<div><div dir="auto">I think the magic for Decl/DeclContext is the overrides of "doit" at the bottom of DeclBase. This is replacing the default casting implementation to make this work. Unfortunately I'm on my phone so I can't get a lot more details.</div><br><div class="gmail_quote"><div>On Wed, Jun 28, 2017 at 6:04 PM Riley Dulin via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:11.0pt">Hello,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">I recently ran into an issue where I wanted to use dyn_cast with a Multiple Inheritance hierarchy. LLVM’s help page on RTTI claims that it can be done, and that Clang’s Decl and DeclContext implement it; however,
when I try to use it I run into odd behavior. Here’s my sample code which doesn’t work:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">```<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">struct Base {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> void *ptr;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> bool hasInfo;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">};<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">struct Info {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> int size;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> static bool classof(const Base *b) { return b->hasInfo; };<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">};<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">struct Child : public Base {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> Child() {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->ptr = this;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->hasInfo = false;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">};<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">struct ChildWithInfo : public Base, public Info {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> ChildWithInfo() {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->ptr = this;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->hasInfo = true;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->size = 10;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">};<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">int main() {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> Base *c = new Child();<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> Base *i = new ChildWithInfo();<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> try {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> if (Info *inf = llvm::dyn_cast<Info>(c)) {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> throw std::string("Casted a child to an info incorrectly");<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> if (Info *inf = llvm::dyn_cast<Info>(i)) {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> if (inf->size != 10) {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> std::ostringstream str;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> str << "Object was sliced: expected 10 but got " << inf->size;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> throw str.str();<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> } else {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> throw std::string("Couldn't cast child with info to info");<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> } catch (std::string &msg) {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> std::cout << msg << std::endl;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> delete c;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> delete i;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">```<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Basically the error is that when a `Base` is cast to an `Info`, it starts reading out of the same offset where `ptr` is stored instead of `size`.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">How can I modify my class setup so that I can use this scheme safely?<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Looking at `Decl` and `DeclContext`, it is unclear to me how they set things up so that the field offsets are correct, although I have a feeling<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">It has something to do with the `LLVM_ALIGNAS` in the class declaration of `Decl`.<br>
<br>
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Regards,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Riley Dulin<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><u></u> <u></u></span></p>
</div>
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</blockquote></div></div><div dir="ltr">-- <br></div><div data-smartmail="gmail_signature">~Craig</div>