the memory address of object between python and c++ are not identical

Question

I am trying to print the memory address of the same object in both c++ and python with pybind11, but I found the returned memory address from both are not identical.

c++ side

class Example {
  public:
    Example() {std::cout << "constuctor" << this << std::endl;}
    ~Example() {std::cout << "destructor " << this << std::endl;}
};

class ABC {
  public:
    static std::unique_ptr<Example> get_example() {
      // std::shared_ptr<Example> ptr = std::make_shared<Example>();
      std::unique_ptr<Example> ptr = std::make_unique<Example>();
      return ptr;
    }
};

void init_example(py::module & m) {
    py::class_<ABC>(m, "ABC")
    .def_static("get_example", &ABC::get_example);
}

python side

example = my_module.ABC.get_example()
print (example)

the output

constuctor0x234cd80
<Example object at 0x7f5493c37928>
destructor 0x234cd80

the memory address from c++ is 0x234cd80, but python is 0x7f5493c37928

any idea?

I don't quite get what you're attempting to print out. You are printing out this in the C++ code, but in actually in get_example, you're creating a std::unique_ptr, and its address is not going to be the same as this. See this example. Issue seems to have nothing to do with Python. — PaulMcKenzie
– PaulMcKenzie, Commented May 8, 2019 at 3:20
my mistake. I thought the address of std::unique_ptr is the same as the address of the object it points to. — alec.tu
– alec.tu, Commented May 8, 2019 at 3:57
But how to print out the memory address o the object in python? — alec.tu
– alec.tu, Commented May 8, 2019 at 3:58
You need to call the get() function, by whatever means Python gives you. — PaulMcKenzie
– PaulMcKenzie, Commented May 8, 2019 at 4:24

Sergei · Accepted Answer · 2019-05-10 08:54:17Z

pybind11 creates a Python object which has a reference to C++ object. So addresses of Python pybind11 wrapper and C++ object are different.

The address in default pybind11 str object representation is the address of python object, not of underlying C++ object or it's smart pointer.

If you need to know address of C++ object add a method to you binding code as @PaulMcKenzie suggested.

C++:

namespace py = pybind11;

PYBIND11_MODULE(example_module, m){
  m.def("add", add);

  py::class_<Example>(m,"Foo")
      .def(py::init())
      .def("get_raw_address",[](Example& foo){ return reinterpret_cast<uint64_t>(&foo);});

}

Python:

example = Example()
print(example)
print("C++ address: %x" % example.get_raw_address())

Output:

constuctor 0x10eff20
<example_module.Foo object at 0x7f51c71d4298>
C++ address: 10eff20
destructor 0x10eff20

PaulMcKenzie · Accepted Answer · 2019-05-08 03:29:38Z

1

I am not familiar with the Python aspect of this question, but focusing on the C++ portion, you are not printing out the correct information.

The std::unique_ptr has a different address than the Example instance that is created, thus the values will be different. If you want to print the address of the item that the unique_ptr is addressing, you need to call the get() function.

Here is a full example showing the differences:

#include <memory>
#include <iostream>

class Example {
  public:
    Example() {std::cout << "constuctor " << this << std::endl;}
    ~Example() {std::cout << "destructor " << this << std::endl;}
};

class ABC {
  public:
    static std::unique_ptr<Example> get_example() 
    {
      std::unique_ptr<Example> ptr = std::make_unique<Example>();
      return ptr;
    }
};

int main()
{
    std::unique_ptr<Example> p = ABC::get_example();
    std::cout << "The unique_ptr address is: " << &p << std::endl;
    std::cout << "The get() function returns: " << p.get() << std::endl;
}

Output:

constuctor 0x555a68bd7c20
The unique_ptr address is: 0x7ffd9fa6c120
The get() function returns: 0x555a68bd7c20
destructor 0x555a68bd7c20

So you need to adjust your Python code to print the return value of get().

answered May 8, 2019 at 3:29

PaulMcKenzie

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2 Comments

alec.tu Over a year ago

I made some changes but the returned memory addresses are still not identical. unique_ptr_address=0x7ffcc78cda70, python_side_address= 0x7ff4c80ab8f0

Lightness Races in Orbit Over a year ago

"So you need to adjust your Python code to print the return value of get()." Or adjust the C++ code, rather?

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