Passing array to a function (and why it does not work in C++)

Question

I have come across some C code that compiles, but I do not understand why. Specifically, I have a C library that has a lot of code using this format:

void get_xu_col(int i_start,                 int n,                 double x[n],                 int n_x,                 int n_u,                 int n_col,                 double xu_col[n_col][n_x + n_u]){     ...  }  int main(){     ...     double xu_col[n_col][n_x + n_u];     get_xu_col( ..., xu_col );     ... } 

What I don't understand is why the compiler allows for sizing in the arrays. To the best of my understanding, either the sizes must be fixed (e.g. xu_col[9][7]) or undefined (e.g. xu_col[][]). In the above code, it appears that the sizes are not compile-time constants.

Is the compiler just ignoring the arguments here? or is it really doing a compile-time check on the dimensions?

If it is the latter, then it seems error-prone to pass the dimensions separately.

The second part of the question is:

Why doesn't the same version work in C++? When I literally change the file extension from .c to .cpp and try to recompile, I get

candidate function not viable: no known conversion from 'double [n_col][n_x + n_u]' to 'double (*)[n_x + n_u]' for 7th argument void get_xu_col(int i_start, int n, double x[n], int n_x, int n_u, int n_col, double xu_col[n_col][n_x + n_u]); 

I would like to know which idiom I should use to convert this code to C++, since apparently the previous idiom was something that works in C, but not C++.

Answer 1

In C, it is possible to use function parameters to define the size of a variable length array parameter as long as the size comes before the array in the parameter list. This is not supported in C++.

Answer 2

The reason it works in C, but not in C++ is simply because it's C code and not C++. The two languages share a history, not a grammar.

The C++ method to pass variable-sized arrays is std::vector, probably by reference if you intend to modify the vector in the function, or by const reference if you don't.