const限定词到decltype

Question

My first try of using decltype

vector<int> vals;
const decltype(&vals[0]) ptr;

for (const auto& val : vals)
    ptr = &val;    

doesn't compile, complaining about assigning to read-only variable ptr . What I'm trying to achieve is ptr to be of type const int* . How can I make it work? Thanks.

Answer 1

std::vector has a const_pointer typedef. You can use that like

decltype(vals)::const_pointer ptr;

to get the correct const pointer to the vector type ( Live Example ).

Answer 2

Just don't declare the pointer before you need it. Then you can deduce its type (in this case, the desired const int* ) in the initialization.

This works:

std::vector<int> val {1,2,3,4};

for (const auto &v : val) {
    auto p = &v;
    std::cout << *p;
}

This fails:

std::vector<int> val {1,2,3,4};

for (const auto &v : val) {
    auto p = &v;
    *p = 7;
}

Answer 3

Your declaration of ptr is basically const (int *) ptr , or int * const ptr . So the pointer is constant, not the objects it points to. You can either make vals const , or at least cast it to const inside the decltype , or use std::vector 's typedefs instead of decltype .

Answer 4

typedef is not textual substitution. decltype is not textual substitution. The modifier const applies to the entire thing it's applied to, it doesn't reach inside the result of decltype to change the left-most thing.

This would work though, because now const is applied to the element type to make the type (const int) , and then ptr is made a pointer to that:

const std::remove_reference<decltype(vals[0])>::type * ptr;

Note that this one wouldn't work, since it uselessly tries to const -qualify a reference type:

std::add_pointer<const decltype(vals[0])>::type ptr;