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<p class="MsoNormal"><span style="font-size:11.0pt">Hello,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></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:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">```<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">struct Base {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> void *ptr;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> bool hasInfo;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">};<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">struct Info {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> int size;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> static bool classof(const Base *b) { return b->hasInfo; };<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">};<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">struct Child : public Base {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> Child() {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->ptr = this;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->hasInfo = false;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">};<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">struct ChildWithInfo : public Base, public Info {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> ChildWithInfo() {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->ptr = this;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->hasInfo = true;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> this->size = 10;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">};<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">int main() {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> Base *c = new Child();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> Base *i = new ChildWithInfo();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> try {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> if (Info *inf = llvm::dyn_cast<Info>(c)) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> throw std::string("Casted a child to an info incorrectly");<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> if (Info *inf = llvm::dyn_cast<Info>(i)) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> if (inf->size != 10) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> std::ostringstream str;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> str << "Object was sliced: expected 10 but got " << inf->size;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> throw str.str();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> } else {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> throw std::string("Couldn't cast child with info to info");<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> } catch (std::string &msg) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> std::cout << msg << std::endl;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> delete c;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> delete i;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">```<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></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`.<o:p></o:p></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?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></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<o:p></o:p></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>
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Riley Dulin<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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