On Wed, Oct 27, 2010 at 7:04 PM, logical american
<[email protected]> wrote:
> Technical and long involved question here involving C++ class behavior,
> please skip for non C++ gurus
>
> This is a question about invoking class methods from const class objects
> without the compiler complaining about ignoring the const qualifier and
> throwing an error.
>
> ----------- start program main.cpp -----------
> 1 | #include <iostream>
> 2 | #include "ui256.hpp"
> 3 |
> 4 | using namespace std;
> 5 |
> 6 | int main()
> 7 | {
> 8 | const ui256 a(7);
> 9 | ui256 b(-7);
> 10 | unsigned int i = 8;
> 11 | int j = -6;
> 12 |
> 13 | cout << "a = " << a << endl;
> 14 | cout << "i = " << i << endl;
> 15 | cout << "j = " << j << endl;
> 16 | cout << endl;
> 17 | cout << "a + i = " << a << " + " << i << " = " << a+i << endl;
> 18 | cout << "a - j = " << a << " - " << j << " = " << a-j << endl;
> 19 | cout << "b + i = " << b << " + " << i << " = " << b+i << endl;
> 20 |
> 21 | return 0;
> 22 | }
> ----------------- end program -----------------
> ui256.hpp and ui256.cpp contain the class ui256 and its methods.
>
> $ g++ -o main main.cpp ui256.cpp -ggdb
> main.cpp: In function ‘int main()’:
> main.cpp:17: error: passing ‘const ui256’ as ‘this’ argument of ‘ui256
> ui256::operator+(const ui256&)’ discards qualifiers
The class is badly declared.
Should have been:
ui256 ui256::operator+(const ui256&) const
or even better to declare a non-member operator+:
class ui256 {
...
friend ui256 operator+(const ui256& a, const ui256& b ) /*no const
here*/ { ... }
};
> main.cpp:18: error: passing ‘const ui256’ as ‘this’ argument of ‘ui256
> ui256::operator-(const ui256&)’ discards qualifiers
>
Same thing, operator- is declared as non-const.
> The operator + is defined as
>
> ui256 operator+(const ui256& rhs);
>
> The compiler complains that the const ui256 a(7) object invoking the +
> operator disregards the const since the operator is working on a
> non-const class (temporary) which is true.
It has nothing to do with temporaries. operator+ is invoked on "a"
which is const, but the operator is not.
>
> If I change the operator to:
>
> ui256 operator+(ui256& rhs);
>
> then the compiler complains that
>
> main.cpp:17: error: no match for ‘operator+’ in ‘a + i’
Try "i+a", it'll be even more unhappy. That's why you declare
non-member operator+.
>
> which is true, there isn't any function signature match to const a in
> this case.
>
> So how is an operator+ for a const object defined?
>
> For the subtraction in line #18, I wrote an outside the class operator
> and the compiler complains about ambiguity, since it is trying to coerce
> a native type to the class type then apply the class operator.
>
> ui256.cpp: In function ‘ui256 operator-(const ui256&, int)’:
> ui256.cpp:1632: warning: ISO C++ says that these are ambiguous, even
> though the worst conversion for the first is better than the worst
> conversion for the second:
> ui256.cpp:1629: note: candidate 1: ui256 operator-(const ui256&, int)
> ui256.hpp:127: note: candidate 2: ui256 ui256::operator-(const ui256&)
The compiler is correct. But you don't need these additional operators.
>
> I just need some way of creating temporaries from constants which invoke
> non-const class methods without the compiler complaining about tossing
> aside the const when using it.
>
> Any ideas of what I am doing wrong here and how to really fix the problem?
>
> -Randall
>
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--
Fedor G Pikus ([email protected])
http://www.pikus.net
http://wild-light.com
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