252023/implicit-conversion-operator-priority/42253006#42253006]. — Oliv, Feb 20 '17 at 17:14

Implicit conversion and user defined conversion

1 Answers1