Refactor der::Input helper methods into new constructors

net/der/input.h

Index: net/der/input.h
diff --git a/net/der/input.h b/net/der/input.h
index 3772d0df927f9b93b2587802ebbddf0dfcfa870d..695e153e80bb656fba556c9883a1bab60bab42a1 100644
--- a/net/der/input.h
+++ b/net/der/input.h
@@ -11,6 +11,7 @@
 #include <string>
 
 #include "base/compiler_specific.h"
+#include "base/strings/string_piece.h"
 #include "net/base/net_export.h"
 
 namespace net {
@@ -51,6 +52,14 @@ class NET_EXPORT_PRIVATE Input {
   // Creates an Input from the given |data| and |len|.
   Input(const uint8_t* data, size_t len);
 
+  // Creates an Input from a base::StringPiece.
+  Input(const base::StringPiece& sp);

[eroman, 2016/01/13 22:54:00]

Mark as explicit

[nharper, 2016/01/13 23:04:47]

Done.

+
+  // Creates an Input from a std::string. The lifetimes are a bit subtle when
+  // using this function: The constructed Input is only valid so long as |s| is
+  // still alive and not mutated.
+  Input(const std::string* s);

[eroman, 2016/01/13 22:54:00]

explicit

[nharper, 2016/01/13 23:04:48]

Done.

+
   // Returns the length in bytes of an Input's data.
   size_t Length() const { return len_; }
 
@@ -66,7 +75,17 @@ class NET_EXPORT_PRIVATE Input {
   // Returns a copy of the data represented by this object as a std::string.
   std::string AsString() const;
 
+  // Returns a StringPiece pointing to the same data as the Input. The resulting
+  // StringPiece cannot outlive the data that was used to construct this Input.

[eroman, 2016/01/13 22:54:00]

nit: cannot --> must not

[nharper, 2016/01/13 23:04:48]

Done.

+  base::StringPiece AsStringPiece() const;
+
  private:
+  // This constructor is deleted to prevent constructing an Input from a
+  // std::string r-value. Since the Input points to memory owned by another
+  // object, such an Input would point to invalid memory. Without this deleted
+  // constructor, a std::string could be passed in to the base::StringPiece
+  // constructor because of StringPiece's implicit constructor.
+  Input(std::string) = delete;

[eroman, 2016/01/13 22:54:00]

Does this need to be const std::string& ?

[nharper, 2016/01/13 23:04:47]

I tested the following, and they all failed to compile:
Input test("test");
---
std::string t = "test";
Input test(t);
---
std::string TestString() { return "hello"; }
Input test(TestString())

What sort of things should I try that might slip past Input(std::string) but not
Input(const std::string&) ?

[eroman, 2016/01/13 23:21:06]

Sounds good. My only other test suggestions would be around the fact that it is
implicit ctor rather than explicit, but I don't anticipate problems there.

   const uint8_t* data_;
   size_t len_;
 };