Difficulty reading a std::string from a LuaTe

[Jon Capps]

Hello, and thanks for directing me to the forum ;)

I'm experimenting with lua_icxx to see how helpful it might be. I'm having
trouble with this code:

LuaTempResult ltr = _interpreter_ptr->eval( "return 'test'" );
std::string ret = ltr;

Based on the examples provided it seems it should work. However, I get the
compiler error:

c:\users\jon\documents\visual studio
2010\projects\dron\dron\script\script.cpp(29): error C2440: 'initializing' :
cannot convert from 'LuaTempResult::Item' to
'std::basic_string<_Elem,_Traits,_Ax>'
1> with
1>
1> No constructor could take the source type, or constructor overload
resolution was ambiguous

When I change the code somewhat, I get an error better describing the
ambiguity:

1>c:\users\jon\documents\visual studio
2010\projects\dron\dron\script\script.cpp(27): error C2593: 'operator =' is
ambiguous
1> c:\program files (x86)\microsoft visual studio
10.0\vc\include\xstring(772): could be 'std::basic_string<_Elem,_Traits,_Ax>
&std::basic_string<_Elem,_Traits,_Ax>::operator =(_Elem)'
1> with
1>
1> c:\program files (x86)\microsoft visual studio
10.0\vc\include\xstring(767): or 'std::basic_string<_Elem,_Traits,_Ax>
&std::basic_string<_Elem,_Traits,_Ax>::operator =(const _Elem *)'
1> with
1>
1> c:\program files (x86)\microsoft visual studio
10.0\vc\include\xstring(762): or 'std::basic_string<_Elem,_Traits,_Ax>
&std::basic_string<_Elem,_Traits,_Ax>::operator =(const
std::basic_string<_Elem,_Traits,_Ax> &)'
1> with
1>
1> c:\program files (x86)\microsoft visual studio
10.0\vc\include\xstring(707): or 'std::basic_string<_Elem,_Traits,_Ax>
&std::basic_string<_Elem,_Traits,_Ax>::operator
=(std::basic_string<_Elem,_Traits,_Ax> &&)'
1> with
1>
1> while trying to match the argument list '(std::string,
LuaTempResult::Item)'

I'm running Win7 64bit, and using MSVC 2010 Express. Am I doing something
wrong? How can I get this to work?

Thanks.

[schoenborno]

Jon, glad you decided to post. On MS VS 2005 the following snippet works fine:

void testDebug()
{
    LuaInterpreter lua;

    LuaTempResult ltr = lua.eval( "'test'" );
    std::string ret = ltr[ 1 ];

    std::cout << ret << std::endl;
}

(This is not relevant to your problem but note that "eval" evaluates an
expression, meaning you do not need to put "return": whatever the expression
evaluates to is returned to the C++ side)

It'll take me a couple days to try it in 2010 as I have been stuck with 2005
for a long time.

You could also try to bypass Item altogether and do

LuaObjRef test = lua.eval("'test'"); 
std::string msg = test;

Also try even simpler

std::string msg = lua.eval("'test'");

Oliver

[schoenborno]

Jon, I also found that the last example I gave you gets a syntax error with
"ambiguous operator=", which I solved by the following:

std::string msg = (string)lua.eval("'test'");

This makes me wonder if the following might be necessary in 2010:

LuaTempResult ltr = lua.eval( "'test'" );
std::string ret = string(ltr);

(you don't need the because when not given, the first item returned is
automatically used, though adding also works).
Oliver

[Jon Capps]

Thanks for the quick replies. I'll try your suggestions at my earliest
opportunity.

Jon

[Jon Capps]

Yes, my errors were ambiguous operator= errors as well. Following your lead, I
found that casting works:

    LuaTempResult ltr = _interpreter_ptr->eval( "'test'" );
    ret = static_cast< std::string >( ltr );

I'm not knowledgeable enough to know why the 2010 compiler is more picky than
the 2005 compiler, but if casting works, then I'm happy with it ;)

Thanks for your help.

Jon

[schoenborno]

Hi Jon, well it's good to hear that there is a workaround, but it certainly
not great news if static_cast is the only way in VS 2010 Express. So you tried

ret = string(ltr);

and that did not work? Did you try

std::string ret( ltr );

I'm very surprised that these would fail if static_cast works.

Oliver

[Jon Capps]

Ah, my apologies. Yes, both those approaches work. I just prefer the C++-style
casting since it's easier to spot when skimming code.

[schoenborno]

Ah that is good news, thanks for checking. Yes I agree that if you are
going to cast, then static_cast<> is better than old-style cast. However I
much prefer not to use casting at all, and instead use constructor or
conversion (the two methods that I asked about in my last post). Cheers,
Oliver