[PATCH] D26319: Disallow StringRef assignment from temporary std::strings.

[Jordan Rose via llvm-commits]

jordan_rose created this revision.
jordan_rose added a reviewer: dexonsmith.
jordan_rose added a subscriber: llvm-commits.
jordan_rose set the repository for this revision to rL LLVM.
jordan_rose added a dependency: D26317: Fix use-of-temporary with StringRef in code coverage.

Similar to https://reviews.llvm.org/D25446, this prevents accidentally referring to temporary storage that goes out of scope by the end of the statement:

  someStringRef = getStringByValue();
  someStringRef = (Twine("-") + otherString).str();

Note that once again the constructor still has this problem:

  StringRef someStringRef = getStringByValue();

because once again we occasionally rely on this in calls:

  takesStringRef(getStringByValue());
  takesStringRef(Twine("-") + otherString);

Still, it's a step, and it caught one real problem in Clang and one in Swift.

Depends on https://reviews.llvm.org/D26317


Repository:
  rL LLVM

https://reviews.llvm.org/D26319

Files:
  include/llvm/ADT/StringRef.h
  unittests/ADT/StringRefTest.cpp


Index: unittests/ADT/StringRefTest.cpp
===================================================================
--- unittests/ADT/StringRefTest.cpp
+++ unittests/ADT/StringRefTest.cpp
@@ -32,14 +32,50 @@
 
 }
 
+// Check that we can't accidentally assign a temporary std::string to a
+// StringRef. (Unfortunately we can't make use of the same thing with
+// constructors.)
+//
+// Disable this check under MSVC; even MSVC 2015 isn't consistent between
+// std::is_assignable and actually writing such an assignment.
+#if !defined(_MSC_VER)
+static_assert(
+    !std::is_assignable<StringRef, std::string>::value,
+    "Assigning from prvalue std::string");
+static_assert(
+    !std::is_assignable<StringRef, std::string &&>::value,
+    "Assigning from xvalue std::string");
+static_assert(
+    std::is_assignable<StringRef, std::string &>::value,
+    "Assigning from lvalue std::string");
+static_assert(
+    std::is_assignable<StringRef, const char *>::value,
+    "Assigning from prvalue C string");
+static_assert(
+    std::is_assignable<StringRef, const char * &&>::value,
+    "Assigning from xvalue C string");
+static_assert(
+    std::is_assignable<StringRef, const char * &>::value,
+    "Assigning from lvalue C string");
+#endif
+
+
 namespace {
 TEST(StringRefTest, Construction) {
   EXPECT_EQ("", StringRef());
   EXPECT_EQ("hello", StringRef("hello"));
   EXPECT_EQ("hello", StringRef("hello world", 5));
   EXPECT_EQ("hello", StringRef(std::string("hello")));
 }
 
+TEST(StringRefTest, EmptyInitializerList) {
+  StringRef S = {};
+  EXPECT_TRUE(S.empty());
+
+  S = {};
+  EXPECT_TRUE(S.empty());
+}
+
 TEST(StringRefTest, Iteration) {
   StringRef S("hello");
   const char *p = "hello";
Index: include/llvm/ADT/StringRef.h
===================================================================
--- include/llvm/ADT/StringRef.h
+++ include/llvm/ADT/StringRef.h
@@ -215,6 +215,15 @@
       return Data[Index];
     }
 
+    /// Disallow accidental assignment from a temporary std::string.
+    ///
+    /// The declaration here is extra complicated so that `stringRef = {}`
+    /// and `stringRef = "abc"` continue to select the move assignment operator.
+    template <typename T>
+    typename std::enable_if<std::is_same<T, std::string>::value,
+                            StringRef>::type &
+    operator=(T &&Str) = delete;
+
     /// @}
     /// @name Type Conversions
     /// @{


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