C++

Load the example 64-bit C++ library in 64-bit Python. To load the 32-bit library in 32-bit Python use "cpp_lib32" as the filename.

Tip

If the file extension is not specified, a default extension, .dll (Windows), .so (Linux) or .dylib (macOS) is used.

Example

Load the example C++ library

>>> from msl.loadlib import LoadLibrary
>>> from msl.examples.loadlib import EXAMPLES_DIR
>>> cpp = LoadLibrary(EXAMPLES_DIR / "cpp_lib64")

By default, ctypes treats all input argument types and the return type of a library function to be a c_int. Therefore, the argtypes and the restype should be defined for each function in the library. A few examples for the C++ library are shown below

>>> from ctypes import c_char_p, c_float, c_int32
>>> cpp.lib.subtract.argtypes = [c_float, c_float]
>>> cpp.lib.subtract.restype = c_float
>>> cpp.lib.reverse_string_v1.argtypes = [c_char_p, c_int32, c_char_p]
>>> cpp.lib.reverse_string_v1.restype = None

Call the add function to calculate the sum of two integers

>>> cpp.lib.add(1, 2)
3

Call the subtract function to calculate the difference between two floats

>>> cpp.lib.subtract(7.1, 2.1)
5.0

Call the reverse_string_v1 function to reverse the characters in a byte string. Python manages the memory of the reversed sting by creating a string buffer of the necessary length

>>> from ctypes import create_string_buffer
>>> original = b"olleh"
>>> reverse = create_string_buffer(len(original))
>>> cpp.lib.reverse_string_v1(original, len(original), reverse)
>>> reverse.raw.decode()
'hello'

C++ Source Code

cpp_lib

cpp_lib.cppcpp_lib.h

// cpp_lib.cpp
// Examples that show how to pass various data types between Python and a C++ library.
//
// Compiled using:
//   g++ cpp_lib.cpp -fPIC -shared -Bstatic -Wall -o cpp_lib64.so
//
#include <math.h>
#include "cpp_lib.h"

int add(int a, int b) {
    return a + b;
}

float subtract(float a, float b) {
    return a - b;
}

double add_or_subtract(double a, double b, bool do_addition) {
    if (do_addition) {
        return a + b;
    } else {
        return a - b;
    }
}

void scalar_multiply(double a, double* xin, int n, double* xout) {
    for (int i = 0; i < n; i++) {
        xout[i] = a * xin[i];
    }
}

void reverse_string_v1(const char* original, int n, char* reversed) {
    for (int i = 0; i < n; i++) {
        reversed[i] = original[n-i-1];
    }
}

char* reverse_string_v2(char* original, int n) {
    char* reversed = new char[n];
    for (int i = 0; i < n; i++) {
        reversed[i] = original[n - i - 1];
    }
    return reversed;
}

// this function is not exported to the shared library
double distance(Point p1, Point p2) {
    double d = sqrt(pow(p1.x - p2.x, 2) + pow(p1.y - p2.y, 2));
    return d;
}

double distance_4_points(FourPoints p) {
    double d = distance(p.points[0], p.points[3]);
    for (int i = 1; i < 4; i++) {
        d += distance(p.points[i], p.points[i-1]);
    }
    return d;
}

double distance_n_points(NPoints p) {
    if (p.n < 2) {
        return 0.0;
    }
    double d = distance(p.points[0], p.points[p.n-1]);
    for (int i = 1; i < p.n; i++) {
        d += distance(p.points[i], p.points[i-1]);
    }
    return d;
}

// cpp_lib.h
// Contains the declaration of exported functions.
//

#if defined(_MSC_VER)
    // Microsoft
    #define EXPORT __declspec(dllexport)
#elif defined(__GNUC__)
    // G++
    #define EXPORT __attribute__((visibility("default")))
#else
#   error "Unknown EXPORT semantics"
#endif

struct Point {
    double x;
    double y;
};

struct FourPoints {
    Point points[4];
};

struct NPoints {
    int n;
    Point *points;
};

extern "C" {

    // a + b
    EXPORT int add(int a, int b);

    // a - b
    EXPORT float subtract(float a, float b);

    // IF do_addition IS TRUE THEN a + b ELSE a - b
    EXPORT double add_or_subtract(double a, double b, bool do_addition);

    // multiply each element in 'x' by 'a'
    EXPORT void scalar_multiply(double a, double* xin, int n, double* xout);

    // reverse a string
    EXPORT void reverse_string_v1(const char* original, int n, char* reversed);

    // reverse a string and return it
    EXPORT char* reverse_string_v2(char* original, int n);

    // calculate the total distance connecting 4 Points
    EXPORT double distance_4_points(FourPoints p);

    // calculate the total distance connecting N Points
    EXPORT double distance_n_points(NPoints p);

}