Editing false - FilterMeisterWiki

When saving, enter the following:

  _____  ______  TTTT    #    #     _____ 
 | ____||___  /   TT     ##  ##    / ____|
 |  _|     / /    TT     # ## #   | |     
 | |___   / /     TT     # ## #   | |     
 |_____|./ /___   TT     #    #   | |____ 
        \_____/   TT     #    #    \_____|
                 TTTT    #    #

Visit Preferences to set your user name

= false =
== Description ==

{{false}} is a Boolean constant representing a logical state of "falsehood". In FilterMeister, any numeric value of 0 or ą0.0, the NUL character constant, and any null pointer value may represent a state of "falsehood". The integral numeric value 0 is reserved as the ''canonical'' representation of "false". Thus, the Boolean constant {{false}} has a numeric value of 0 when evaluated in a numeric context.

Note that ''any'' zero or null value represents "falsehood", so the integer constant 0, the floating point constants 0.0, -0.0, 0.0L, and -0.0L, the pointer constant {{((NULL))}}, and the character constant '\0' (NUL) will all evaluate as false in a Boolean or logical context. A null string ("" or "\0"), however, will evaluate as true, since the value of a character string is its address, and by convention no object in FM may be allocated at address 0.

=== Caution ===
A common cause of mistakes in C or FM programming is to assume that any true expression has the value 1 (or {{((true))}}). It is bad programming style, and a frequent cause of subtle errors, to code, for example:
<code>
    if (flag==true) break;
</code>
since this code will ''not'' break if the value of {{flag}} is 77, which also represents truth. Unless the programmer is specifically testing for the value 1, the preferred idiom is:
<code>
    if (flag) break;
</code>
which will break when {{flag}} contains ''any'' true value (i.e., any value but 0, ą0.0, '\0', or {{((NULL))}}), and not just in the case that {{flag}} has the value 1.

Note that this caution does not apply to the Boolean constant {{false}}, since the canonical numeric value of {{false}} is 0.  The only other representation of falsity is a null pointer or a NUL character, and the numeric value of these is always 0 in FM. Thus the following code fragment, while poor style, will always break correctly when {{flag}} has the value 0, ą0.0, {{false}}, '\0', or {{((NULL))}}.
<code>
    if (flag==false) break;
</code>
Nevertheless, the preferred coding style is:
<code>
    if (!flag) break;
</code>

== Example ==
<code>
%ffp

OnFilterStart:{
    bool flag = false;

Info("The integer value of false is %d", flag);

if (flag) Info("This should not appear");

return false;
}
</code>

This snippet will display a single message box with the message:
<code>
    The integer value of false is 0
</code>

== Also see ==
::((true)), ((NULL)), ((Constants))

Summary of change:
Add document to category: