std::vector and oder containers as userdata

[FilippoLeon]

So I'd like to expose automatically std::vector and similar to luabridge, but not as lua table (like including Vector.h) (which incurs in a lot of copies and performance impacts for big vectors), but just a userdata with accessors.

Is there a way to do this generically without having to register every single instance manually?

Also, ideally I would also support the same syntax lua tables, I think it is easy to overload # and index metamethods, however, if I want to implement ipairs support I need to have multiple return values from luabridge.

Any idea about how best to do it?

[kunitoki]

I can try to come up with some code later how to do it easily.

[FilippoLeon]

I can give it a try myself, could be fun. I just need a starting point.

[kunitoki]

An initial starting point:

#include <vector>

template <class T>
void registerVector(luabridge::Namespace luaNamespace, const char* className)
{
    using U = std::vector<T>;

    luaNamespace
        .beginClass<U>(className)
            .addConstructor([](void* ptr) { return new(ptr) U(); })
            .addFunction("push_back", luabridge::nonConstOverload<const float&>(&U::push_back))
            .addFunction("at", luabridge::constOverload<std::size_t>(&U::at))
            .addFunction("size", &U::size)
        .endClass()
        .endNamespace();
}

TEST_F(VectorTests, RegisterCustomTest)
{
    auto localNamespace = luabridge::getGlobalNamespace(L).beginNamespace("test");

    registerVector<float>(std::move(localNamespace), "FloatVector");

    runLua("a = test.FloatVector(); a:push_back(1.0); result = a:size()");
    EXPECT_EQ(1, result<int>());
}

[FilippoLeon]

Actually I was thinking about a way to have it registered automatically. Say I have a class with a std::vector member, and I expose this. If I include Vector.h then everything is cool, but I get a lua table that uses the stack. If I leave it alone, I get an unregistered userdata that I have to find out myself and register.

What I'd like to have is a way to combine both worlds, and have a usertype that is generically registered once, if that makes sense.

[kunitoki]

It's hard for luabridge to understand when a std::vector<T> should be used as a lua table or as userdata.

If you registered a FloatVector type in luabridge it should be automatically found when you expose a std::vector<float> to lua, but you shouldn't include Vector.h as that will make it think the std::vector<float> is a convertible to lua table.

[totalgee]

This (registerVector<type>()) makes sense, it's similar to what I'm used to from ChaiScript, where there are helpers to "bootstrap" various std library containers (actually, this is initially what I expected including Vector.h would provide). In ChaiScript, you need to explicitly register specific container types you want exposed to the scripting language (for each templated type), but it's as easy as a call to a helper, similar to what you propose above:

https://github.com/ChaiScript/ChaiScript/blob/develop/include/chaiscript/dispatchkit/bootstrap_stl.hpp#L425

[totalgee]

@kunitoki I tried your registerVector() helper, and it seems to work for the basics (adding an element). One minor thing to change is this line:

            .addFunction("push_back", luabridge::nonConstOverload<const T&>(&U::push_back))

(use <const T&> as shown as the template parameter, rather than <const float&> as you had in your example)

But what more is needed to expose via registerVector(), to allow iteration over this vector type in Lua (e.g. using for k = 1, #vec do ...)? If I expose __len to give the result of std::vector::size(), then calling #vec in Lua seems to work to get the size)... How can I do something similar for indexing? I don't know enough Lua to be sure what's the best way to do this for userdata... I tried exposing a version of __index as shown below, but so far only caused crashes. (-;

    template <class T>
    void registerVector(luabridge::Namespace luaNamespace, const char* className)
    {
        using U = std::vector<T>;

        luaNamespace
            .beginClass<U>(className)
            .addConstructor([](void* ptr) { return new(ptr) U(); })
            .addFunction("push_back", luabridge::nonConstOverload<T const&>(&U::push_back))
            .addFunction("at", luabridge::constOverload<std::size_t>(&U::at))
            .addFunction("size", &U::size)
            // The following seems to work, so #vec gives the correct size in Lua:
            .addFunction("__len", &U::size)
            // The following is not correct to allow vec[n] indexing; I get a crash on push_back():
            //.addFunction("__index", luabridge::nonConstOverload<std::size_t>(&U::at))
            .endClass()
            .endNamespace();
    }

[kunitoki]

There are some tests that show how to deal with __index and __newindex metamethods registration https://github.com/kunitoki/LuaBridge3/blob/master/Tests/Source/ClassTests.cpp#L2328

You'll probably have to register a lambda that matches the way lua table indexing work.

As this is a request, i will look at providing these stl containers as userdata registration methods.