Question

I am trying to do some OO functionality in Lua via the C API. In my Lua script I have the following:

Parser = {}
Parser.__index = Parser

function Parser:create(url)
    local self = {}
    print ("creating")
    self.baseUrl = url
    setmetatable(self, Parser)
    return self
end

function Parser:Foo()
    print ("baseUrl: " .. self.baseUrl)
end

p = Parser:create("http://www.google.com")
p:Foo()

If I run this from the command line, it works fine and I see the following output:

creating
baseUrl: http://www.google.com

Now, if I comment out the last two lines and try the following through the C API

// <load the state and lua file>
lua_getglobal(L, "Parser");
lua_getfield(L, -1, "create");
lua_pushstring(L, "http://www.google.com");

if (lua_pcall(L, 1, 1, 0) != 0)
{
  // handle error
}

This works. I see “creating” in my standard output as expected. As I understand it, the new Parser object is now on top of the stack. If I them immediately try the following:

lua_getfield(L, -1, "Foo");
if (lua_pcall(L, 0, 0, 0) != 0)
{
  logger()->error("-- %1", lua_tostring(L, -1));
}

I get the following error: attempt to index local ‘self’ (a nil value)

Can anyone tell me what I’m doing wrong and how to get the function to run as expected?

Thank you!

Answer 1

The definition function Parser:Foo() ... end is equivalent to:

Parser.Foo = function(self)
    print ("baseUrl: " .. self.baseUrl)
end

That is — Foo is a function that takes one argument. When you call lua_pcall(L, 0, 0, 0) you are passing 0 arguments. Change it to lua_pcall(L, 1, 0, 0) and everything should work. (You will have to also change the pcall to create to correctly pass 2 arguments rather than 1).