[ADT] Teach StringRef to take basic features from std::string_view (NFC)

[kazutakahirata]

This patch teaches StringRef to take basic features from
std::string_view, including:

• npos
• iterator types
• empty, begin, end, rbegin, rend

I'm not replacing performance-sensitive functions like find and
operator== for now so that this patch will likely stick.

[llvmbot]

@llvm/pr-subscribers-llvm-adt

@llvm/pr-subscribers-llvm-support

Author: Kazu Hirata (kazutakahirata)

Changes

This patch teaches StringRef to take basic features from
std::string_view, including:

• npos
• iterator types
• empty, begin, end, rbegin, rend

I'm not replacing performance-sensitive functions like find and
operator== for now so that this patch will likely stick.

---

Full diff: https://github.com/llvm/llvm-project/pull/113797.diff

2 Files Affected:

• (modified) llvm/include/llvm/ADT/StringRef.h (+17-26)
• (modified) llvm/lib/Support/StringRef.cpp (+2-4)

diff --git a/llvm/include/llvm/ADT/StringRef.h b/llvm/include/llvm/ADT/StringRef.h
index f879bbf7164fd6..e59a3f10ee76df 100644
--- a/llvm/include/llvm/ADT/StringRef.h
+++ b/llvm/include/llvm/ADT/StringRef.h
@@ -50,14 +50,14 @@ namespace llvm {
   /// general safe to store a StringRef.
   class LLVM_GSL_POINTER StringRef {
   public:
-    static constexpr size_t npos = ~size_t(0);
+    static constexpr size_t npos = std::string_view::npos;
 
-    using iterator = const char *;
-    using const_iterator = const char *;
-    using size_type = size_t;
-    using value_type = char;
-    using reverse_iterator = std::reverse_iterator<iterator>;
-    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+    using iterator = std::string_view::iterator;
+    using const_iterator = std::string_view::const_iterator;
+    using size_type = std::string_view::size_type;
+    using value_type = std::string_view::value_type;
+    using reverse_iterator = std::string_view::reverse_iterator;
+    using const_reverse_iterator = std::string_view::const_reverse_iterator;
 
   private:
     std::string_view View;
@@ -106,17 +106,10 @@ namespace llvm {
     /// @name Iterators
     /// @{
 
-    iterator begin() const { return data(); }
-
-    iterator end() const { return data() + size(); }
-
-    reverse_iterator rbegin() const {
-      return std::make_reverse_iterator(end());
-    }
-
-    reverse_iterator rend() const {
-      return std::make_reverse_iterator(begin());
-    }
+    iterator begin() const { return View.begin(); }
+    iterator end() const { return View.end(); }
+    reverse_iterator rbegin() const { return View.rbegin(); }
+    reverse_iterator rend() const { return View.rend(); }
 
     const unsigned char *bytes_begin() const {
       return reinterpret_cast<const unsigned char *>(begin());
@@ -137,7 +130,7 @@ namespace llvm {
     [[nodiscard]] constexpr const char *data() const { return View.data(); }
 
     /// empty - Check if the string is empty.
-    [[nodiscard]] constexpr bool empty() const { return size() == 0; }
+    [[nodiscard]] constexpr bool empty() const { return View.empty(); }
 
     /// size - Get the string size.
     [[nodiscard]] constexpr size_t size() const { return View.size(); }
@@ -145,13 +138,13 @@ namespace llvm {
     /// front - Get the first character in the string.
     [[nodiscard]] char front() const {
       assert(!empty());
-      return data()[0];
+      return View.front();
     }
 
     /// back - Get the last character in the string.
     [[nodiscard]] char back() const {
       assert(!empty());
-      return data()[size() - 1];
+      return View.back();
     }
 
     // copy - Allocate copy in Allocator and return StringRef to it.
@@ -222,7 +215,7 @@ namespace llvm {
     [[nodiscard]] std::string str() const {
       if (!data())
         return std::string();
-      return std::string(data(), size());
+      return std::string(View);
     }
 
     /// @}
@@ -246,9 +239,7 @@ namespace llvm {
     /// @name Type Conversions
     /// @{
 
-    constexpr operator std::string_view() const {
-      return std::string_view(data(), size());
-    }
+    constexpr operator std::string_view() const { return View; }
 
     /// @}
     /// @name String Predicates
@@ -288,7 +279,7 @@ namespace llvm {
     /// \returns The index of the first occurrence of \p C, or npos if not
     /// found.
     [[nodiscard]] size_t find(char C, size_t From = 0) const {
-      return std::string_view(*this).find(C, From);
+      return View.find(C, From);
     }
 
     /// Search for the first character \p C in the string, ignoring case.
diff --git a/llvm/lib/Support/StringRef.cpp b/llvm/lib/Support/StringRef.cpp
index 4f5fcb4857e805..ba45a625c0a8db 100644
--- a/llvm/lib/Support/StringRef.cpp
+++ b/llvm/lib/Support/StringRef.cpp
@@ -216,9 +216,7 @@ size_t StringRef::rfind_insensitive(char C, size_t From) const {
 ///
 /// \return - The index of the last occurrence of \arg Str, or npos if not
 /// found.
-size_t StringRef::rfind(StringRef Str) const {
-  return std::string_view(*this).rfind(Str);
-}
+size_t StringRef::rfind(StringRef Str) const { return View.rfind(Str); }
 
 size_t StringRef::rfind_insensitive(StringRef Str) const {
   size_t N = Str.size();
@@ -251,7 +249,7 @@ StringRef::size_type StringRef::find_first_of(StringRef Chars,
 /// find_first_not_of - Find the first character in the string that is not
 /// \arg C or npos if not found.
 StringRef::size_type StringRef::find_first_not_of(char C, size_t From) const {
-  return std::string_view(*this).find_first_not_of(C, From);
+  return View.find_first_not_of(C, From);
 }
 
 /// find_first_not_of - Find the first character in the string that is not

[MaskRay]

Seems that there is no conclusion on the approach. #113775 is reverted
https://discourse.llvm.org/t/migrating-llvm-stringref-to-std-string-view/82785

[dwblaikie]

Seems that there is no conclusion on the approach. #113775 is reverted https://discourse.llvm.org/t/migrating-llvm-stringref-to-std-string-view/82785

As mentioned in that thread, I think it's still good to converge the APIs, even if they never get deduplicated.