[QUESTION] How to write the binding code for a c++ function that takes char * and return a std::string

[ruiyang00]

Hello All, I have a cipher function inside the RC4 class that takes char* and returns std::string. I am wondering how should I write

the binding code. Thank you!!! I am trying return the string to python code. I have searched the Issues, I do not see any solution to my situation!!!

[bstaletic]

Pybind doesn't support pointers-as-arrays in bindings. Wrap that function call in a lambda that:

• takes a py::str or py::bytes
• extracts the char* with C API
• calls cipher

[YannickJadoul]

@bstaletic I thought there was an exception for char * and const char * conversions from str (and maybe bytes)?

What's the error you get?

[ruiyang00]

Below is my c++ code:

class RC4{
          
           char* cipher(char *text) {
                int i = 0;
                int j = 0;
                 char* ciphertext = new char[strlen(text)];
 
                for(size_t n = 0, len = strlen(text); n < len; n++) {
                     i = (i + 1) % N;
                     j = (j + S[i]) % N;

                     swap(&S[i], &S[j]);

                     int rnd = S[(S[i] + S[j]) % N];
               
                     std::cout<<"This is rnd: "<< rnd<< "\n";
            
                     ciphertext[n] = rnd ^ text[n];    

                  }
                  return ciphertext;
            }

}

below is my binding code:

PYBIND11_MODULE(example, m) {
py::class_<RC4>(m, "RC4")
        .def(py::init<>())
        .def("cipher", [](RC4 &self) -> py::bytes{
            return py::bytes(self.cipher())
        }); 
}

I am getting an error

[YannickJadoul]

@ruiyang00, well, if you read the error, it says you need to pass an argument to the RC4::cipher function. And you need a semicolon. These are plain C++ errors, unrelated to pybind11.

[ruiyang00]

@YannickJadoul @rolandd @dekken
Really appreciate your guys time!
I am very new to pybind11. I do not know how to intercept the args passed from python to C++.
in python

r = RC4();
r.cipher("hello");

In pybind11:
How should I get the hello in pybind11 binding and pass "hello" string to my C++ cipher()?
becuase there was a issue with the utf-8 encoding between C++ and python. I read the pybind11 documentation, but there isn't much info that fit my situation.

[YannickJadoul]

I read the pybind11 documentation, but there isn't much info that fit my situation.

https://pybind11.readthedocs.io/en/stable/basics.html#creating-bindings-for-a-simple-function

all details regarding the function’s parameters and return value were automatically inferred using template metaprogramming.

I.e., make sure the function (in the form of a lambda, in your case) you're binding takes a parameter of the right type.

pybind11 automatically converts these types for you: https://pybind11.readthedocs.io/en/stable/advanced/cast/strings.html

[ruiyang00]

@YannickJadoul thank you! I think we can close this Issue!

[YannickJadoul]

Thanks for letting me know, @ruiyang00! :-)