[lldb-dev] small Editline wrapper cleanup req for feedback

Greg Clayton via lldb-dev lldb-dev at lists.llvm.org
Tue May 4 14:42:43 PDT 2021

As long as the solution matches "EditLine *" (C struct type from edit line library) to back to the C++ instance of "Editline" (lower case ell in "line" from LLDB). It should be easy to do with a template.

I am fine with any new solution that makes it easier to add new commands. I would rather have a templated function argument over a template argument if possible. I am thinking of something like:

createEditLineCommandDescriptor(“Command1", “Command1 help", &Foo::Foo1);
createEditLineCommandDescriptor(“Command2", “Command2 help", &Foo::Bar);

as I find it more readable.


> On Apr 30, 2021, at 9:35 PM, Neal Sidhwaney via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> Some comments in https://reviews.llvm.org/rGfd89af6880f33ead708abe2f7d88ecb687d4e0d2 <https://reviews.llvm.org/rGfd89af6880f33ead708abe2f7d88ecb687d4e0d2> prompted me to look more into potential simplifications of our EditLine wrapper and I wanted to run this by anyone who is interested before making the changes.
> Right now we set a bunch of callbacks in libedit that are captureless lambdas implicitly converted to C function pointers.  The lambdas look up an instance of our Editline class and invoke member functions.  The boilerplate that could be generated by templates is something like the following:
> class Foo { // Imagine this is our Editline class that wraps libedit
> public:
>   unsigned char Foo1(int ch) {  // These are member functions invoked by lambdas we pass to libedit
>     return 'a';
>   }
>   unsigned char Bar(int ch) {
>     return 'b';
>   }
>   unsigned char Baz(int ch) {
>     return 'c';
>   }
> };
> typedef unsigned char (*elFnPtr)(EditLine*, int);  // Signature of callbacks libedit takes (note Edit__L__ine is libedit, and Edit__l__ine is our wrapper)
> typedef unsigned char (Foo::*FooMemPtr)(int ch);   // Signature of member functions invoked
> template<FooMemPtr callback, typename Fn = elFnPtr>
> tuple<const char*, const char*, Fn> createEditLineCommandDescriptor(const char* command, const char* helpText) {
>   return make_tuple(command, helpText, [] (EditLine*, int ch) {
>     cout << ch;
>     Foo foo;
>     ((&foo)->*callback)('a');
>     return (unsigned char) ch;
>   });
> }
> auto editlineCommands = {
>   createEditLineCommandDescriptor<&Foo::Foo1>(“Command1", “Command1 help"),
>   createEditLineCommandDescriptor<&Foo::Bar>(“Command2", “Command2 help")
> };
> for (auto editlineCommand : editLineCommands) {
>  // call into libedit to add editlineCommand, e.g.:
>  el_set(EL_ADDFN, editlineCommand.get<0>(), editLineCommand.get<1>(), editLineCommand.get<2>());
> }
> The pointer to member function is a type parameter because otherwise the compiler complains about the lambda needing to capture it, in which case we could not pass it to libedit.
> I also plan to look into the wchar_t/char preprocessor logic that the original comment brought up but then I got distracted by shiny template stuff ;-)
> Thanks!
> Neal
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