C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Question on delete [] vs just plain delete          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message DamonChong Guest Posted: Sat Jan 29, 2005 9:39 am    Post subject: Question on delete [] vs just plain delete Hi, I am new to c++. I recently spend an enormous among of time troubleshooting a seeminly innocuous piece of code. Although I narrow down this piece of code as the culprit but I don't understand why. Can some guru help to enlighten me? Thank you. // I created an array of pointers to object pointers: Object ** obs = new Object * [9]; // After that I populate this array: for(int i=0; i<9; i++){ obs[i] = new Object(i); } // After using them i deleted the array: for(int i=0; i<9; i++){ delete obs[i]; } delete [] obs; delete obs; // this is the problem, during execution the program hanged. Apparently, the above caused undefined behaviour. But I don't know why i shouldn't do that. Thanks again for any sharing! Cheers, Damon Back to top Efrat Regev Guest Posted: Sat Jan 29, 2005 11:11 am    Post subject: Re: Question on delete [] vs just plain delete "DamonChong" <so_excited (AT) excite (DOT) com> wrote Quote: Hi, I am new to c++. I recently spend an enormous among of time troubleshooting a seeminly innocuous piece of code. Although I narrow down this piece of code as the culprit but I don't understand why. Can some guru help to enlighten me? Thank you. While the gurus answer more difficult questions, I'll field this one. Quote: // I created an array of pointers to object pointers: Object ** obs = new Object * [9]; /////////// Great, you've allocated an array of pointers to Object Quote: // After that I populate this array: for(int i=0; i<9; i++){ obs[i] = new Object(i); /////////// And now you've allocated the objects. Quote: } // After using them i deleted the array: for(int i=0; i<9; i++){ delete obs[i]; /////////// Now you've deallocated the objects. Quote: } delete [] obs; /////////// And now you've deallocated the array. /////////// Good. So you've deallocated everything you've allocated. Nothing left to deallocate. /////////// and yet.... Quote: delete obs; // this is the problem, during execution the program hanged. /////////// So the previous line hangs since you're deallocating something which you shouldn't Back to top Peter Koch Larsen Guest Posted: Sat Jan 29, 2005 11:20 am    Post subject: Re: Question on delete [] vs just plain delete Hi Damon Efrat already answered your post, but I will recommend you do not use new/delete as a newbie C++ programmer. Instead focus on the standard library and just don't use pointers. Pointers are difficult and better not used until you understand the basics of C++ - including the standard library. By going this route, you will also learn how little use there actually is of pointers in modern C++. /Peter "DamonChong" <so_excited (AT) excite (DOT) com> skrev i en meddelelse news:1106991572.867157.21520 (AT) c13g2000cwb (DOT) googlegroups.com... Quote: Hi, I am new to c++. I recently spend an enormous among of time troubleshooting a seeminly innocuous piece of code. Although I narrow down this piece of code as the culprit but I don't understand why. Can some guru help to enlighten me? Thank you. // I created an array of pointers to object pointers: Object ** obs = new Object * [9]; // After that I populate this array: for(int i=0; i<9; i++){ obs[i] = new Object(i); } // After using them i deleted the array: for(int i=0; i<9; i++){ delete obs[i]; } delete [] obs; delete obs; // this is the problem, during execution the program hanged. Apparently, the above caused undefined behaviour. But I don't know why i shouldn't do that. Thanks again for any sharing! Cheers, Damon Back to top ajk Guest Posted: Sat Jan 29, 2005 2:47 pm    Post subject: Re: Question on delete [] vs just plain delete On 29 Jan 2005 01:39:32 -0800, "DamonChong" <so_excited (AT) excite (DOT) com> wrote: Quote: delete [] obs; delete obs; // this is the problem, during execution the program normally you use delete [] on any array you allocate to tell the compiler that the ptr you are deleting is an array. it is just one of those not obvious c++ rules you need to learn :) in any case I would suggest you use vector<> instead of array since it is safer and more convenient. check up the online help/book for info. STL in general can help you a lot. /ajk -- "Those are my principles. If you don't like them I have others." Groucho Marx. Back to top DamonChong Guest Posted: Sat Jan 29, 2005 5:42 pm    Post subject: Re: Question on delete [] vs just plain delete Thanks alot. Kind of silly I suppose to make such mistake. ;P Back to top Jon Guest Posted: Sat Jan 29, 2005 7:58 pm    Post subject: Re: Question on delete [] vs just plain delete While I would agree that pointers are maybe not obvious for a beginner to say that there is "little use" of pointers in modern c++ is just plain wrong in my opinion. Of course it maybe depends on what you define as "modern c++" but that is another question... "Peter Koch Larsen" <pkspaml (AT) mailme (DOT) dk> wrote Quote: Hi Damon Efrat already answered your post, but I will recommend you do not use new/delete as a newbie C++ programmer. Instead focus on the standard library and just don't use pointers. Pointers are difficult and better not used until you understand the basics of C++ - including the standard library. By going this route, you will also learn how little use there actually is of pointers in modern C++. /Peter "DamonChong" <so_excited (AT) excite (DOT) com> skrev i en meddelelse news:1106991572.867157.21520 (AT) c13g2000cwb (DOT) googlegroups.com... Hi, I am new to c++. I recently spend an enormous among of time troubleshooting a seeminly innocuous piece of code. Although I narrow down this piece of code as the culprit but I don't understand why. Can some guru help to enlighten me? Thank you. // I created an array of pointers to object pointers: Object ** obs = new Object * [9]; // After that I populate this array: for(int i=0; i<9; i++){ obs[i] = new Object(i); } // After using them i deleted the array: for(int i=0; i<9; i++){ delete obs[i]; } delete [] obs; delete obs; // this is the problem, during execution the program hanged. Apparently, the above caused undefined behaviour. But I don't know why i shouldn't do that. Thanks again for any sharing! Cheers, Damon Back to top Efrat Regev Guest Posted: Sat Jan 29, 2005 8:17 pm    Post subject: Re: Question on delete [] vs just plain delete "DamonChong" <so_excited (AT) excite (DOT) com> wrote Quote: Thanks alot. Kind of silly I suppose to make such mistake. ;P No problem. BTW, explicit memory management is both very powerful, and very prone to making "silly" mistakes (which I don't think are silly at all). Consequently, perhaps you might want to read Peter Koch Larsen's response (with which I completely agree). Specifically, you could write your code this way: #include <vector> std::vector<Object> obs[9]; for(int i = 0; i < 9; ++i) obs[i] = Object(i); Or if Object doesn't have a default constructor, #include #include <boost/shared_ptr.hpp> std::vector<boost::shared_ptr obs(9); for(int i = 0; i < 9; ++i) obs[i].reset(new Object(i)); (You could google for boost smart_ptr for the above). Back to top Bradley Guest Posted: Sun Jan 30, 2005 1:00 am    Post subject: Re: Question on delete [] vs just plain delete My question is why dynamically allocate an array of Object pointers. Why not just just Object *obparray[9]; ?? Is memory that sparse now days? "DamonChong" <so_excited (AT) excite (DOT) com> wrote Quote: Hi, I am new to c++. I recently spend an enormous among of time troubleshooting a seeminly innocuous piece of code. Although I narrow down this piece of code as the culprit but I don't understand why. Can some guru help to enlighten me? Thank you. // I created an array of pointers to object pointers: Object ** obs = new Object * [9]; // After that I populate this array: for(int i=0; i<9; i++){ obs[i] = new Object(i); } // After using them i deleted the array: for(int i=0; i<9; i++){ delete obs[i]; } delete [] obs; delete obs; // this is the problem, during execution the program hanged. Apparently, the above caused undefined behaviour. But I don't know why i shouldn't do that. Thanks again for any sharing! Cheers, Damon Back to top Efrat Regev Guest Posted: Sun Jan 30, 2005 9:23 am    Post subject: Re: Question on delete [] vs just plain delete "Bradley" <bnorth1 (AT) kc (DOT) rr.com> wrote Quote: My question is why dynamically allocate an array of Object pointers. Why not just just Object *obparray[9]; ?? Is memory that sparse now days? It's possible that Object doesn't have a default constructor. Back to top Efrat Regev Guest Posted: Sun Jan 30, 2005 9:24 am    Post subject: Re: Question on delete [] vs just plain delete "Efrat Regev" <efrat_regev (AT) yahoo (DOT) com> wrote Quote: "Bradley" <bnorth1 (AT) kc (DOT) rr.com> wrote in message news:flWKd.230262$ye4.191747 (AT) twister (DOT) rdc-kc.rr.com... My question is why dynamically allocate an array of Object pointers. Why not just just Object *obparray[9]; ?? Is memory that sparse now days? It's possible that Object doesn't have a default constructor. Oops! my bad - I misunderstood your post. Back to top Old Wolf Guest Posted: Sun Jan 30, 2005 8:06 pm    Post subject: Re: Question on delete [] vs just plain delete Efrat Regev wrote: Quote: Specifically, you could write your code this way: #include <vector std::vector for(int i = 0; i < 9; ++i) obs[i] = Object(i); Or if Object doesn't have a default constructor, #include #include std::vector obs(9); for(int i = 0; i < 9; ++i) obs[i].reset(new Object(i)); (You could google for boost smart_ptr for the above). You don't need to go to those lengths; this would do, if Object is copyable: .. std::vector .. for(int i = 0; i != 9; ++i) .. obs.push_back( Object(i) ); Back to top msalters Guest Posted: Mon Jan 31, 2005 1:27 pm    Post subject: Re: Question on delete [] vs just plain delete Efrat Regev wrote: Quote: Specifically, you could write your code this way: #include <vector std::vector for(int i = 0; i < 9; ++i) obs[i] = Object(i); Actually, you mean std::vector which is one vector with initially 9 elements, and not 9 vectors with initially no elements. With your [9] declaration, the compiler will complain that it can't assign Object(0) to an empty vector of Objects. You could also be a bit clearer, and write the size only once: #include <vector> std::vector<Object> obs; // empty for(int i = 0; i < 9; ++i) obs.pushback( Object(i) ); // add one element at a time. Now, there's only one occurence of 9 - if the vector should ever change size, you can do so in one place. 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