C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  returning reference by dereferencing a pointer          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message fiatlux Guest Posted: Tue Nov 22, 2005 1:17 am    Post subject: returning reference by dereferencing a pointer Hello, Here is the short version of the problem (that might be enough!?): The use of the following member function causes the program to segfault: double& operator[](unsigned int index) { double *ret = gsl_vector_ptr(vct_, index); // from gsl library // std::cout << "foo" << std::endl; return *ret; } The behavior changes with optimization (-O3), and it also doesn't segfault when I uncomment the line in the middle. Can someone help me understand what's going on? Thanks, Matt Here is the longer version with the background. I would like to use the notation m[i][j] = 42.0; to assign a matrix's element. I have built a class matrix around the gsl_matrix* struct of the gsl library. I have in there a member function pseudo_vector_t operator[](unsigned int index){...} which returns a vector corresponding to a row in the matrix. The pseudo vector class is a wrapper around gsl_vector*, which has a copy constructor, and overloads the operator[]. The pseudo vector is just a way to allow the notation m[i][j]. double& operator[](unsigned int index) { double *ret = gsl_vector_ptr(vct, index); return *ret; } I've tried the direct solution ( return *gsl_vector_ptr(vct, index); ) but it segfaults too. I've tried double* const ret = ..., but it fails too Someone talked to me about the return register pointing out of the stack, but I don't quite understand what he's talking about, and can't think of the beginning of an answer. I'd greatly appreciate help on that matter (I've been very humbled by that problem!) Matt [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Stanimir Kabaivanov Guest Posted: Tue Nov 22, 2005 4:05 pm    Post subject: Re: returning reference by dereferencing a pointer Hi Matt, Quote: Here is the short version of the problem (that might be enough!?): The use of the following member function causes the program to segfault: double& operator[](unsigned int index) { double *ret = gsl_vector_ptr(vct_, index); // from gsl library // std::cout << "foo" << std::endl; return *ret; } Well, first of all, as far as I'm acquainted with gsl docs, gsl_vector_prt can also return null pointer (and invoke error handler) if your index points out of the 0..n-1 vector index range. Perhaps it would be a nice idea to handle such situation and report an error (throw an exception). Current code will try to dereference null pointer otherwise. Quote: The behavior changes with optimization (-O3), and it also doesn't segfault when I uncomment the line in the middle. Can someone help me understand what's going on? Is your compiler GCC? with a better optimization (that is larger N after -0) it can skip part of your code, or rearange it the way it likes, but I don't belive optimization is your problem. Quote: Here is the longer version with the background. I would like to use the notation m[i][j] = 42.0; to assign a matrix's element. I have built a class matrix around the gsl_matrix* struct of the gsl library. I have in there a member function pseudo_vector_t operator[](unsigned int index){...} which returns a vector corresponding to a row in the matrix. The pseudo vector class is a wrapper around gsl_vector*, which has a copy constructor, and overloads the operator[]. The pseudo vector is just a way to allow the notation m[i][j]. double& operator[](unsigned int index) { double *ret = gsl_vector_ptr(vct, index); return *ret; } I've tried the direct solution ( return *gsl_vector_ptr(vct, index); ) but it segfaults too. I've tried double* const ret = ..., but it fails too Have you created (allocated) the vector in question) in accordance with GSL requirements? Also how is your GSL compiled/built? Are code generation settings different from the settings you use for your class? Best regards, Stanimir Kabaivanov [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top fiatlux Guest Posted: Tue Nov 22, 2005 7:28 pm    Post subject: Re: returning reference by dereferencing a pointer Hello Stanimir, I don't do much error checking (I will in the future), but I know that the index is within range. I have verified (printf not shown) that the index is strictly smaller than vector_->size. Quote: Well, first of all, as far as I'm acquainted with gsl docs, gsl_vector_prt can also return null pointer (and invoke error handler) if your index points out of the 0..n-1 vector index range. Perhaps it would be a nice idea to handle such situation and report an error (throw an exception). Current code will try to dereference null pointer otherwise. The behavior changes with optimization (-O3), and it also doesn't segfault when I uncomment the line in the middle. Can someone help me understand what's going on? Is your compiler GCC? with a better optimization (that is larger N after -0) it can skip part of your code, or rearange it the way it likes, but I don't belive optimization is your problem. I use g++ I looked at the gsl source, and it seems they use '-g -O2'. Using the same parameters for compilation, I still have the segfault. Quote: Have you created (allocated) the vector in question) in accordance with GSL requirements? Also how is your GSL compiled/built? Are code generation settings different from the settings you use for your class? I have created the matrix using proper allocation methods (gsl_matrix_calloc...) The matrix class returns a pseudo_vector, whose purpose is just to expose the matrix row in a convenient way. The pseudo_vector just has a copy constructor, which looks like pseudo_vector(const gsl_vector* _vector) { vector_ = _vector;} where vector_ is a member variable of pseudo_vector of type gsl_vector* The only other member functions of pseudo_vector are those {const} double& operator[] functions that are causing me so much grief. Thanks for your response, Cheers, Matt [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top fiatlux Guest Posted: Wed Nov 23, 2005 1:53 pm    Post subject: Re: returning reference by dereferencing a pointer I think the problem lies not in the return reference, but elsewhere (gsl_matrix_view allocates in the stack and gets out of scope). Thanks to those who provided comments, Matthieu [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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