# The variable parameter type

The application can register functions that take a reference to a variable type, which means that the function can receive a reference to a variable of any type. This is useful when making generic containers.

When a function is registered with this special parameter type, the function will receive both the reference and an extra argument with the type id of the variable type. The reference refers to the actual value that the caller sent, i.e. if the expression is an object handle then the reference will refer to the handle, not the actual object.

    // An example usage with a native function
    engine->RegisterGlobalFunction("void func_c(?&in)", asFUNCTION(func_c), asCALL_CDECL);

    void func_c(void *ref, int typeId)
    {
        // Do something with the reference

        // The type of the reference is determined through the type id
    }

    // An example usage with a generic function
    engine->RegisterGlobalFunction("void func_g(?&in)", asFUNCTION(func_g), asCALL_GENERIC);

    void func_g(asIScriptGeneric *gen)
    {
        void *ref = gen->GetArgAddress(0);
        int typeId = gen->GetArgTypeId(0);

        func_c(ref, typeId);
    }

The variable type can also be used with `out` references, but not with `inout` references. Currently it can only be used with global functions, object constructors, and object methods. It cannot be used with other behaviours and operators.

The variable type is not available within scripts, so it can only be used to register application functions.

See also:
  any object and dictionary object for examples