Re: template operator== not working

"Alf P. Steinbach" <>
Thu, 08 May 2008 22:09:19 +0200
* Ben Voigt [C++ MVP]:

Alf P. Steinbach wrote:

* Ben Voigt [C++ MVP]:

You might not see that example as typed but if( a * b = c) ... does
occure because of typing mistakes. The compiler will catch this.

And why would you want to prevent that?

Because the side-effect of the assignment operator is to change a
temporary that is immediately thrown away, obviously not intended.

The programmer meant to use the comparison operator == instead.

Did she?

Anyway, the techniques for dealing with
inadvertent-assignment-in-condition for built-in types (namely high
warning level for compilation, writing constants on left side, code
inspection, systematic testing, etc.) apply here also.

"writing constants on left side"?

That's exactly what is going on here!

Nope. An expression is not necessarily a constant, and in particular, in C++,
an rvalue is not necessarily a constant. For class types it is in most cases
not constant, e.g. the std::vector<T>().swap(v) idiom for clearing a vector v
relies on this, and the rvalue non-constness is also used in the code below.

Applying my well-known telepathic powers, I think you're perhaps confusing an
expectation about the result of '*' in e.g. mathematics, with operator results
in C++.

The technique of using constants on the lhs has two common cases: literal, or
something named and declared 'const', which for the above and a class Rational
would mean

   Rational const product = a*b;
   if( product = c ) // Oops.

Unfortunately, in this context, operator= is one of those that can't be defined
as a free-standing function. If it could then the above would not even be
potential problem, because a temporary can't be bound to reference to non-const.
  With member operator= you can't directly prohibit assignment to non-const
rvalue (which is generally preferable, especially for optimization).

An alternative is to change the syntax for assignment, so that '=' simply isn't
valid. Examples: 'a.value() = b', or 'a.assignFrom( b )', or 'a ^= b'. The
'^=' syntax on the assumption that exclusive-or assignment isn't useful on its
own for this class.

Example that illustrates all three assignment syntaxes mentioned above (I would
not, however, do this, because I don't think =/== is a real problem):

template< typename T >
class AssignableRef
     T* myRef;
     AssignableRef( T& v ): myRef( & v ) {}
     T& operator=( T const& v ) { return myRef->assignFrom( v ); }

class Rational
     Rational& operator=( Rational const& ); // No such.

     Rational( int = 0 ) {}
     void swap( Rational& ) {}

     Rational& assignFrom( Rational const other )
         Rational( other ).swap( *this );
         return *this;

     Rational& operator^=( Rational const& other )
         return assignFrom( other );

     AssignableRef<Rational> value()
         return AssignableRef<Rational>( *this );

     Rational& operator*=( Rational const& ) { return *this; }
     bool operator==( Rational const& ) { return true; }

Rational operator*( Rational const& a, Rational const& b )
     return Rational( a ) *= b;

int main()
     Rational a, b, c;

     a ^= b; // OK, this is an assignment.
     a.assignFrom( b ); // OK, this is also an assignment.
     a.value() = b; // OK. this is also an assignment.

     if( a*b == c ) {} // OK, this is a comparision.
     //if( a*b = c ) {} // Oops, does not compile (which is OK).

The question is though, is the potential error detection really worth the
non-standard notation? If it isn't, then that means the potential error isn't
really that much of a problem in practice. And then the cost of a 'const'
result, in particular prohibiting optimization, is counter-indicated.

There are also other alternatives, such as the extremely dirty trick (formally
UB if you use the standard library) of redefining 'if' so that it /requires/ a
'bool' value. But that extremely dirty trick doesn't help with ?:, nor does it
help with assignment to bool... Another alternative: it might be that the
compiler can produce a warning that can be turned into an error, but this
alternative is compiler-specific. A third is, as mentioned, the convention
always writing something const on the left hand side of '==', although the logic
of that is questionable, for what if you forget, how can you detect that?

Best: simply don't write '=' instead of '==', and test the software so that any
such slip-ups are detected.

If '=' instead of '==' passes all tests, then, after all, it doesn't matter.

Fixing that possible problem for e.g. class Rational amounts to
nothing compared to the rest of code, and as mentioned it does have a
cost, so, pain but no gain.

Cheers, & hth.,

- Alf

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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