# Using C++ Member Function with rb\_protect **URL:** \ **Category:** Ruby\ **Created:** [August 8, 2006, 10:33pm UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964 "2006-08-08T22:33:38Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![agnathan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/919ad9/32.png) [@agnathan](https://www.ruby-forum.com/u/agnathan)\ **Post date:** [August 8, 2006, 10:33pm UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964/1 "2006-08-08T22:33:38Z")
I’m having problems using a C++ member function with rb\_protect. Here is the code: class Sandbox { public: Sandbox() { sandbox\_ = rb\_funcall(rb\_const\_get(rb\_cObject, rb\_intern (“Sandbox”)), rb\_intern(“safe”), 0); } virtual ~Sandbox() { } VALUE eval(char \* code) { int status = 0; VALUE val = rb\_protect(&Sandbox::eval\_protect, rb\_str\_new2(code), &status); if (status) { /\* handle error… \*/ } return val; } private: VALUE eval\_protect(VALUE code) { return rb\_funcall(sandbox\_, rb\_intern(“eval”), 1, code); } VALUE sandbox\_; }; The compile fails with: test.cpp: In member function ?VALUE Sandbox::eval(char\*)?: test.cpp:18: error: cannot convert ?VALUE (Sandbox::_)(VALUE)? to ?VALUE (_)(VALUE)? for argument ?1? to ?VALUE rb\_protect(VALUE (_) (VALUE), VALUE, int_)? $ ruby1.8 -v ruby 1.8.5 (2006-08-06) [powerpc-darwin8.7.0] $ g++ --version powerpc-apple-darwin8-g+±4.0.1 (GCC) 4.0.1 (Apple Computer, Inc. build 5341) Copyright © 2005 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. Any ideas? Thanks, – Daniel ---
**Author:** ![agnathan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/919ad9/32.png) [@agnathan](https://www.ruby-forum.com/u/agnathan)\ **Post date:** [August 8, 2006, 10:48pm UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964/2 "2006-08-08T22:48:40Z")
On 8/8/06, Daniel H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I’m having problems using a C++ member function with rb\_protect. Here > is the code: What happens if you remove the & in this line: VALUE val = rb\_protect(&Sandbox::eval\_protect, ---
**Author:** ![agnathan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/919ad9/32.png) [@agnathan](https://www.ruby-forum.com/u/agnathan)\ **Post date:** [August 8, 2006, 11:06pm UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964/3 "2006-08-08T23:06:41Z")
Is it possible to make Sandbox::eval\_protect a static function? Because as an instance method, it’s prototype is not VALUE(_)(VALUE) but rather something like VALUE(_)(Sandbox&, VALUE). ---
**Author:** ![agnathan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/919ad9/32.png) [@agnathan](https://www.ruby-forum.com/u/agnathan)\ **Post date:** [August 8, 2006, 11:00pm UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964/4 "2006-08-08T23:00:40Z")
On Aug 8, 2006, at 4:45 PM, Francis C. wrote: > What happens if you remove the & in this line: > > VALUE val = rb\_protect(&Sandbox::eval\_protect, I get this compile error: test.cpp: In member function ?VALUE Sandbox::eval(char\*)?: test.cpp:15: error: argument of type ?VALUE (Sandbox::)(VALUE)? does not match ?VALUE (\*)(VALUE)? – Daniel ---
**Author:** ![agnathan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/919ad9/32.png) [@agnathan](https://www.ruby-forum.com/u/agnathan)\ **Post date:** [August 8, 2006, 11:30pm UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964/5 "2006-08-08T23:30:42Z")
> Any ideas? Also, have you tried casting to a void \* ? ---
**Author:** ![agnathan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/919ad9/32.png) [@agnathan](https://www.ruby-forum.com/u/agnathan)\ **Post date:** [August 8, 2006, 11:30pm UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964/6 "2006-08-08T23:30:42Z")
> Any ideas? I’m not sure if it will help, but for my C++ extensions, I’ve had to do this type of trickery: typedef VALUE (ruby\_method)(…); when doing this: rb\_define\_method(some\_ruby\_object, “initialize”, (ruby\_method\*) &some\_object\_initialize, 0); ---
**Author:** ![agnathan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/919ad9/32.png) [@agnathan](https://www.ruby-forum.com/u/agnathan)\ **Post date:** [August 8, 2006, 11:36pm UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964/7 "2006-08-08T23:36:43Z")
On Wed, Aug 09, 2006 at 05:57:53AM +0900, Daniel H. wrote: > not match e$B!Fe(BVALUE (\*)(VALUE)e$B!Ge(B > Of course it won’t work that way, being a member function (it is given a > reference to this too…). You’d have to do something like this (it’s been a while without writing any C++, so take this with a grain of salt): class Sandbox { public: Sandbox() { sandbox\_ = rb\_funcall(rb\_const\_get(rb\_cObject, rb\_intern(“Sandbox”)), rb\_intern(“safe”), 0); } virtual ~Sandbox() { } VALUE eval(char \* code) { int status = 0; VALUE args[2]; args[0] = (VALUE)this; args[1] = rb\_str\_new2(code); VALUE val = rb\_protect(&call\_eval\_protect, (VALUE)args, &status); if (status) { /\* handle error… \*/ } return val; } private: static VALUE call\_eval\_protect(VALUE args) { Sandbox \*me = ((Sanbox \*)args)[0]; VALUE code = ((VALUE \*)args)[1]; return rb\_funcall(me-\>sandbox\_, rb\_intern(“eval”), 1, code); } VALUE sandbox\_; }; ---
**Author:** ![agnathan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/919ad9/32.png) [@agnathan](https://www.ruby-forum.com/u/agnathan)\ **Post date:** [August 9, 2006, 8:29pm UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964/8 "2006-08-09T20:29:18Z")
On Wed, 2006-08-09 at 06:03 +0900, Francis C. wrote: > Is it possible to make Sandbox::eval\_protect a static function? Because as > an instance method, it’s prototype is not VALUE(_)(VALUE) but rather > something like VALUE(_)(Sandbox&, VALUE). Specifically, VALUE (Sandbox::_)(VALUE). Pointers to non-static methods are actually a totally separate family of types to function pointers and cannot be called without first using the ._ or -\>\* operators to “bind” them. This is similar to the difference between UnboundMethod and Proc/Method in Ruby. -mental ---
**Author:** ![agnathan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/919ad9/32.png) [@agnathan](https://www.ruby-forum.com/u/agnathan)\ **Post date:** [August 9, 2006, 12:00am UTC](https://www.ruby-forum.com/t/using-c-member-function-with-rb-protect/101964/9 "2006-08-09T00:00:43Z")
On Aug 8, 2006, at 5:34 PM, Mauricio F. wrote: > Of course it won’t work that way, being a member function (it is > given a > reference to this too…). > > You’d have to do something like this (it’s been a while without > writing any > C++, so take this with a grain of salt): Thanks, I ended up with something similar. class Sandbox { public: Sandbox() { sandbox\_ = rb\_funcall(rb\_const\_get(rb\_cObject, rb\_intern (“Sandbox”)), rb\_intern(“safe”), 0); } virtual ~Sandbox() { } VALUE eval(char \* code) { int status = 0; VALUE args[2]; args[0] = reinterpret\_cast(this); args[1] = rb\_str\_new2(code); VALUE val = rb\_protect(eval\_protect, reinterpret\_cast(args), &status); if (status) { } return val; } ``` VALUE & sandbox() { return sandbox_; } ``` private: VALUE sandbox\_; static VALUE eval\_protect(VALUE data) { VALUE \*args = reinterpret\_cast\(data); return rb\_funcall(reinterpret\_cast\(args[0])- > sandbox(), > rb\_intern(“eval”), > 1, > args[1]); > } > }; – Daniel