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SBM ModScript, Part 12 - Class Inheritance

SBM ModScript, Part 12 - Class Inheritance

After looking into ChaiScript classes, I discovered that ChaiScript does not currently support class inheritance. This is common for scripting languages. However, as ChaiScript is such a powerful language, I was wondering how hard it would be to implement it. I came up with the following two options.

Option 1

Option 1 was updated on June 11, 2018 to introduce the "this.This" object, which allows for methods in the base class to invoke overrides on the child class.

def _getBaseVarName( obj ) {
  return "_${obj.get_type_name()}_base";
}

def _getBase( obj ) {
  return eval("obj.${_getBaseVarName(obj)}");
}

def _setThis( obj, ancestor ) {
  ancestor.This := obj;
  if ( eval( "!ancestor.${_getBaseVarName(ancestor)}.is_var_undef()" ) ) {
    _setThis( obj, ancestor._getBase() );
  }
}

def _addMethod( obj, funcname, containedFunc ) {
  var pts = containedFunc.get_param_types();
  
  // since we checked arity above, we know there will be at least 2 entries, so this is safe
  if ( pts[1].is_type_undef() || pts[1].name() == "Dynamic_Object" ) { // if parameter is for a Dynamic_Object class, this is true
    // ensure there are no guards on the parameters (pts[0] is return, pts[1] is "this"
    if ( pts.size() > 2 ) {
      for( var ii = 2; ii < pts.size(); ++ii ) {
        if ( pts[ii].name() != "Object" ) {
          return false;
        }
      }
    }
    
    var paramStr = ""
    for ( var ii = 1; ii < containedFunc.get_arity(); ++ii ) {
      if ( ii != 1 ) {
        paramStr += ',';
      }
      paramStr += "v${ii}";
    }
    eval( "def ${obj.get_type_name()}::${funcname}(${paramStr}){ this._getBase().${funcname}(${paramStr}); }" );
    return true;
  }
  return false;
}

def _verifyAndAddMethod( obj, base, funcname, func ) {
  var sParamVals = "";
  for ( var x = 1; x < func.get_arity(); ++x ) {
    sParamVals += ",0";
  }
  
  if ( eval("call_exists( func, base ${sParamVals} );") && // method exists on base object
       !eval("call_exists( func, obj ${sParamVals} );") && // method does not exist on this object (override)
       !func.has_guard() &&                                // method may not have guards
       !func.get_contained_functions()[0].has_guard() &&   // method may not have guards
       funcname != "This" &&                               // ignore "This" attribute
       !_addMethod( obj, funcname, func ) ) {
    for ( containedFunc : func.get_contained_functions() ){
      if ( !containedFunc.get_contained_functions().empty() && _addMethod( obj, funcname, containedFunc ) ) {
        break;
      }
    }
  }
}

def _setup( obj ) {
  _setThis( obj, obj );
}

def _inherit( obj, base ) {
  obj._getBase() = base;
  _setup( obj );
  
  global _inheritance;
  if ( _inheritance.is_var_undef() ) {
    _inheritance = Map();
  }
  
  // only set up this class inheritance once
  if ( _inheritance.count( obj.get_type_name() ) == 0 ) {
    _inheritance[obj.get_type_name()] = base.get_type_name();
    
    // must invoke get_functions every time, cannot filter and save list, as class definitions might occur after we 
    // hit this for the first time
    for ( func : get_functions() ) {
      if ( func.second.get_arity() == -1 ) {
        for ( inner : func.second.get_contained_functions() ) {
          if ( inner.get_arity() > 0 && !inner.get_contained_functions().empty() ) {
            _verifyAndAddMethod( obj, base, func.first, inner );
          }
        }
      }
      else if ( func.second.get_arity() > 0 && !func.second.get_contained_functions().empty() ) {
        _verifyAndAddMethod( obj, base, func.first, func.second );
      }
    }
  }
}
Usage:
class Base {
  attr id;
  def Base() { _setup( this ); }
  def Base( ID ) { _setup( this ); this.id = ID; }
  def go( x ){ print("go(x) | dynamicType: ${this.This.get_type_name()} | x: ${x} | id: ${this.This.id}"); }
  def go2( x ){ print( "go2(x) | hardcodedType: Base | x: ${x} | id: ${this.This.id}" ); }
  def go3( x, y ){ print( "go3(x,y) | hardcodedType: Base | x: ${x} | x: ${y} | id: ${this.This.id}" ); }
};

class Child {
  def Child() { 
    _inherit( this, Base() );
  }
  
  def Child( ID ) {
    _inherit( this, Base( ID ) );
  }
  
  // override that (optionally) invokes base
  def go2( x ){ print( "Child: ${x}: ${this.This.id}" ); this._getBase().go2( x ); }
};

class GrandChild {
  def GrandChild() { 
    _inherit( this, Child() );
  }
  
  def GrandChild( ID ) {
    _inherit( this, Child( ID ) );
  }
  
  // override that (optionally) invokes _base
  def go3( x, y ){ print( "go3(x,y) | dynamicType: ${this.get_type_name()} | x: ${x} | x: ${y}" );  this._getBase().go3( x,y ); }
};
What Does It Do?
  • In any given base class (class which does not inherit from another class, but will be inherited from), invoke "_setup(this);".
    • This will set up a "this.This" attribute on the object.
  • In the constructors of child classes, invoke "_inherit( this, )".
    • The class of the base object is used to determine the parent class of this object. It is important to use the same base class for every constructor for the child class. 
    • You can invoke any of the parent's constructors, allowing you to initialize the parent object as desired.
  • Behind the scenes of the _inherit() function
    • It will look at all the attributes and methods of the parent class and, if it doesn't find an override in the child class, it will define a method in the child class that invokes the method from the parent class. As such, any given child class object will have access to all the attributes and methods of the parent object.
    • The base object passed to _inherit is stored as a member on the child object. It is accessible via the "this._getBase()" method. This allows you to invoke the parent class method directly if desired.
    • Each object of type GrandChild will look like this (simplified):
      • GrandChild class
        • this = theGrandChildObj
        • this.This = theGrandChildObj
        • this._getBase() = theChildObj (Child class)
          • this = theChildObj
          • this.This = theGrandChildObj
          • this._getBase() = theBaseObj (Base class)
            • this = theBaseObj
            • this.This = theGrandChildObj
            • this._getBase() = undefinedObj
    • As you can see, in the GrandChild constructor, we create an object of type Child (which gets stored in the "this._getBase()" location). The Child constructor creates an object of type Base which gets stored in the Child object's "this._getBase()" location. Therefore, in any given method, you can call the parent object's version of the method by invoking this._getBase().method().
    • Each object will point to itself via the "this" attribute. Be careful when you use "this", as it is safer to use "this.This" to ensure polymorphic overrides are invoked.
    • Each object will point to the actual object via the "this.This" attribute. As such, any given method can polymorphically invoke the methods of the child object.
      • Do NOT use "this.This" when invoking "this._getBase()".
  • Watch out for guards on methods or parameters. It is just too complex to support class method overrides with guards, so they are not supported in this implementation. Also, each class is still separate as far as the ChaiScript class engine is concerned, so a function like "foo( Base bar )" will not recognize an object of type "Child". As long as you leave the guards out, all objects should act polymorphically correct.
Notes
  • Make sure to invoke _inherit() in every constructor on the child class, and pass in an object from the parent class.
  • Make sure to invoke _setup() in every class intended for usage as a base class. This will set up the "this.This" pointer, which allows the base class methods to invoke the overridden methods in the child class.
  • Attributes and Methods added to the parent class after a child class object has been created will never be visible to objects of the child class.
  • No classes in the class hierarchy may have methods with guards on the parameters or on the functions.
  • Objects of the child class will not convert to the base class, do not try to pass an object of type Child to a function like foo( Base bar ).
  • When invoking a method from the base class, code inside the base class method will treat the "this" object as if it is of the base class. Calls in the base method to other methods that have overrides in the child will not invoke the child override if you use the "this" object. To get the child methods to invoke from base methods, use the "this.This" object.
  • Classes built in to ModScript cannot be inherited from.

 

Option 2

There is another option, which has different benefits and drawbacks. The biggest drawback is that you cannot use the ChaiScript "class" syntax.

global _classInheritence_ = ["VirtualClass" : ""];
global _vtable_ = Map();

// All "classes" will be instances of VirtualClass, but with a different
// value for _classname_. It knows how to look up the stuff for the
// fake "class".
class VirtualClass{
  attr _classname_;
  def VirtualClass( sClass ) { this._classname_ = sClass; }
  
  // a method for invoking the function in the parent class
  def _callBase( classname, funcName, params ) {
    // verify "classname" is base of "this"
    var classnameSearch = this._classname_;
    while ( !classnameSearch.empty() ) {
      if ( classnameSearch == classname ) {
        break;
      }
      classnameSearch = _classInheritence_[classnameSearch];
    }
    if ( classnameSearch.empty() ) {
      Ext.LogErrorMsg( __FILE__ + ":" + __LINE__ + ": could not find base class ${classname} for ${this._classname_}");
      throw(0);
    }
    var f = _findVMethod( "${funcName}:${params.size()+1}", classname );
    var p2 = [this];
    params.for_each( back_inserter( p2 ) );
    return call( f, p2 );
  }
}

def _findVMethod( sFuncWithArrity, classname ) {
  var virtFuncArrity = _vtable_.at( sFuncWithArrity );
  var classnameSearch = classname;
  while ( !classnameSearch.empty() ) {
    if ( virtFuncArrity.count( classnameSearch ) == 1 ) {
      return virtFuncArrity[ classnameSearch ];
    }
    classnameSearch = _classInheritence_[classnameSearch];
  }
  Ext.LogErrorMsg( __FILE__ + ":" + __LINE__ + ": could not find function ${sFuncWithArrity} for class ${classname}" );
  throw(0);
}

def VClass( sClass, sParentClass ) {
  if ( _classInheritence_.count(sClass) == 0 ) {
    //eval( "global ${sClass} = fun(){ return VirtualClass( \"${sClass}\" ); };" );
    eval( "def ${sClass}(){ return VirtualClass( \"${sClass}\" ); }" );
    _classInheritence_[sClass] = sParentClass;
  }
}

def VClass( sClass ) {
  VClass( sClass, "VirtualClass" );
}

def VMethod( sClass, sFunc, f ) {
  if ( f.get_arity() < 1 ) {
    return;
  }
  else if ( f.has_guard() ) {
    Ext.LogErrorMsg( __FILE__ + ":" + __LINE__ + ": registerVirtual failed for function ${sFunc}, virtual function cannot have guard" );
    throw(0);
  }
  else {
    // check the params of f, reject if any params have a guard
    for ( p : f.get_param_types() ){
      if ( p.name() != "Object" ) {
        Ext.LogErrorMsg( __FILE__ + ":" + __LINE__ + ": registerVirtual failed for function ${sFunc}, virtual function cannot have guard on parameter" );
        throw(0);
      }
    }
  }
  
  var sFuncWithArrity = "${sFunc}:${f.get_arity()}";
  
  if ( _vtable_.count( sFuncWithArrity ) == 0 ) {
    _vtable_[sFuncWithArrity] = Map();
    var sParams = "";
    for ( var v = 1; v < f.get_arity(); ++v ) {
      if ( v != 1 ) {
        sParams += ',';
      }
      sParams += "v${v}";
    }
    
    // add a method to the VirtualClass that will look up the method for this "class"
    eval( 
"def VirtualClass::${sFunc}(${sParams}){ 
  /* _findVMethod throws if not found */
  var f = _findVMethod( \"${sFuncWithArrity}\", this._classname_ );
  return f( this ${sParams.empty() ? "" : ","} ${sParams} );
}" );
  }
  
  _vtable_[sFuncWithArrity][sClass] = f;
}

def VClassAttr( sClass, sMember ) {
  VMethod( sClass, sMember, eval( "fun(this){ return this._${sClass}_${sMember}; }" ) );
}
Usage:
VClass( "Base" );
VMethod( "Base", "go", fun(this, x){ print( "${this._classname_}: ${x}" ); } );
VMethod( "Base", "go2", fun(this, x){ print( "Base: ${x}" ); } );
VClassAttr( "Base", "id" );

VClass( "Child", "Base" );
VMethod( "Child", "go2", fun(this, x){ 

// optional call to base
this._callBase( "Base", "go2", );
} ); VClass( "GrandChild", "Child" ); VMethod( "GrandChild", "go2", fun(this, x){ this._callBase( "Base", "go2", ); } );

 

Notes
  • All objects are instances of the VirtualBase class
    • All methods and attributes for each class are added to the _vtable_ Map.
    • Each object has an attribute that tells it which class it is.
    • Function calls will look up the method for the current class, and if no override is found, will search up the class tree to find the method or attribute.
    • As opposed to Option 1, when invoking a method from the base class, code inside the base class method will treat the object as if it is of the child class. Calls in the base method to other methods that have overrides in the child will invoke the child override. As such, there is no need for a "this.This" attribute.
  • No virtual methods may have methods with guards on the parameters or on the functions.
  • All objects are of type VirtualBase, do not try to pass an object of type Child to a function like foo( Base bar ), but you can pass any objects from this hierarchy to a function like  foo( VirtualBase bar ).
  • Classes built in to ModScript cannot be inherited from.
  • Option 2 is faster than Option 1.
  • this._callBase() requires that you tell it the name of the base class, and it checks to ensure that the base class you select is an ancestor of the current class.

 

SBM ModScript Blog Series

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