https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98933
Bug ID: 98933
Summary: P0784R7 example support: constexpr new
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: markus.kuehni at triviso dot ch
Target Milestone: ---
Trying to recreate the
[P0784R7](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0784r7.html)
code example.
<source>:13:10: error: 'mark_immutable_if_constexpr' is not a member of 'std'
13 | std::mark_immutable_if_constexpr(this->ps);
Removed std::mark_immutable_if_constexpr(this->ps);
<source>:22:31: error: no matching function for call to 'S<char>::S(const char
[7])'
22 | constexpr S<char> str("Hello!");
| ^
Added std::add_const<T>::type for constructor argument p.
<source>:22:31: in 'constexpr' expansion of 'S<char>("Hello!")'
<source>:11:7: error: call to non-'constexpr' function 'void* operator
new(std::size_t, void*)'
11 | new(this->ps+k) T{p[k]};
Replaced with std::construct_at() and std::destroy_at() for symmetry, see
[91369](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91369).
In file included from
/opt/compiler-explorer/gcc-trunk-20210202/include/c++/11.0.0/memory:64,
from <source>:1:
/opt/compiler-explorer/gcc-trunk-20210202/include/c++/11.0.0/bits/allocator.h:171:50:
error: 'S<char>("Hello!")' is not a constant expression because it refers to a
result of 'operator new'
171 | return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp)));
Now I'm stuck.
A cleaned-up version is here:
https://godbolt.org/z/KWMqna
```
#include <memory>
#include <new>
template<typename T> struct S : std::allocator<T> {
std::size_t sz;
T *ps;
template<std::size_t N>
constexpr S(typename std::add_const<T>::type (&p)[N]) :
sz { N }, ps {this->allocate(N)} {
for (std::size_t k = 0; k<N; ++k) {
std::construct_at(ps+k, p[k]);
}
}
constexpr ~S() {
for (std::size_t k = 0; k < this->sz; ++k) {
std::destroy_at(ps+k);
}
this->deallocate(this->ps, this->sz);
}
};
constexpr S<char> str("Hello!");
// str ends up pointing to a static array
// containing the string "Hello!".
```
