How can I pass a type by value to Lua instead of a constant reference?

[3lyrion]

My bindings return Vector3f const&, but I want to always return Vector3f

[freddysae0]

LuaBridge decides by the C++ return type: T / const T are pushed as a copy (Lua lifetime), while T& / const T& / pointers are pushed as a reference to the C++ object (C++ lifetime, and read-only for const). See Passing Objects in the docs. So you need to register something that returns Vector3f by value. The simplest way is a lambda:

.beginClass<Entity>("Entity")
    // const& -> Lua gets a const reference to e.position
    .addFunction("getPosition", &Entity::getPosition)
    // by value -> Lua gets its own copy
    .addFunction("getPositionCopy", [](const Entity* e) { return e->getPosition(); })
    // works for properties too
    .addProperty("position", [](const Entity* e) { return e->getPosition(); })
.endClass()

A lambda without a trailing return type deduces Vector3f (the const& is dropped), so LuaBridge pushes a copy.

If you have many such getters, a small adapter avoids writing a lambda per binding:

template <auto Getter> struct ByValue;

template <class C, class R, R (C::*Getter)() const>
struct ByValue<Getter> {
    static std::decay_t<R> get(const C* self) { return (self->*Getter)(); }
};

// usage
.addFunction("getPosition", &ByValue<&Entity::getPosition>::get)

(I first tried a generic lambda (const auto*), but LuaBridge can't deduce a signature from it and the overload fails. The template above deduces the class and return type from the member pointer.)

I compiled and ran this against current LuaBridge3 (1c0d0ef) + Lua 5.4.8 with MSVC, C++20:

const& : write ok? false  err: no writable member 'x'
getPositionCopy: copy.x=42.0  entity.x still=1.0
getPositionCopy2: copy.x=42.0  entity.x still=1.0      <- ByValue adapter
property: p.y=99.0  entity.y still=2.0
after C++ sets x=7 -> const& R.x=7.0                     <- the const& version aliases the C++ object

One related gotcha I hit while testing: in LuaBridge3, addProperty("x", &Vector3f::x) with a single member pointer registers a read-only property. If you want the copies to be modifiable from Lua, register the fields with addPropertyReadWrite("x", &Vector3f::x) (or addProperty("x", &Vector3f::x, &Vector3f::x)). Otherwise even a copy reports 'x' is read-only.