Serious bug: char*-valued functions that return nullptr make system go BOOM

[rpatters1]

I have finally nailed down the actual situation that is causing a breaking change in my scripts. I apologize for two previous long-winded issues that were wrong (and now closed).

Consider this class and its registration:

class foo
{
public:
   foo() {}
   const char * nullconst() { return nullptr; }
   char * null() { return nullptr; }
   void twochars (const char* one, const char* two = nullptr)
   {
      std::string one_two = one;
      if (two) one_two += " another parameter was provided: " + two;
   }
};

   luabridge::getGlobalNamespace(L)
     .beginNamespace("foobar")
        .beginClass<foo>("foo")
           .addConstructor<void(*)(void)>()
           .addFunction("nullconst", &foo::nullconst)
           .addFunction("null", &foo::null)
           .addFunction("twochars", &foo::twochars)
        .endClass()
     .endNamespace();

Now consider the following Lua:

local foo = foobar.foo()
local nullconst = foo:nullconst() -- nullconst comes back nil on LB2 but "" on LB3: this is a breaking change
local null = foo:null() -- null doesn't come back. system goes >>BOOM<< on both LB2 and LB3

See comments. My opinion is that both local variables nullconst and null should be nil after this code runs.

[rpatters1]

I edited the original description to add the twochars function to illustrate another breaking change from LB2 that is related. The 2nd parameter of twochars is optional in C++ and takes nullptr as the default. With LB2 it is possible to call this function as

foo:twochars("aaa", nil)

However on LB3 it receives a runtime error: "Error decoding argument 3: The lua object can't be cast to desired type". This does not seem acceptable to me. To support the twochars function, I have to either modify it to handle empty strings the same as nullptr, which is not possible (or at least undesirable) with 3rd party libraries. Besides, nullptr and "" are not the same thing. The other route is that I would have to find all the needles in my haystack that need proxy functions: a laborious and error-prone nightmare.

I don't think this is an acceptable breaking change.

[kunitoki]

Fixed in latest master