ywkaras commented on PR #9482:
URL: https://github.com/apache/trafficserver/pull/9482#issuecomment-1608277276
How about this approach?
```
wkaras ~/STUFF/GET_SERVICE
$ cat x.cc
#include <cstdint>
namespace ts
{
// Returns offset from Base1 to Base2 in any instance of Derived.
//
template <class Derived, class Base1, class Base2>
std::ptrdiff_t base_to_base_offset()
{
// Don't use 0 for dummy address of derived, since compiler may treat null
address specially.
//
auto addr = reinterpret_cast<Derived *>(0x100);
return reinterpret_cast<char *>(static_cast<Base2 *>(addr)) -
reinterpret_cast<char *>(static_cast<Base1 *>(addr));
}
} // end namespace ts
enum class X_SERVICE {
Y1, Y2, Y3, NUM
};
template <class>
struct X_Service;
static std::ptrdiff_t const X_SERVICE_NONE{PTRDIFF_MIN};
class X
{
public:
template <class C>
C const * get_service() const
{
auto ofs = _service_ofs()[std::size_t(X_Service<C>::value)];
if (X_SERVICE_NONE == ofs) {
return nullptr;
} else {
return reinterpret_cast<C const *>(reinterpret_cast<char const
*>(this) + ofs);
}
}
template <class C>
C * get_service()
{
std::ptrdiff_t ofs = _service_ofs()[std::size_t(X_Service<C>::value)];
if (X_SERVICE_NONE == ofs) {
return nullptr;
} else {
return reinterpret_cast<C *>(reinterpret_cast<char *>(this) + ofs);
}
}
private:
static std::ptrdiff_t const * const _service_ofs_table;
// Returns array (A), indexed by service (S), where A[S] is the offset
from base class X to the base class for service
// S in the same object, or NO_SERVICE if there is no base class in the
object for service S.
//
virtual std::ptrdiff_t const * _service_ofs() const { return
_service_ofs_table; }
int dummy_data;
};
std::ptrdiff_t const * const X::_service_ofs_table{[]() -> std::ptrdiff_t *
{
static std::ptrdiff_t tbl[std::size_t(X_SERVICE::NUM)];
tbl[std::size_t(X_SERVICE::Y1)] = X_SERVICE_NONE;
tbl[std::size_t(X_SERVICE::Y2)] = X_SERVICE_NONE;
tbl[std::size_t(X_SERVICE::Y3)] = X_SERVICE_NONE;
return tbl;
}()};
struct Y1
{
int dummy_data;
};
template <>
struct X_Service<Y1>
{
static X_SERVICE const value{X_SERVICE::Y1};
};
struct Y2
{
int dummy_data;
};
template <>
struct X_Service<Y2>
{
static X_SERVICE const value{X_SERVICE::Y2};
};
struct Y3
{
int dummy_data;
};
template <>
struct X_Service<Y3>
{
static X_SERVICE const value{X_SERVICE::Y3};
};
class D1 : public X, public Y1, public Y3
{
public:
int dummy_data;
private:
static std::ptrdiff_t const * const _service_ofs_table;
// Returns array (A), indexed by service (S), where A[S] is the offset
from base class X to the base class for service
// S in the same object, or NO_SERVICE if there is no base class in the
object for service S.
//
std::ptrdiff_t const * _service_ofs() const override { return
_service_ofs_table; }
};
std::ptrdiff_t const * const D1::_service_ofs_table{[]() -> std::ptrdiff_t *
{
static std::ptrdiff_t tbl[std::size_t(X_SERVICE::NUM)];
tbl[std::size_t(X_SERVICE::Y1)] = ts::base_to_base_offset<D1, X, Y1>();
tbl[std::size_t(X_SERVICE::Y2)] = X_SERVICE_NONE;
tbl[std::size_t(X_SERVICE::Y3)] = ts::base_to_base_offset<D1, X, Y3>();
return tbl;
}()};
#include <cassert>
int main()
{
D1 d1;
X *xp = &d1;
assert(xp->get_service<Y1>() == static_cast<Y1 *>(&d1));
assert(xp->get_service<Y2>() == nullptr);
assert(xp->get_service<Y3>() == static_cast<Y3 *>(&d1));
return 0;
}
wkaras ~/STUFF/GET_SERVICE
$ gcc -Wall -Wextra -pedantic -std=c++17 x.cc -lstdc++
wkaras ~/STUFF/GET_SERVICE
$ ./a.out
wkaras ~/STUFF/GET_SERVICE
$
```
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