How to convert “pointer to pointer type” to const?

Question

With the following code

void TestF(const double ** testv){;}
void callTest(){
    double** test;
    TestF(test);
}

I get this:

Answer 1

It is correct that a double ** cannot be implicitly converted to a const double **. It can be converted to a const double * const *, though.

Imagine this scenario:

const double cd = 7.0;
double d = 4.0;
double *pd = &d;
double **ppd = &pd;
const double **ppCd = ppd;  //this is illegal, but if it were possible:
*ppCd = &cd;  //now *ppCd, which is also *ppd, which is pd, points to cd
*pd = 3.14; // pd now points to cd and thus modifies a const value!

So, if your function does not intend to modify any of the pointers involved, change it to take a const double * const *. If it intends to do modifications, you must decide whether all the modifications it does are safe and thus const_cast can be used, or whether you really need to pass in a const double **.