This changes when returning value objects

Christoffer Green · 2013-03-26T22:30:51

compiler: mingw 4.7.2 os: Windows 7 angelscript: official 2.26.1 I have a problem that I cannot solve, I am hoping that I am just registering something wrong. The short story of the symptoms: ---------------------- When registering a struct as a value type the value of 'this' changes by a few bytes if a called member function of that struct is returning another registered struct value type, 'this' does not change when returning nothing (void) or when returning float. This bug goes away if at least 3 int (for example) member variables are added to the registered value struct that is returned, if so then 'this' does not change. ---------------------- Here is a small example that illustrates the problem. We have 2 structs (Dummy and Test) and we register them in angelscript. 'Test' has a debug printing of 'this' at every function that gets run from it. We then run these lines in angelscript: void main() { Test test; test.noReturnValue(); test.floatReturnValue(); test.getDummy(); } This run outputs: Constructor self address: 0x60e7c4 This print from constructor: 0x60e7c4 This print from noReturnValue function: 0x60e7c4 This print from floatReturnValue function: 0x60e7c4 This print from getDummy function: 0x60e7bc The application also crashes at the end of the run. The last lines shows that 'this' changes slightly due to that function call returning a 'Dummy' value. Here is an example program that is as small as I could make it that still shows the problem (this is c++11 code). #include <angelscript.h> #include <add_on/scriptstdstring/scriptstdstring.h> #include <add_on/scriptbuilder/scriptbuilder.h> #include <stdio.h> #include <assert.h> #include <iostream> void MessageCallback(const asSMessageInfo *msg, void *param) { const char *type = "ERR "; if( msg->type == asMSGTYPE_WARNING ) type = "WARN"; else if( msg->type == asMSGTYPE_INFORMATION ) type = "INFO"; printf("%s (%d, %d) : %s : %s\n", msg->section, msg->row, msg->col, type, msg->message); } void print(const std::string &in) { std::cout << in << std::endl; } struct Dummy { static void Constructor(Dummy *self) {new(self) Dummy();} static void Destructor(Dummy *memory) {memory->~Dummy();} //int a, b, c; // uncomment this line for it to work again //int a, b; // will not work, requires at least 3 extra vars }; struct Test { Test() { std::cout << "This print from constructor: " << this << std::endl; } static void Constructor(Test *self) {std::cout << "Constructor self address: " << self << std::endl;new(self) Test();} static void Destructor(Test *memory) {memory->~Test();} void noReturnValue() { std::cout << "This print from noReturnValue function: " << this << std::endl; } float floatReturnValue() { std::cout << "This print from floatReturnValue function: " << this << std::endl; return 5.6; } Dummy getDummy() { std::cout << "This print from getDummy function: " << this << std::endl; return Dummy(); } }; template<typename T> void reg(asIScriptEngine *engine, std::string name) { int r = engine->RegisterObjectType(name.c_str(), sizeof(T), asOBJ_VALUE | asOBJ_APP_CLASS_CDAK ); assert( r >= 0 ); r = engine->RegisterObjectBehaviour(name.c_str(), asBEHAVE_CONSTRUCT, std::string("void f()").c_str(), asFUNCTION(T::Constructor), asCALL_CDECL_OBJLAST); assert( r >= 0 ); r = engine->RegisterObjectBehaviour(name.c_str(), asBEHAVE_DESTRUCT, "void f()", asFUNCTION(T::Destructor), asCALL_CDECL_OBJLAST); assert( r >= 0 ); } std::string app = R"(void main() { Test test; test.noReturnValue(); test.floatReturnValue(); test.getDummy(); })"; int main() { asIScriptEngine *engine = asCreateScriptEngine(ANGELSCRIPT_VERSION); int r = engine->SetMessageCallback(asFUNCTION(MessageCallback), 0, asCALL_CDECL); assert( r >= 0 ); RegisterStdString(engine); r = engine->RegisterGlobalFunction("void print(const string &in)", asFUNCTION(print), asCALL_CDECL); assert( r >= 0 ); reg<Dummy>(engine, "Dummy"); reg<Test>(engine, "Test"); r = engine->RegisterObjectMethod("Test", "Dummy getDummy()", asMETHOD(Test,getDummy), asCALL_THISCALL);assert( r >= 0 ); r = engine->RegisterObjectMethod("Test", "void noReturnValue()", asMETHOD(Test,noReturnValue), asCALL_THISCALL);assert( r >= 0 ); r = engine->RegisterObjectMethod("Test", "float floatReturnValue()", asMETHOD(Test,floatReturnValue), asCALL_THISCALL);assert( r >= 0 ); CScriptBuilder builder; r = builder.StartNewModule(engine, "MyModule"); if( r < 0 ) {printf("Unrecoverable error while starting a new module.\n");return 0;} r = builder.AddSectionFromMemory("TestSection", app.c_str());if( r < 0 ) {printf("Please correct the errors in the script and try again.\n");return 0;} r = builder.BuildModule(); if( r < 0 ) {printf("Please correct the errors in the script and try again.\n");return 0;} asIScriptModule *mod = engine->GetModule("MyModule"); asIScriptFunction *func = mod->GetFunctionByDecl("void main()"); if( func == 0 ) {printf("The script must have the function 'void main()'. Please add it and try again.\n");return 0;} asIScriptContext *ctx = engine->CreateContext(); ctx->Prepare(func); r = ctx->Execute(); if( r != asEXECUTION_FINISHED and r == asEXECUTION_EXCEPTION) {printf("An exception '%s' occurred. Please correct the code and try again.\n", ctx->GetExceptionString());} }

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Started by Heuristics March 11, 2013 08:41 AM

10 comments, last by WitchLord 11 years, 8 months ago

Heuristics

Author

147

March 26, 2013 08:47 AM

Oh, that is worrying. Is it possible for end user like myself know how a value type should be registered? Is there a method end users can use to deduce how it should be done in all cases?

WitchLord

4,860

March 26, 2013 10:30 PM

Until I'm able to confirm otherwise the way to register the value types is to follow the documentation. Fortunately the case you reported is probably not that frequently occurring, and can easily be remedied by adding an explicit constructor to the class if needed.

Unfortunately the ABI is not something that is standardized. Every combination of compiler, operative system and CPU has its own ABI. With AngelScript I keep a common interface so that a type can be registered the same way on all platforms, but unfortunately the C++ language doesn't quite provide all the information needed to automatically determine how the compiler handles a type in the ABI.

The new template functions introduced with C++11 does help a bit, but it seems that they are not perfect either.

In this case the best would probably be to have some code generator that can parse the C++ code and determine the correct way to register the class that way. Some attempts to implement something like this has been done before, but I'm not sure if any of these attempts managed a perfect implementation, nor do I know of any that is maintained to this day.

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This changes when returning value objects

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