`std::string` is not instrumented properly on MacOS

[codeinred]

Applications compiled on MacOS use an instance of std::string::~string() linked in from the C++ standard library. This means that std::string lacks annotates.

Take the following program as an example:

#include <string>
#include <vector>

struct my_object {
    std::string       s;
    std::vector<char> v;

    template <size_t N>
    my_object(char const (&cstr)[N]) : s(cstr)
                                     , v(cstr, cstr + N) {}
};

int main() { my_object a = "01234556789 this is a string that's too big for SSO etc"; }

If we dump out the stats for my_object, we see this:

// Totals for my_object sizeof=48 bytes
// └── 112 bytes across 2 allocs and 1 instance
struct my_object
{
  std::string       s;
  std::vector<char> v; // 56 bytes across 1 alloc : std::vector<char>

  ~my_object();
  // directly owned (or unannotated):
  // └── 56 bytes across 1 alloc
};

Both my_object::s and my_object::v should show 56 bytes, but only v has recorded bytes.

Since std::string's destructor isn't annotated, the 56 bytes owned by my_object::s are instead treated as directly owned.