Semicolon on a function parameters

Question

matrix_* matrix_insert_values(int n; double a[][n], int m, int n) {     matrix_* x = matrix_new(m, n);     for (int i = 0; i < m; i++)         for (int j = 0; j < n; j++)             x->v[i][j] = a[i][j];     return x; } 

Example of my test matrix

double in[][3] = {     { 12, -51,   4},     {  6, 167, -68},     { -4,  24, -41},     { -1, 1, 0},     { 2, 0, 3}, }; 

I'm a bit lost, I can't figure out what int n; is inside my argument declaration, it works over C but C++ doesn't allow this implementation. I want to understand how this is working because I'm going to migrate this code to C++.

Answer 1

It is a seldom-used feature from C99 GNU extension (GCC documentation) that is used to forward-declare parameters used in VLA declarators.

matrix_* matrix_insert_values(int n; double a[][n], int m, int n); 

Do you see how int n appears twice? The first int n; is just a forward declaration of the actual int n, which is at the end. It has to appear before double a[][n] because n is used in the declaration of a. If you were okay with rearranging parameters, you could just put n before a and then you wouldn't need this feature

matrix_* matrix_insert_values_rearranged(int m, int n, double a[][n]); 

Note about C++ compatibility

To be clear, the GNU extension is just the forward declaration of function parameters. The following prototype is standard C:

// standard C, but invalid C++ matrix_* matrix_insert_values_2(int m, int n, double a[][n]); 

You cannot call this function from C++, because this code uses variable length arrays, which are not supported in C++. You would have to rewrite the function in order to be able to call it from C++.