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.NET Forum / Languages / Managed C++ / July 2006one new char question
Hi, all I use the following codes to create a char array with only three elements, /------------------------ char *strHead = new char[0]; bytesin=(DWORD)strlen(strHead) /--------------------------- but why the values of bytesin is 24, why the value is not zero? Could any body tell me why? Thanks in advance! Joseph > Hi, all > I use the following codes to create a char array with only three [quoted text clipped - 8 lines] > > Could any body tell me why? Undefined behavior. You've allocated an array of 0 bytes and then called a function that searches for the first 0-valued byte in that array - immediately over-indexing the array and reading whatever garbage lies beyond the array in memory. What were you trying to do? If you want an array of 3 characters, allocate an array of 3 characters. But even then, you can't just apply strlen to a freshly allocated array - the newly allocated array has undefined content (although under Windows it will frequently contain 0's). In order to have a defined result, you need to initialize the contents of the array before using a function like strlen on it. -cd Thanks, Carl Please see my code as below: //--------------------------------------- 1: CString s; 2: int i_num=2; 3: s.Format("%d",i_num); 4: char *strHead = new char[3]; 5: strHead ="XX"; 6: strcat(strHead,s.GetBuffer()); 7: bytesin= (DWORD)strlen(strHead); 8: WriteFile(hReadFile, strHead,bytesin,&bytesout,NULL); 9: delete strHead; //------------------------------------ When I execute line 6 and 9, it will turn out an unprocessed exception like the following format: ReadDatFiles.exe in 0x10215657 (msvcr71d.dll) unprocessed exception: 0xC0000005: writing location 0x0043209e access confliction. Why? Thanks in advance Joseph >> Hi, all >> I use the following codes to create a char array with only three [quoted text clipped - 22 lines] > > -cd error 1: strHead ="XX"; correction: strcpy(strHead, "XX"); error 2: strcat(strHead,s.GetBuffer()); correction: char *strHead = new char[3 + 1]; please note that you need a char to store the "\0". > Thanks, Carl > [quoted text clipped - 48 lines] >> >> -cd Many thanks to u & Carl! Joseph > error 1: strHead ="XX"; > correction: strcpy(strHead, "XX"); [quoted text clipped - 59 lines] >>> >>> -cd > Thanks, Carl > > Please see my code as below: in addition to other errors previously noted: > //--------------------------------------- > [quoted text clipped - 7 lines] > 8: WriteFile(hReadFile, strHead,bytesin,&bytesout,NULL); > 9: delete strHead; delete [] strHead; But why mix in a char array when you're already using CString? Better yet, why not use the C++ standard string class? -cd > 1: CString s; > 2: int i_num=2; [quoted text clipped - 5 lines] > 8: WriteFile(hReadFile, strHead,bytesin,&bytesout,NULL); > 9: delete strHead; You want to write "XX2" to a file? CString s; int i_num=2; s.Format("%s%d", "XX", i_num); WriteFile(hReadFile, s, s.GetLength(), &bytesout, NULL); Yes, thanks Mihajlo! My new question is : when I use the following questions to write to a file, it will replace the string already in that file, but I do want to insert a string to that file , not just write and replace. Could any body tell me the best solution, thanks in advance! Joseph "Mihajlo Cvetanoviæ" <mac@RnEeMtOsVeEt.co.yu> ???? news:OCU6tBxqGHA.1852@TK2MSFTNGP03.phx.gbl... >> 1: CString s; >> 2: int i_num=2; [quoted text clipped - 12 lines] > s.Format("%s%d", "XX", i_num); > WriteFile(hReadFile, s, s.GetLength(), &bytesout, NULL); > Yes, thanks Mihajlo! > > My new question is : when I use the following questions to write to a > file, it will replace the string already in that file, but I do want to > insert a string to that file , not just write and replace. Could any body > tell me the best solution, thanks in advance! You can't insert into a file. You can only append to the end, or overwrite. To append, you just need to seek to the end of the file before writing to it (see SetFilePointer if you're using WriteFile to write). If you truly need to insert other than at the end, then you need to re-write the entire file (or at least all of it that comes after the starting point of your insertion). Typically, editing programs (e.g. notepad) that need to support insert/delete write an entirely new file every time you click "Save". -cd Got it, thanks Carl! >> Yes, thanks Mihajlo! >> [quoted text clipped - 16 lines] > > -cd Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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