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authorChris Larson <chris_larson@mentor.com>2010-02-25 09:42:28 -0700
committerChris Larson <chris_larson@mentor.com>2010-02-26 09:12:55 -0700
commit74f50fbca194c9c72bd2a540f4b9de458cb08e2d (patch)
tree797aca668fdaf799520f2722b9dfd4a01290a287 /doc
parent5b6ccb16c6e71e23dac6920cd2df994d67c2587b (diff)
downloadbitbake-74f50fbca194c9c72bd2a540f4b9de458cb08e2d.tar.gz
Implement ??= operator
??= is a lazy, conditional assignment. Whereas a ?= immediately assigns to the variable if the variable has not yet been set, ??= does not apply the default assignment until the end of the parse. As a result, the final ??= for a given variable is used, as opposed to the first as in ?=. Note that the initial implementation relies upon finalise() to apply the defaults, so a "bitbake -e" without specifying a recipe will not show the defaults as set by ??=. Moving application of the default into getVar adds too large a performance hit. We may want to revisit this later. Signed-off-by: Chris Larson <chris_larson@mentor.com>
Diffstat (limited to 'doc')
-rw-r--r--doc/manual/usermanual.xml8
1 files changed, 7 insertions, 1 deletions
diff --git a/doc/manual/usermanual.xml b/doc/manual/usermanual.xml
index c3403449f..450ac77d7 100644
--- a/doc/manual/usermanual.xml
+++ b/doc/manual/usermanual.xml
@@ -91,7 +91,13 @@ share common metadata between many packages.</para></listitem>
<section>
<title>Setting a default value (?=)</title>
<para><screen><varname>A</varname> ?= "aval"</screen></para>
- <para>If <varname>A</varname> is set before the above is called, it will retain it's previous value. If <varname>A</varname> is unset prior to the above call, <varname>A</varname> will be set to <literal>aval</literal>.</para>
+ <para>If <varname>A</varname> is set before the above is called, it will retain it's previous value. If <varname>A</varname> is unset prior to the above call, <varname>A</varname> will be set to <literal>aval</literal>. Note that this assignment is immediate, so if there are multiple ?= assignments to a single variable, the first of those will be used.</para>
+ </section>
+ <section>
+ <title>Setting a default value (??=)</title>
+ <para><screen><varname>A</varname> ??= "somevalue"</screen></para>
+ <para><screen><varname>A</varname> ??= "someothervalue"</screen></para>
+ <para>If <varname>A</varname> is set before the above, it will retain that value. If <varname>A</varname> is unset prior to the above, <varname>A</varname> will be set to <literal>someothervalue</literal>. This is a lazy version of ??=, in that the assignment does not occur until the end of the parsing process, so that the last, rather than the first, ??= assignment to a given variable will be used.</para>
</section>
<section>
<title>Immediate variable expansion (:=)</title>