Assorted tweaks to folly/String.h

Summary: [Folly] Assorted tweaks to `folly/String.h`. * Use `void_t` in the definition of `IsConvertible`. * Remove `AllConvertible`. Callers can use `StrictConjunction` instead. * Move the `static_assert`s from the header to the source file. Reviewed By: andrewjcg, ot Differential Revision: D6501515 fbshipit-source-id: 472ecf23bf7f06be211480b0aceea95f7e60dc21

Assorted tweaks to folly/String.h
Summary: [Folly] Assorted tweaks to `folly/String.h`. * Use `void_t` in the definition of `IsConvertible`. * Remove `AllConvertible`. Callers can use `StrictConjunction` instead. * Move the `static_assert`s from the header to the source file. Reviewed By: andrewjcg, ot Differential Revision: D6501515 fbshipit-source-id: 472ecf23bf7f06be211480b0aceea95f7e60dc21
c01c18b2 · Yedidya Feldblum · Facebook Github Bot · 3764b633 · c01c18b2 · c01c18b2
Commit c01c18b2 authored Dec 11, 2017 by Yedidya Feldblum Committed by Facebook Github Bot Dec 11, 2017
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Showing with 20 additions and 31 deletions

folly/String-inl.h folly/String-inl.h +2 -1

folly/String.cpp folly/String.cpp +6 -0

folly/String.h folly/String.h +12 -30

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--- a/folly/String-inl.h
+++ b/folly/String-inl.h
@@ -401,7 +401,8 @@ void splitTo(const Delim& delimiter,

 template <bool exact, class Delim, class... OutputTypes>
 typename std::enable_if<
-    AllConvertible<OutputTypes...>::value && sizeof...(OutputTypes) >= 1,
+    StrictConjunction<IsConvertible<OutputTypes>...>::value &&
+        sizeof...(OutputTypes) >= 1,
    bool>::type
 split(const Delim& delimiter, StringPiece input, OutputTypes&... outputs) {
  return detail::splitFixed<exact>(

--- a/folly/String.cpp
+++ b/folly/String.cpp
@@ -29,6 +29,12 @@

 namespace folly {

+static_assert(IsConvertible<float>::value, "");
+static_assert(IsConvertible<int>::value, "");
+static_assert(IsConvertible<bool>::value, "");
+static_assert(IsConvertible<int>::value, "");
+static_assert(!IsConvertible<std::vector<int>>::value, "");
+
 static inline bool is_oddspace(char c) {
  return c == '\n' || c == '\t' || c == '\r';
 }

--- a/folly/String.h
+++ b/folly/String.h
@@ -34,6 +34,7 @@
 #include <folly/Portability.h>
 #include <folly/Range.h>
 #include <folly/ScopeGuard.h>
+#include <folly/Traits.h>

 // Compatibility function, to make sure toStdString(s) can be called
 // to convert a std::string or fbstring variable s into type std::string
@@ -468,41 +469,22 @@ void splitTo(const Delim& delimiter,
 * Note that this will likely not work if the last field's target is of numeric
 * type, in which case folly::to<> will throw an exception.
 */
-template <class T, class Enable = void>
-struct IsConvertible {
-  enum { value = false };
-};
+namespace detail {
+template <typename Void, typename OutputType>
+struct IsConvertible : std::false_type {};

-template <class T>
+template <typename OutputType>
 struct IsConvertible<
-    T,
-    decltype(static_cast<void>(
-        parseTo(std::declval<folly::StringPiece>(), std::declval<T&>())))> {
-  enum { value = true };
-};
-
-template <class... Types>
-struct AllConvertible;
-
-template <class Head, class... Tail>
-struct AllConvertible<Head, Tail...> {
-  enum { value = IsConvertible<Head>::value && AllConvertible<Tail...>::value };
-};
-
-template <>
-struct AllConvertible<> {
-  enum { value = true };
-};
-
-static_assert(AllConvertible<float>::value, "");
-static_assert(AllConvertible<int>::value, "");
-static_assert(AllConvertible<bool>::value, "");
-static_assert(AllConvertible<int>::value, "");
-static_assert(!AllConvertible<std::vector<int>>::value, "");
+    void_t<decltype(parseTo(StringPiece{}, std::declval<OutputType&>()))>,
+    OutputType> : std::true_type {};
+} // namespace detail
+template <typename OutputType>
+struct IsConvertible : detail::IsConvertible<void, OutputType> {};

 template <bool exact = true, class Delim, class... OutputTypes>
 typename std::enable_if<
-    AllConvertible<OutputTypes...>::value && sizeof...(OutputTypes) >= 1,
+    StrictConjunction<IsConvertible<OutputTypes>...>::value &&
+        sizeof...(OutputTypes) >= 1,
    bool>::type
 split(const Delim& delimiter, StringPiece input, OutputTypes&... outputs);