Re: value vs. const ref, char* vs char[4] - part 2

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

comp.lang.c++

Date:

Tue, 5 Feb 2008 00:56:43 +0100

Message-ID:

<60pn08F1r9vfbU1@mid.individual.net>

Rick wrote:

Is the following appropriate behavior? It certainly isn't what I
expected.

So your expectations are wrong. :-)

#include <iostream>

using namespace std;

template<typename T> bool fun(const T& value) {
   cout << "In fun(const T&);" << endl;

}

template<typename T> bool fun(T value) {
   cout << "In fun(T);" << endl;

}

int main(int argc, char** argv) {
   fun(static_cast<const int&>(10) );

}

$ CC test.C
"test.C", line 14: Error: Overloading ambiguity between
"fun<int>(const int&)" and "fun<int>(int)".
1 Error(s) detected.
$
$
$ g++ test.C
test.C: In function `int main(int, char**)':
test.C:14: error: call of overloaded `fun(const int&)' is ambiguous
test.C:5: note: candidates are: bool fun(const T&) [with T = int]
test.C:9: note: bool fun(T) [with T = int]
$

I would have said that if I tell the compiler explicitly that I
want a const int&, there is no ambiguity. Something in the C++
standard apparently says that this is correct behavior, but it
seems wrong to me.

Your problem is that there is not such thing as "a const int&",
because that is just a reference to an object that exists elsewhere.
It can't exist by itself.

Try this instead, assuming a class Person:

Person Robert; // A person called Robert

Person& Bob = Robert; // Another name for Robert

fun(Robert);
fun(Bob);

Do you want different overloads called depending on what name we use
for the same person? No!

Now this:

const number ten = 10;

const number& also_ten = ten;

fun(ten);
fun(also_ten);

Do you want different overloads called depending on what name we use
for the number 10? Not really!

Bo Persson