Write a function that will return string after fetching message using FormatMessage

Question

I wanted to write a function that will take error_code as argument and fetch the error message and the return the message. But for FormatMessage the memory allocated is cleared by using LocalFree(err_msg). Not sure how can this be dont along with returning.

static char* return_message(int error_code) {
   LPTSTR err_msg;
   FormatMessage( FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM |
            FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_MAX_WIDTH_MASK,
            0, error_code, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
            (LPTSTR)&err_msg, 0, 0);
  return err_msg;

I want to have a method similar to above. While in above case if we return err_msg it goes out of scope. Can anyone please give the proper function for this?

Answer 1

Seeing as you're using C++ you can copy the resulting message into an std::string instance, free the C-string and return the copy. std::string's destructor will take care of deallocation when its no longer used.

#include <string>
#include <windows.h>

static std::string return_message(int error_code) {
   char* tmp_msg;
   FormatMessageA( FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM |
            FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_MAX_WIDTH_MASK,
            0, error_code, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
            (LPSTR)&tmp_msg, 0, 0);

   std::string err_msg(tmp_msg);
   LocalFree(tmp_msg);
   return err_msg;
}