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

[Neal Sidhwaney via lldb-dev]

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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