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digitalmars.D.bugs - [Issue 13489] New: Boolean semantics of floating point types should


          Issue ID: 13489
           Summary: Boolean semantics of floating point types should use
                    "<> 0"
           Product: D
           Version: D1 & D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: DMD
          Assignee: nobody puremagic.com
          Reporter: david.eckardt sociomantic.com

Floating point numbers can be implicitly cast to bool so they can be used as
if() and assert() arguments. In this case NaN evaluates to true.
I'm working at Sociomantic where this behaviour didn't reveal a bug and caused
us to attempt to spend a monetary amount of $9223372036854.775808:

import stdc.math: lround, fabs;

ulong calcAmount ( double x )
    assert(x); // succeeds for NaN
    // do calculation with x
    return lround(fabs(x)); // returns 9223372036854775808 if x is NaN

It happens because currently for a floating point variable x "cast(bool)x"
behaves like "(x != 0)". I suggest changing that to "(x <> 0)".

Rationale: The difference between "!=" and "<>" is subtle, many programmers
aren't aware of it, and even the ones who are do this mistake easily, as it
happened to me.
"x != 0" is appropriate for integer and pointer types. For floating point types
one has always to respect NaN, and "!=" doesn't do that so, unless NaN is
anticipated, one should use "<>" instead. This applies in most use cases and,
since it is the initial floating point value, NaN is encountered mostly caused
by a bug of using a variable without assigning it before. This very notorious
bug should be detected by "assert(x)", and I'd be surprised if most existing
code that does "if (x)" (or otherwise uses floating point values as boolean)
works as it should with NaN.

Sep 17 2014