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

问题

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?

回答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 *

回答2:

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 **.

来源：https://stackoverflow.com/questions/19910296/how-to-convert-pointer-to-pointer-type-to-const

标签

c++

const

const-cast