我們可以在不使用C風格的vtables進行分配的情況下使用類型擦除。
首先,在一個私人的命名空間中的虛函數表的細節:
namespace details {
template<class R, class...Args>
using call_view_sig = R(void const volatile*, Args&&...);
template<class R, class...Args>
struct call_view_vtable {
call_view_sig<R, Args...> const* invoke = 0;
};
template<class F, class R, class...Args>
call_view_sig<R, Args...>const* get_call_viewer() {
return [](void const volatile* pvoid, Args&&...args)->R{
F* pf = (F*)pvoid;
return (*pf)(std::forward<Args>(args)...);
};
}
template<class F, class R, class...Args>
call_view_vtable<R, Args...> make_call_view_vtable() {
return {get_call_viewer<F, R, Args...>()};
}
template<class F, class R, class...Args>
call_view_vtable<R, Args...>const* get_call_view_vtable() {
static const auto vtable = make_call_view_vtable<F, R, Args...>();
return &vtable;
}
}
模板iteslf。這就是所謂的call_view<Sig>
,類似於std::function<Sig>
:
template<class Sig>
struct call_view;
template<class R, class...Args>
struct call_view<R(Args...)> {
// check for "null":
explicit operator bool() const { return vtable && vtable->invoke; }
// invoke:
R operator()(Args...args) const {
return vtable->invoke(pvoid, std::forward<Args>(args)...);
}
// special member functions. No need for move, as state is pointers:
call_view(call_view const&)=default;
call_view& operator=(call_view const&)=default;
call_view()=default;
// construct from invokable object with compatible signature:
template<class F,
std::enable_if_t<!std::is_same<call_view, std::decay_t<F>>{}, int> =0
// todo: check compatibility of F
>
call_view(F&& f):
vtable(details::get_call_view_vtable< std::decay_t<F>, R, Args... >()),
pvoid(std::addressof(f))
{}
private:
// state is a vtable pointer and a pvoid:
details::call_view_vtable<R, Args...> const* vtable = 0;
void const volatile* pvoid = 0;
};
在這種情況下,vtable
是有點冗餘;一個只包含指向單個函數的指針的結構。當我們有不止一次手術時,我們正在擦拭這是明智的;在這種情況下,我們不這樣做。
我們可以用該操作替換vtable
。上述虛函數表上面的工作一半可以去除,實現更簡單:
template<class Sig>
struct call_view;
template<class R, class...Args>
struct call_view<R(Args...)> {
explicit operator bool() const { return invoke; }
R operator()(Args...args) const {
return invoke(pvoid, std::forward<Args>(args)...);
}
call_view(call_view const&)=default;
call_view& operator=(call_view const&)=default;
call_view()=default;
template<class F,
std::enable_if_t<!std::is_same<call_view, std::decay_t<F>>{}, int> =0
>
call_view(F&& f):
invoke(details::get_call_viewer< std::decay_t<F>, R, Args... >()),
pvoid(std::addressof(f))
{}
private:
details::call_view_sig<R, Args...> const* invoke = 0;
void const volatile* pvoid = 0;
};
,它仍然有效。
通過一些重構,我們可以從存儲器(所有者或非存儲器)拆分調度表(或多個函數),以從擦除操作類型中分離類型擦除的值/引用語義。
作爲一個例子,一個只能移動擁有的可調用函數應該重用幾乎所有的上述代碼。被刪除的數據存在於智能指針中,void const volatile*
或std::aligned_storage
可以與您在刪除對象上的操作分開。
如果需要值語義,可以如下擴展類型擦除:
namespace details {
using dtor_sig = void(void*);
using move_sig = void(void* dest, void*src);
using copy_sig = void(void* dest, void const*src);
struct dtor_vtable {
dtor_sig const* dtor = 0;
};
template<class T>
dtor_sig const* get_dtor() {
return [](void* x){
static_cast<T*>(x)->~T();
};
}
template<class T>
dtor_vtable make_dtor_vtable() {
return { get_dtor<T>() };
}
template<class T>
dtor_vtable const* get_dtor_vtable() {
static const auto vtable = make_dtor_vtable<T>();
return &vtable;
}
struct move_vtable:dtor_vtable {
move_sig const* move = 0;
move_sig const* move_assign = 0;
};
template<class T>
move_sig const* get_mover() {
return [](void* dest, void* src){
::new(dest) T(std::move(*static_cast<T*>(src)));
};
}
// not all moveable types can be move-assigned; for example, lambdas:
template<class T>
move_sig const* get_move_assigner() {
if constexpr(std::is_assignable<T,T>{})
return [](void* dest, void* src){
*static_cast<T*>(dest) = std::move(*static_cast<T*>(src));
};
else
return nullptr; // user of vtable has to handle this possibility
}
template<class T>
move_vtable make_move_vtable() {
return {{make_dtor_vtable<T>()}, get_mover<T>(), get_move_assigner<T>()};
}
template<class T>
move_vtable const* get_move_vtable() {
static const auto vtable = make_move_vtable<T>();
return &vtable;
}
template<class R, class...Args>
struct call_noalloc_vtable:
move_vtable,
call_view_vtable<R,Args...>
{};
template<class F, class R, class...Args>
call_noalloc_vtable<R,Args...> make_call_noalloc_vtable() {
return {{make_move_vtable<F>()}, {make_call_view_vtable<F, R, Args...>()}};
}
template<class F, class R, class...Args>
call_noalloc_vtable<R,Args...> const* get_call_noalloc_vtable() {
static const auto vtable = make_call_noalloc_vtable<F, R, Args...>();
return &vtable;
}
}
template<class Sig, std::size_t sz = sizeof(void*)*3, std::size_t algn=alignof(void*)>
struct call_noalloc;
template<class R, class...Args, std::size_t sz, std::size_t algn>
struct call_noalloc<R(Args...), sz, algn> {
explicit operator bool() const { return vtable; }
R operator()(Args...args) const {
return vtable->invoke(pvoid(), std::forward<Args>(args)...);
}
call_noalloc(call_noalloc&& o):call_noalloc()
{
*this = std::move(o);
}
call_noalloc& operator=(call_noalloc const& o) {
if (this == &o) return *this;
// moveing onto same type, assign:
if (o.vtable && vtable->move_assign && vtable == o.vtable)
{
vtable->move_assign(&data, &o.data);
return *this;
}
clear();
if (o.vtable) {
// moveing onto differnt type, construct:
o.vtable->move(&data, &o.data);
vtable = o.vtable;
}
return *this;
}
call_noalloc()=default;
template<class F,
std::enable_if_t<!std::is_same<call_noalloc, std::decay_t<F>>{}, int> =0
>
call_noalloc(F&& f)
{
static_assert(sizeof(std::decay_t<F>)<=sz && alignof(std::decay_t<F>)<=algn);
::new((void*)&data) std::decay_t<F>(std::forward<F>(f));
vtable = details::get_call_noalloc_vtable< std::decay_t<F>, R, Args... >();
}
void clear() {
if (!*this) return;
vtable->dtor(&data);
vtable = nullptr;
}
private:
void* pvoid() { return &data; }
void const* pvoid() const { return &data; }
details::call_noalloc_vtable<R, Args...> const* vtable = 0;
std::aligned_storage_t< sz, algn > data;
};
,我們創建的內存界緩衝區對象存儲在該版本僅支持移動語義。收件人擴展到複製語義應該是顯而易見的。
這比std::function
的優勢在於,如果您沒有足夠的空間來存儲相關對象,則會出現硬編譯器錯誤。作爲一種非分配類型,您可以在性能關鍵代碼中使用它,而不會冒分配延遲的風險。
測試代碼:
void print_test(call_view< void(std::ostream& os) > printer) {
printer(std::cout);
}
int main() {
print_test([](auto&& os){ os << "hello world\n"; });
}
Live example與測試的所有3。