c++ - 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:

'TestF' : cannot convert parameter 1 from 'double **' to 'const double **'

I cannot understand why. Why test cannot be silently casted to const double**? Why should I do it explicitly? I know that

TestF(const_cast<const double**>(test)) 

makes my code correct, but I feel this should be unnecessary.

Are there some key concepts about const that I'm missing?

Answer 1

The language allows implicit conversion from double ** to const double *const *, but not to const double **. The conversion you attempt would implicitly violate the rules of const correctness, even though it is not immediately obvious.

The example in the [de-facto standard] C++ FAQ illustrates the issue

https://isocpp.org/wiki/faq/const-correctness#constptrptr-conversion

Basically, the rule is: once you add const at some level of indirection, you have to add const to all levels of indirection all the way to the right. For example, int ***** cannot be implicitly converted to int **const ***, but it can be implicitly converted to int **const *const *const *