Why does calling std::string.c_str() on a function that returns a string not work?

Question

I have the following code:

std::string getString() {
    std::string str(\"hello\");
    return str;
}

int main() {
    const char* cStr = getString().c_str         


        

Answer 1

As mentioned by others you are using a pointer to temporary after it has already been deleted - this is a classic example of heap after free use.

What I can add to others' answers is that you can easily detect such usage with gcc's or clang's address sanitizers.

Example:

#include 
#include 

std::string get()
{
  return "hello";
}

int main()
{
  const char* c = get().c_str();
  std::cout << c << std::endl;
}

sanitizer output:

=================================================================
==2951==ERROR: AddressSanitizer: heap-use-after-free on address 0x60300000eff8 at pc 0x7f78e27869bb bp 0x7fffc483e670 sp 0x7fffc483de20
READ of size 6 at 0x60300000eff8 thread T0
    #0 0x7f78e27869ba in strlen (/usr/lib64/libasan.so.2+0x6d9ba)
    #1 0x39b4892ba0 in std::basic_ostream >& std::operator<<  >(std::basic_ostream >&, char const*) (/usr/lib64/libstdc++.so.6+0x39b4892ba0)
    #2 0x400dd8 in main /tmp/tmep_string/main.cpp:12
    #3 0x39aa41ed5c in __libc_start_main (/lib64/libc.so.6+0x39aa41ed5c)
    #4 0x400c48  (/tmp/tmep_string/a.out+0x400c48)

0x60300000eff8 is located 24 bytes inside of 30-byte region [0x60300000efe0,0x60300000effe)
freed by thread T0 here:
    #0 0x7f78e27ae6ea in operator delete(void*) (/usr/lib64/libasan.so.2+0x956ea)
    #1 0x39b489d4c8 in std::basic_string, std::allocator >::~basic_string() (/usr/lib64/libstdc++.so.6+0x39b489d4c8)
    #2 0x39aa41ed5c in __libc_start_main (/lib64/libc.so.6+0x39aa41ed5c)

previously allocated by thread T0 here:
    #0 0x7f78e27ae1aa in operator new(unsigned long) (/usr/lib64/libasan.so.2+0x951aa)
    #1 0x39b489c3c8 in std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator const&) (/usr/lib64/libstdc++.so.6+0x39b489c3c8)
    #2 0x400c1f  (/tmp/tmep_string/a.out+0x400c1f)

SUMMARY: AddressSanitizer: heap-use-after-free ??:0 strlen
Shadow bytes around the buggy address:
  0x0c067fff9da0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff9db0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff9dc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff9dd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff9de0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c067fff9df0: fa fa fa fa fa fa fa fa fa fa fa fa fd fd fd[fd]
  0x0c067fff9e00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff9e10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff9e20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff9e30: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff9e40: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
==2951==ABORTING