Question

I am facing a memory leak in placement new of standard library string.

Below i have given the code where the leak is showing.

string string1("new string");
char _string[sizeof(string)];
new(_string) string(string1);

The leak is found using dbx, and it is shown as below

Actual leaks report    (actual leaks:            1  total size:         52 bytes)

  Total     Num of  Leaked     Allocation call stack
  Size      Blocks  Block
                    Address
==========  ====== =========== =======================================
        52       1    0x43f68  operator new < std::basic_string<char,std::char_traits<char>,std::allocator<char> >::__getRep < std::basic_string<char,std::char_traits<char>,std::allocator<char> >::basic_string < main


Possible leaks report  (possible leaks:          0  total size:          0 bytes)

Is this the real memory leakage or is the dbx intrepreting this as leak?

Answer 1

You still need to call the destructor for the string object you created by placement new.

std::string allocates the storage for the chars it stores on the heap (unless you specify a custom allocator, which might be what you’re after here), and you’re leaking that. (sizeof(string) is a constant, doesn’t depend on what is stored in the string.)