Using "extern C" from C++ with vector for ctypes

Question

My goal is to use ctypes to call from within Python a C++ library I'm creating separately, where I am passing numpy vectors to the C++ routines via pointers. So Python would allocate the memory and just pass the address, and then the C++ routine would access the data and perform its calculations and return the results.

Since I'm new to ctypes, right now I'm gradually building a working toy example. I'm starting off writing the C++ code and creating a C extern interface that the Python wrapper code will use. I haven't even started on the Python wrapper code yet but have some experience with that aspect already.

Here is my example which includes a constructor and a simple total function.

// foo.cpp //
#include <iostream>
#include <string>
#include <vector>
using namespace std;

class Data {
public:
  Data(vector<double>*);
  void PrintTotal(void);
private:
  vector<double>* myContents;
};
Data::Data(vector<double>* input) {
  std::cout << "Entered constructor in C++" << std::endl;
  myContents = input;
}
void Data::PrintTotal(void) {
  double total = 0;
  for (int i=0; i<myContents->size(); i++){
    total += (*myContents)[i];
  }
  std::cout << "Hello, my total is " << total << std::endl;
}

extern "C" {
  Data* Data_new(double (**input)) {return new Data(input);}
  void Print_Total(Data* pData) {pData->PrintTotal();}
}

Note in particular that I'm using the vector class from the STL, which may be part of the problem described below. The idea is that the class Data holds a pointer to the data and does not replicate the data.

When I try to compile this using the command

g++ -c -fPIC foo.cpp -o foo.o

on our Linux machine, I get the following error:

foo.cpp: In function âData* Data_new(double**)â:
foo.cpp:26: error: no matching function for call to âData::Data(double**&)â
foo.cpp:13: note: candidates are: Data::Data(std::vector<double, std::allocator<double> >*)
foo.cpp:6: note:                 Data::Data(const Data&)

This seems pretty clear to me: it's saying that the way I call the Data constructor in the third to last line (#26) in foo.cpp does not coincide with the constructor I wrote in the C++ part of the code. In both the C++ code and the C extern code, I'm trying to say that the input is a pointer to a vector/array of doubles. I've tried other options on line #36 but it still doesn't compile.

How do I write the extern C part while still using the vector class (which I will find useful when I code the real algorithm) in the C++ part? Am I running into trouble because vector is part of the STL, which doesn't work well with extern C?

Thanks in advance.

If you want the Date_new and Print_total functions to be usable from a C program, then that won't work. Even when using extern "C" the C program will not be able to use classes like Data. — Some programmer dude

razeh razeh · Accepted Answer · 2012-08-29T00:29:13

First, let's answer your question. You should change your constructor so that it takes the same arguments as Data_new, and converts the input into a std::vector. However, it looks like there are a couple of conceptual issues that you need to focus on first:

Arrays and vectors are not equivalent: if your input points to an array, but you want to work with a vector, you'll need to copy all of the elements from the array into the vector (or ask a vector constructor to do it for you). You need to do the copy because all STL containers hold onto copies of the elements that are put in them.
You can construct the vector using two iterators, with:

myContents(data, data + numberOfElements);

But this brings up the second conceptual point --- there is nothing in the signature of your code that tells you how large the input array is.
In Data you're holding onto a pointer to a vector, but you never free it. This is a memory leak. That being said, you should probably just hold onto a vector, not a pointer to a vector.

Using "extern C" from C++ with vector for ctypes

1 Answers