Passing const T& as this to a constexpr member function - c++

Question

I am trying to fix some library code where a boiled-down minimal version looks like this:
#include <iostream>
template <typename RangeT>
struct formatter {
constexpr void format(const RangeT& values) {
for (auto it = values.begin(), end = values.end(); it != end; ++it) {
std::cout << it << "\n";
}
}
};
template <typename RangeT, typename Formatter>
struct type_erased {
static void format(const void* arg) {
Formatter f;
f.format(*static_cast<const RangeT*>(arg));
}
};
struct view {
int count_;
constexpr View(int count) : count_(count) {}
constexpr int
begin() { return 0; }
constexpr int
end() { return -1; }
};
int
main()
{
View view(5);
void* ptr = static_cast<void*>(&view);
type_erased<View, formatter<View>>::format(ptr);
}
The above code does not compile in GCC since:
../src/view.cpp: In instantiation of ‘constexpr void formatter<RangeT>::format(const RangeT&) [with RangeT = View]’:
../src/view.cpp:21:9: required from ‘static void type_erased<RangeT, Formatter>::format(const void*) [with RangeT = View; Formatter = formatter<View>]’
../src/view.cpp:43:41: required from here
../src/view.cpp:11:15: error: passing ‘const View’ as ‘this’ argument discards qualifiers [-fpermissive]
11 | for (auto it = values.begin(), end = values.end(); it != end; ++it) {
| ^~
../src/view.cpp:31:5: note: in call to ‘constexpr int View::begin()’
31 | begin() { return 0; }
| ^~~~~
../src/view.cpp:11:36: error: passing ‘const View’ as ‘this’ argument discards qualifiers [-fpermissive]
11 | for (auto it = values.begin(), end = values.end(); it != end; ++it) {
| ^~~
../src/view.cpp:34:5: note: in call to ‘constexpr int View::end()’
34 | end() { return -1; }
What are the rules regarding this in constexpr member function? Is it subject to the rules specified for function parameters or are there special constraints?
How would I go about fixing this error? If it would only be the formatter struct I would use RangeT&& and std::move since views are by definition copyable in O(1) as far as I know. I don't know how to do that with the type erasure step in the mix though...
Thanks in advance,
Richard

Answer 1

I don't think this has anything to do with constexpr.
You have a reference to const RangeT, and you're trying to invoke non-const member functions on it (begin() and end()).
Provide const overloads (and/or cbegin()/cend() variants) if you want to permit that.

Answer 2

In your code as begin and end are not const functions the this pointer can't point to a const object without "discarding qualifiers".
By making the functions const then the this pointer can point to a const object.
https://en.cppreference.com/w/cpp/language/member_functions#const-.2C_volatile-.2C_and_ref-qualified_member_functions
https://en.cppreference.com/w/cpp/language/this
https://godbolt.org/z/3_wKh9