As has been noted in a recent thread, WebKit strives to make the code clear and
understandable rather than have embedded comments that can become obsolete.
One common practice that I find quite opaque is for classes to have functions
returning pointers which can never return 0, especially when such functions are
often chained.
For example, is this call safe?
int fontSize = node->document()->frame()->page()->settings()->minimumFontSize();
(Yes, I could call document()->settings() directly, but I want to illustrate
all the different styles of writing accessors.)
No it is not:
Node::document() returns m_document, with an assert documenting that it never
returns 0
Document::frame() has a comment stating that it CAN return 0
Frame::page() CAN return 0 - I think. It just returns "m_page" with no
comments - is this the same situation as Node::document(), so m_page is never
0, but nobody bothered to assert? I deduce that m_page can be 0 because the
detachFromPage() function which sets m_page to 0 is defined next in the file
and I happened to look down and notice it.
Page::settings() cannot return 0, since it returns m_settings.get() and
m_settings is an OwnPtr, so I know its lifetime matches that of the Page.
So the correct way to write this is:
int fontSize = 0;
Frame* frame = node->document()->frame();
if (frame) {
Page* page = frame->page();
if (page)
fontSize = page->settings()->minimumFontSize();
}
But the only way to know this is to read the code for each accessor! And
forget about memorizing which is which: Document::settings() can return 0, but
Page::settings() can't, so whether "settings()" needs to be checked or not
depends on the context.
Is there a reason we don't return a reference from functions that are
guaranteed to be non-zero? Then the above would become:
int fontSize = node->document().frame()->page()->settings().minimumFontSize();
And the error is easy to spot: all the dereferences without checking for 0 are
dangerous. And the compiler will enforce this.
I find the common use of things like "document()->frame()" without checking the
return value of document() is a big impediment to understanding the code,
because I'm never sure if I can take it as gospel or if someone has been sloppy
and forgotten to check return values. And in turn it leads to sloppiness,
since it's easy to forget which accessors do require null checks and write an
invalid chain such as the one above.
Joe
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