Call CPP function from Python

Tips for CPP Programming

Mangle

Due to the functionality of overloading in cpp, compiler will mangle the function name, so that you cannot call function by the name you defined, so you have to tell compiler this line should be compiled by c instead of c++ by extern "C"

ctypes API

ctypes only support c api(not c++), so class with initializer and destructor or other member functions aren't supported. If you still want to call c++-based class object, you need to implement functions as proxy, for example:

class Rule
{
    private:
        float amtThresh;
    public:
        Rule(){amtThresh = 0;}
        Rule(const float amtThresh_in){amtThresh = amtThresh_in;}
        ~Rule(){}
        void run()
        {
            std::cout << "Threshold of amount:" << amtThresh << std::endl;
        }
};
Rule* NewRule(float amtThresh_in)
{
    return new Rule(amtThresh_in);
}

void RunRule(Rule* rulePtr)
{
    rulePtr->run();
}

Notations for Types

String

python -> c++: Before sent string from python to c++, you must change format of python-string, for example:
```
inString = "hello".encode('utf-8')
ctypes.c_char_p(inString)
```
In c++ side, you need to accept the string by point of char, then youn consider it as std::string
```
char * const inCharPtr;
std::string inputString = inCharPtr;
```
c++ -> python: Just like sent string from python to c++, changing format is necessary:
```
const char *GetCharPtr(std::string outString){return outString.c_str();}
```
In python side, you only need to define format at the first time, then you can get return string directly
```
libFoo.GetCharPtr.restype = ctypes.c_char_p
```

Compile

gcc -c -fPIC cppSource.cpp -o cppSource.o

Generate liberary

g++ cppSource.o -shared -o libfoo.so

Tips for Python Programming

import ctypes
libFoo = ctypes.CDLL("./libfoo.so")

Sometimes you may need to cast python types into c types, like ctypes.c_int(1) or others.

Yorwxue/aml-rules