Added StringPiece overloads for base::JoinString.

base/strings/string_util.h

Index: base/strings/string_util.h
diff --git a/base/strings/string_util.h b/base/strings/string_util.h
index e5cce8fd97e56f93e0717250b2b4bb60851c4806..2916848a270b3b7d7dbe15afcbf805c491d5fe2f 100644
--- a/base/strings/string_util.h
+++ b/base/strings/string_util.h
@@ -429,11 +429,28 @@ BASE_EXPORT char16* WriteInto(string16* str, size_t length_with_null);
 BASE_EXPORT wchar_t* WriteInto(std::wstring* str, size_t length_with_null);
 #endif
 
-// Does the opposite of SplitString().
+// Does the opposite of SplitString()/SplitStringPiece(). Joins a vector or list
+// of strings into a single string, inserting |separator| (which may be empty)
+// in between all elements.
+//
+// Prefer the StringPiece variant if possible, to avoid unnecessary copying.

[dcheng, 2017/02/22 07:22:18]

I don't think it prevents copies though? It all depends on what you already have
as your input type: if you already have std::string objects, converting to
StringPiece won't help.

[danakj, 2017/02/22 15:10:46]

Right, the setup code to make the vector may have less copies, but that's not
related to this API per se.

[Matt Giuca, 2017/02/23 04:48:06]

Right, JoinString doesn't create any more/less copies based on which overload.

The point I'm trying to get across is that the caller should be structured to
take advantage of StringPiece where possible, so you avoid copying in the caller
before calling JoinString. I rewrote this to be a bit clearer about that, but
it's also quite verbose. Do you think I should say this? Happy to just delete
the paragraph also.

 BASE_EXPORT std::string JoinString(const std::vector<std::string>& parts,
                                    StringPiece separator);
 BASE_EXPORT string16 JoinString(const std::vector<string16>& parts,
                                 StringPiece16 separator);
+BASE_EXPORT std::string JoinString(const std::vector<StringPiece>& parts,
+                                   StringPiece separator);
+BASE_EXPORT string16 JoinString(const std::vector<StringPiece16>& parts,
+                                StringPiece16 separator);
+// Explicit initializer_list overloads are required to break ambiguity when used
+// with a literal initializer list (otherwise the compiler would not be able to
+// decide between the two other overloads).
+BASE_EXPORT std::string JoinString(
+    const std::initializer_list<StringPiece>& parts,

[dcheng, 2017/02/22 07:22:18]

FWIW, it seems much more common to pass by value (as this is just a pointer +
size object, like StringPiece)

[Matt Giuca, 2017/02/23 04:48:06]

Done.

+    StringPiece separator);
+BASE_EXPORT string16
+JoinString(const std::initializer_list<StringPiece16>& parts,
+           StringPiece16 separator);
 
 // Replace $1-$2-$3..$9 in the format string with values from |subst|.
 // Additionally, any number of consecutive '$' characters is replaced by that