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 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()
.
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
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