C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Design question with memcpy with a method having a char* pa          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message fernandez.dan@gmail.com Guest Posted: Wed Jul 27, 2005 11:22 pm    Post subject: Design question with memcpy with a method having a char* pa Hi, I have a question dealing if I should do a memcopy or pointer assigment. An example with a simplication of my class class MyClass { public: void methodA(char* data, long size); private: long m_size; char* m_data; }; My question is dealing with my methodA. What is the appropriate strategy for assigning member variable m_data from methodA. I'm not sure if I should do a memcpy so I don't have to worry about the scope of the pointer. Example of the two different implementations of methodA are void MyClass::methodA(char* data, long size) { if(data == NULL || size == 0) return; if(m_data != NULL) delete m_data; m_data = new char[size]; m_size = size; memcpy(m_data, data, size); } or void MyClass::methodA(char* data, long size) { if(data == NULL || size == 0) return; if(m_data != NULL) delete m_data; m_data = data; m_size = size; } Hopefully with this simple example you kind of get my question. Thanks for any input or advice on this point. Danny [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Shantanu Garg Guest Posted: Thu Jul 28, 2005 11:21 am    Post subject: Re: Design question with memcpy with a method having a char Quote: My question is dealing with my methodA. What is the appropriate strategy for assigning member variable m_data from methodA. I'm not sure if I should do a memcpy so I don't have to worry about the scope of the pointer. Example of the two different implementations of methodA are void MyClass::methodA(char* data, long size) { if(data == NULL || size == 0) return; if(m_data != NULL) delete m_data; m_data = new char[size]; m_size = size; memcpy(m_data, data, size); } or void MyClass::methodA(char* data, long size) { if(data == NULL || size == 0) return; if(m_data != NULL) delete m_data; m_data = data; m_size = size; } The answer is it depends. If the 'char* data' is being allocated on heap and only purpose of this is to just pass it in methodA the second approach is good. Otherwise the first aproach is good. In general case we should avoid the second case, since we dont want to impose anything to the caller of 'methodA'. The first approach is good, clean and will always work irresoective of the way caller allocates 'data' or use it afterwards. -Shantanu Garg [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top lampard Guest Posted: Thu Jul 28, 2005 11:49 am    Post subject: Re: Design question with memcpy with a method having a char* [email]fernandez.dan (AT) gmail (DOT) com[/email] wrote: Quote: Hi, I have a question dealing if I should do a memcopy or pointer assigment. An example with a simplication of my class class MyClass { public: void methodA(char* data, long size); private: long m_size; char* m_data; }; My question is dealing with my methodA. What is the appropriate strategy for assigning member variable m_data from methodA. I'm not sure if I should do a memcpy so I don't have to worry about the scope of the pointer. Example of the two different implementations of methodA are This approach looks safer to me with the only exception that the parameter should be declared as a const variable. With this approach you need not care what happens to the passed in paramter in the caller. //void MyClass::methodA(const char* data, long size) Quote: void MyClass::methodA(char* data, long size) { if(data == NULL || size == 0) return; if(m_data != NULL) delete m_data; m_data = new char[size]; m_size = size; memcpy(m_data, data, size); } or This approach is very dangerous, as you may delete the memory pointed to by the caller, unless if it is what you intend to do. Try calling the same method twice with the same data, you may find unhandled exception. Quote: void MyClass::methodA(char* data, long size) { if(data == NULL || size == 0) return; if(m_data != NULL) delete m_data; m_data = data; m_size = size; } Hopefully with this simple example you kind of get my question. Thanks for any input or advice on this point. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Owen Guest Posted: Thu Jul 28, 2005 11:50 am    Post subject: Re: Design question with memcpy with a method having a char* Smart pointer should help you. http://www.boost.org/libs/smart_ptr/smart_ptr.htm Good luck! Owen. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Stephan Brönnimann Guest Posted: Thu Jul 28, 2005 11:56 am    Post subject: Re: Design question with memcpy with a method having a char* [email]fernandez.dan (AT) gmail (DOT) com[/email] wrote: Quote: Hi, I have a question dealing if I should do a memcopy or pointer assigment. An example with a simplication of my class class MyClass { public: void methodA(char* data, long size); private: long m_size; char* m_data; }; My question is dealing with my methodA. What is the appropriate strategy for assigning member variable m_data from methodA. I'm not sure if I should do a memcpy so I don't have to worry about the scope of the pointer. Example of the two different implementations of methodA are void MyClass::methodA(char* data, long size) { if(data == NULL || size == 0) return; if(m_data != NULL) delete m_data; m_data = new char[size]; m_size = size; memcpy(m_data, data, size); } or void MyClass::methodA(char* data, long size) { if(data == NULL || size == 0) return; if(m_data != NULL) delete m_data; m_data = data; m_size = size; } Hopefully with this simple example you kind of get my question. Thanks for any input or advice on this point. Danny Unless you have very good reasons, you should copy `data' in void MyClass::methodA(char* data, long size); Because you don't modify `data' you might take the argument as const char* data. Rationale: Think about the implementation of MyClass's copy constructor, assignment operator and destructor: If you don't copy m_data from the right-hand-side two (or more) objects will share the same data and both of it will delete it in the destructor which is undefined behavior. If you really want to transfer the ownership of `data' then you should consider to use std::auto_ptr<char> for MyClass::methodA(...). regards Stephan Brönnimann [email]broeni (AT) osb-systems (DOT) com[/email] Open source rating and billing engine for communication networks. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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