Question

I have written a C++ application and within it, I need to call a math function that was written in C. The prototype looks like:

void Jacobi_Cyclic_Method(double *eigenvalues, double *eigenvectors, double *A, int n);

My problem is that I can’t seem to pass the function double * (for instance both eigenvectors and A are multi-dimensional arrays. The C++ way to pass those things seems to be

double [][size]

I have read about extern C but I don’t think it applies here since I am not interfacing with an object but with source itself. How can I send that C function my multi-dimensional arrays defined as such:

double [100][100] A;
double [100][100] eigenvectors;
double [100] eigenvalues;

Trying to compile I get:

error: no matching function for call to ‘MathEngine::Jacobi_Cyclic_Method(double 
[100], double [100][100], double [100][100], int)’
mathEngine.h:9: note: candidates are: static void    
MathEngine::Jacobi_Cyclic_Method(double*, double*, double*, int)

Answer 1

I’m assuming that your C function requires multi-dimensional arrays for some of its parameters and that the prototype is written with pointers to doubles for the array/matrix parameters where the integer parameter indicates the size of each dimension (I guess the matrices are square?). If this is the case then you can pass a pointer to the first element of each array/matrix like this:

Jacobi_Cyclic_Methods(&eigenvalues[0], &eigenvectors[0][0], &A[0][0], 100);

Your initial attempt doesn’t work because the type of, for instance, eigenvectors is double[100][100] which decays to double (*)[100], not double *.

This post addresses the issue of pointers and multi-dimensional arrays.