c++ - Why this causing Heap corruption? -

why piece of code causing heap corruption when trying delete[] fullname variable?

#include <iostream> #include <cstring> #include "stdafx.h"  using namespace std;   int main() {     const int size = 255;     char* firstname = new char[size];     char* lastname = new char[size];     cin.getline(firstname, size, '\n');     cin.getline(lastname, size, '\n');      int fullnamesize = strlen(firstname) + strlen(lastname) + 2;     cout << "fullname size: " << fullnamesize;     char* fullname = new char[fullnamesize];     strcpy_s(fullname, size, firstname);     strcat_s(fullname, size, " ");     strcat_s(fullname, size, lastname);      cout << "full name: " << fullname << "\n" ;      delete[] fullname;     delete[] firstname;     delete[] lastname;        cout << "hello";     return 0; } 

i cannot find wrong dellocation...

what's wrong it? goes wrong @ strcpy_s. because strcpy_s overwrites target buffer first (to size specified in function call). see https://msdn.microsoft.com/en-us/library/td1esda9.aspx -

the debug versions of these functions first fill buffer 0xfe. disable behavior, use _crtsetdebugfillthreshold.

(that quite subtle admit!)

and have wrong size target buffer memory overwritten you've allocated enough memory fullname, size parameters passing larger.

why crash @ delete[] rather strncpy?

there's nothing wrong deallocation @ delete[]. problem has occurred before there.

the program has no idea you've passed wrong size in. strcpy function overwrites memory management structure in heap. doesn't cause problems until delete[] call. delete tries use information , discovers it's corrupt. crash.

in big program type of problem complete nightmare.

what's right approach?

using safe string functions great idea, you'll need read manual them ensure right result. , more careful size passing them.

*ps. old strcpy , char * approach important understand, in long run, robust code, investigate std::string*