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Registering a function

This article aims to explain the way functions are registered with AngelScript, and some of the differences between C++ and AngelScript that the developer needs to be aware of in order to be successful in registering the application interface that the scripts will use. The principles learned here are used in several locations, such as RegisterGlobalFunction, RegisterObjectMethod, RegisterObjectBehaviour, etc.

# How to get the address of the application function or method

The macros asFUNCTION, asFUNCTIONPR, asMETHOD, and asMETHODPR have been implemented to facilitate the task of getting the function pointer and passing them on to the script engine.

The asFUNCTION takes the function name as the parameter, which works for all global functions that do not have any overloads. If you use overloads, i.e. multiple functions with the same name but with different parameters, then you need to use asFUNCTIONPR instead. This macro takes as parameter the function name, parameter list, and return type, so that the C++ compiler can resolve exactly which overloaded function to take the address of.

// Global function
void globalFunc();
r = engine->RegisterGlobalFunction("void globalFunc()", asFUNCTION(globalFunc), asCALL_CDECL); assert( r >= 0 );
// Overloaded global functions
void globalFunc2(int);
void globalFunc2(float);
r = engine->RegisterGlobalFunction("void globalFunc2(int)", asFUNCTIONPR(globalFunc2, (int), void), asCALL_CDECL); assert( r >= 0 );
asFUNCTIONPR
#define asFUNCTIONPR(f, p, r)
Returns an asSFuncPtr representing the function specified by the name, parameter list,...
Definition angelscript.h:684
asCALL_CDECL
@ asCALL_CDECL
A cdecl function.
Definition angelscript.h:226
asFUNCTION
#define asFUNCTION(f)
Returns an asSFuncPtr representing the function specified by the name.
Definition angelscript.h:675
asGetTypeTraits
asUINT asGetTypeTraits()
Returns the appropriate flags for use with RegisterObjectType.
Definition angelscript.h:1007

The same goes for asMETHOD and asMETHODPR. The difference between these and asFUNCTION/asFUNCTIONPR is that the former take the class name as well as parameter.

class Object
{
// Class method
void method();
// Overloaded method
void method2(int input);
void method2(int input, int &output);
// Const method
int getAttr(int) const;
};
// Registering the class method
r = engine->RegisterObjectMethod("object", "void method()", asMETHOD(Object,method), asCALL_THISCALL); assert( r >= 0 );
// Registering the overloaded methods
r = engine->RegisterObjectMethod("object", "void method2(int)", asMETHODPR(Object, method2, (int), void), asCALL_THISCALL); assert( r >= 0 );
r = engine->RegisterObjectMethod("object", "void method2(int, int &out)", asMETHODPR(Object, method2, (int, int&), void), asCALL_THISCALL); assert( r >= 0 );
// Registering a const method
r = engine->RegisterObjectMethod("object", "int getAttr(int) const", asMETHODPR(Object, getAttr, (int) const, int), asCALL_THISCALL); assert( r >= 0 );
asCALL_THISCALL
@ asCALL_THISCALL
A thiscall class method.
Definition angelscript.h:232
asMETHOD
#define asMETHOD(c, m)
Returns an asSFuncPtr representing the class method specified by class and method name.
Definition angelscript.h:740
asMETHODPR
#define asMETHODPR(c, m, p, r)
Returns an asSFuncPtr representing the class method specified by class, method name,...
Definition angelscript.h:742

Note
  asMETHOD doesn't work well for classes with multiple inheritances. On some compilers the method pointer ends up referring to the wrong base class due to a limitation in the C++ compiler. The solution then is to use the asMETHODPR macro.

It is possible to register a class method to be called from the script as if it was a global function. This is commonly done when exposing a singleton to the script interface, as the singleton's methods then look like ordinary global functions. When this is done the application must have a reference to the object at the time of the registration and the application must guarantee that the object is alive until it is no longer possible for the scripts to call the method.

class MySingleton
{
// Class method to be called from script as if a global function
void MyGlobalFunc(int arg1, int arg2);
};
MySingleton single;
// Registering the singleton's method as if a global function
r = engine->RegisterGlobalFunction("void MyGlobalFunc(int, int)", asMETHOD(MySingleton, MyGlobalFunc), asCALL_THISCALL_ASGLOBAL, &single); assert( r >= 0 );
asCALL_THISCALL_ASGLOBAL
@ asCALL_THISCALL_ASGLOBAL
A thiscall class method registered as a global function.
Definition angelscript.h:230

# Calling convention

AngelScript accepts most common calling conventions that C++ uses, i.e. cdecl, stdcall, and thiscall. There is also a generic calling convention that can be used for example when native calling conventions are not supported on the target platform.

All functions and behaviours must be registered with the asCALL_CDECL, asCALL_STDCALL, asCALL_THISCALL, or asCALL_GENERIC flags to tell AngelScript which calling convention the application function uses. The special conventions asCALL_CDECL_OBJLAST and asCALL_CDECL_OBJFIRST can also be used wherever asCALL_THISCALL is accepted, in order to simulate a class method through a global function. Functor objects can also be used to emulate global functions with the convention asCALL_THISCALL_ASGLOBAL, or class methods with the conventions asCALL_THISCALL_OBJFIRST and asCALL_THISCALL_OBJLAST.

If the incorrect calling convention is given on the registration you'll very likely see the application crash with a stack corruption whenever the script engine calls the function. cdecl is the default calling convention for all global functions in C++ programs, so if in doubt try with asCALL_CDECL first. The calling convention only differs from cdecl if the function is explicitly declared to use a different convention, or if you've set the compiler options to default to another convention.

For class methods there is only the thiscall convention, except when the method is static, as those methods are in truth global functions in the class namespace. Normal methods, virtual methods, and methods for classes with multiple inheritance are all registered the same way, with asCALL_THISCALL.

Classes with virtual inheritance are not supported natively, and for these it will be necessary to create wrapper functions. These wrapper functions can either be implemented manually, or the template based automatic wrappers from the add-on can be used.

See also
  The generic calling convention

# A little on type differences

AngelScript supports most of the same types that C++ has, but there are differences that you'll need to know when registering functions, methods, and behaviours. Make sure you read and understand the article Datatypes in AngelScript and C++.

When registering functions that take references, you must make sure to inform the correct keyword that informs the intention of the data in the reference. For example, a parameter reference that is meant to be used as input should have the keyword 'in' defined after the & character, and an output reference should have the keyword 'out'. Reference types can be passed as both input and output references, in which case the keyword 'inout' can be used, or simply no keyword at all. Value types on the other hand cannot use 'inout' references, as AngelScript cannot guarantee that the reference will be valid during the whole execution of the function.

When registering functions that take pointers, you need to determine what the pointer represents. If the pointer is to a value type though, then it can only be registered as a reference. If the pointer is to a reference type, then it can be registered as an object handle, or as a plain reference. If you choose to use the object handle then you need to pay attention to the reference counter in the type so you don't get problems with memory leaks or crashes due to objects being destroyed too early.

# Virtual inheritance is not supported

Registering class methods for classes with virtual inheritance is not supported due to the high complexity involved with them. Each compiler implements the method pointers for these classes differently, and keeping the code portable would be very difficult. This is not a great loss though, as classes with virtual inheritance are relatively rare, and it is easy to write simple proxy functions where the classes to exist.

class A { void SomeMethodA(); };
class B : virtual A {};
class C : virtual A {};
class D : public B, public C {};
// Need a proxy function for registering SomeMethodA for class D
void D_SomeMethodA_proxy(D *d)
{
// The C++ compiler will resolve the virtual method for us
d->SomeMethodA();
}
// Register the global function as if it was a class method,
// but with calling convention asCALL_CDECL_OBJLAST
engine->RegisterObjectMethod("D", "void SomeMethodA()", asFUNCTION(D_SomeMethodA_proxy), asCALL_CDECL_OBJLAST);
asCALL_CDECL_OBJLAST
@ asCALL_CDECL_OBJLAST
A cdecl function that takes the object pointer as the last parameter.
Definition angelscript.h:234

If you have a lot of classes with virtual inheritance, you should probably think about writing a template proxy function, so you don't have to manually write all the proxy functions.