Borrowing C++ string into Python safely

test.cc

#include <string>

class Foo {
 public:
  Foo() : bar("foobar") {}
  std::string bar;
};

extern "C" void CreateFoo(void** handle) {
  *handle = static_cast<void*>(new Foo());
}

extern "C" void FooGetContent(void* handle, const char** out) {
  *out = static_cast<Foo*>(handle)->bar.c_str();
}

extern "C" void FooChangeContent(void* handle) {
  static_cast<Foo*>(handle)->bar = "meow";
}

test.py

import ctypes

lib = ctypes.cdll.LoadLibrary('./test.dylib')
foo = ctypes.c_void_p()
lib.CreateFoo(ctypes.byref(foo))

val = ctypes.c_char_p()   # borrowed pointer from C++
lib.FooGetContent(foo, ctypes.byref(val))
val2 = val.value.decode('utf-8')   # this makes a copy
print(val.value)
print(val2)
lib.FooChangeContent(foo)
print(val.value)   # this pointer now refers to "meow"
print(val2)        # this value is still "foobar"

It's okay to borrow const char* from C++, as long as we call .decode() right away from the Python side.