Passing table with subtable to Lua function from C++

Question

I'm trying to pass table with subtable to Lua function as argument from C++.

Here's my code that doesn't work but shows what I'm trying to do.

    class DragInfo{
    public:
        std::vector <std::string> files;
        glm::vec2 position;
    };

    //a callback that passes DragInfo to a Lua function as a table which has 2 subtables
    void callDragged(DragInfo &info)
    {
        lua_getglobal(L, "dragged");
        if (!lua_isfunction(L, -1))
        {
            lua_pop(L, 1);
            return;
        }
        lua_newtable(L);
        for (size_t i = 0; i < info.files.size(); ++i)
        {
            lua_pushinteger(L, static_cast<lua_Integer>(i + 1));
            lua_pushstring(L, info.files[i].c_str());
            lua_settable(L, -3);
        }
        lua_pushnumber(L, static_cast<lua_Number>(info.position.x));
        lua_setfield(L, -2, "x");
        lua_pushnumber(L, static_cast<lua_Number>(info.position.y));
        lua_setfield(L, -2, "y");

        if (lua_pcall(L, 1, 0, 0))
            std::cout << "Error : " <<  lua_tostring(L, -1) << std::endl;
    }

And in Lua, I want to be able to, for example..

function dragged(info)
   for i=1, #info.files do
       print("dragged filename: " .. info.files[i])
   end
   print("dragged position: " .. info.position.x .. " " .. info.position.y)
end

And the result can be something like

dragged filename: background.jpg
dragged filename: apple.png
dragged position: 425 32

How should I correct my C++ function so it works properly like the example?

Henri Menke · Accepted Answer · 2018-07-08 10:36:10Z

2

It is rather simple. Just create subtables and assign those to fields in the outermost table.

I also recommend that you raise an error if dragged is not a function rather than doing nothing.

// a callback that passes DragInfo to a Lua function as a table which has 2
// subtables
void callDragged(DragInfo &info) {
    lua_getglobal(L, "dragged");
    if (!lua_isfunction(L, -1)) {
        lua_pop(L, 1);
        lua_pushstring(L, "argument is not a function");
        lua_error(L);
        return;
    }

    // outermost table
    lua_newtable(L);

    // subtable "files"
    lua_newtable(L);
    for (size_t i = 0; i < info.files.size(); ++i) {
        lua_pushinteger(L, i + 1);
        lua_pushstring(L, info.files[i].c_str());
        lua_settable(L, -3);
    }
    lua_setfield(L, -2, "files");

    // subtable "position"
    lua_newtable(L);
    lua_pushnumber(L, info.position.x);
    lua_setfield(L, -2, "x");
    lua_pushnumber(L, info.position.y);
    lua_setfield(L, -2, "y");
    lua_setfield(L, -2, "position");

    if (lua_pcall(L, 1, 0, 0) != 0) {
        std::cout << "Error : " << lua_tostring(L, -1) << '\n';
        lua_pop(L, 1);
    }
}

edited Jul 8, 2018 at 10:36

answered Jul 8, 2018 at 9:45

Henri Menke

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4 Comments

Zack Lee Over a year ago

Thank you so much for your answer. Don't I need to put lua_pop(L, 1) inside if (!lua_isfunction(L, -1)) before return?

Henri Menke Over a year ago

@ZackLee Since the error ends the program it doesn't really matter but technically yes. I put it there now, just in case the function is invoked within pcall.

Zack Lee Over a year ago

Thanks! Last question: does calling lua_setglobal(L, "name") also requires lua_pop(L,1) afterwards?

Henri Menke Over a year ago

@ZackLee “Pops a value from the stack and sets it as the new value of global” lua.org/manual/5.3/manual.html#lua_setglobal

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