Issue 6378 - [emscripten] Make DependentString constexpr

compiled/String.h

Index: compiled/String.h
diff --git a/compiled/String.h b/compiled/String.h
index 70086f847b913460c906d285ff85c9f8013f8a67..de3d2021ef32fafccc70e2b5c05d8421c417bb2b 100644
--- a/compiled/String.h
+++ b/compiled/String.h
@@ -76,14 +76,12 @@ protected:
   value_type* mBuf;
   size_type mLen;
 
-  explicit String(value_type* buf, size_type len, size_type flags)
+  constexpr explicit String(value_type* buf, size_type len, size_type flags)
       : mBuf(buf), mLen((len & LENGTH_MASK) | flags)
   {
   }
 
-  ~String()
-  {
-  }
+  ~String() = default;

[Eric, 2018/03/15 17:05:00]

We're not defining copy or move constructors, so there doesn't seem to be any
need to define a default destructor. Omitting this would make the destructor
public, but there's no downside to that.

[René Jeschke, 2018/03/15 17:41:23]

What was the reason to make the dtor protected in the beginning?

[Eric, 2018/03/15 18:33:54]

Don't know. If there's a good reason, it deserves a one-line comment. Sergei?
Hubert?

[sergei, 2018/03/16 13:43:26]

Such class hierarchy caused questions from the beginning but we agreed to go
with it for the sake of convenience and performance. However it's not supposed
that an instance of `String` can live without either `DependentString` or
`OwnedString`, so having of the protected destructor ensures that no one can
forget to call the destructor of `OwnedString`.

 
   void reset(value_type* buf, size_type len, size_type flags)
   {
@@ -92,17 +90,17 @@ protected:
   }
 
 public:
-  size_type length() const
+  constexpr size_type length() const

[Eric, 2018/03/15 17:05:00]

constexpr implies const, so the trailing declaration can be dropped here and in
several places below.

[René Jeschke, 2018/03/15 17:41:23]

No, it does not. 'constexpr' on an object declaration implies 'const', not on a
function declaration.

[Eric, 2018/03/15 18:33:54]

Bah. Misread the spec. Never mind.

   {
     return mLen & LENGTH_MASK;
   }
 
-  bool empty() const
+  constexpr bool empty() const
   {
     return !(mLen & LENGTH_MASK);
   }
 
-  const value_type* data() const
+  constexpr const value_type* data() const
   {
     return mBuf;
   }
@@ -113,7 +111,7 @@ public:
     return mBuf;
   }
 
-  const value_type& operator[](size_type pos) const
+  constexpr const value_type& operator[](size_type pos) const
   {
     return mBuf[pos];
   }
@@ -124,7 +122,7 @@ public:
     return mBuf[pos];
   }
 
-  bool is_writable() const
+  constexpr bool is_writable() const
   {
     return (mLen & FLAGS_MASK) == READ_WRITE;
   }
@@ -194,12 +192,12 @@ public:
     return npos;
   }
 
-  bool is_invalid() const
+  constexpr bool is_invalid() const
   {
     return (mLen & FLAGS_MASK) == INVALID;
   }
 
-  bool is_deleted() const
+  constexpr bool is_deleted() const
   {
     return (mLen & FLAGS_MASK) == DELETED;
   }
@@ -245,17 +243,23 @@ inline std::ostream& operator<<(std::ostream& os, const String& str)
 class DependentString : public String
 {
 public:
-  explicit DependentString()
+  constexpr explicit DependentString()
       : String(nullptr, 0, INVALID)
   {
   }
 
+  template <int N1>
+  constexpr explicit DependentString(const value_type (&buf)[N1])
+      : String(const_cast<value_type*>(buf), N1 - 1, READ_ONLY)
+  {
+  }
+
   explicit DependentString(value_type* buf, size_type len)
       : String(buf, len, READ_WRITE)
   {
   }
 
-  explicit DependentString(const value_type* buf, size_type len)
+  constexpr explicit DependentString(const value_type* buf, size_type len)
       : String(const_cast<value_type*>(buf), len, READ_ONLY)
   {
   }
@@ -313,7 +317,7 @@ inline std::ostream& operator<<(std::ostream& os, const DependentString& str)
 }
 #endif
 
-inline DependentString operator "" _str(const String::value_type* str,
+inline constexpr DependentString operator "" _str(const String::value_type* str,

[Eric, 2018/03/15 17:05:01]

constexpr on functions implies inline, so it could be dropped.

[René Jeschke, 2018/03/15 17:41:23]

Right. Done.

     String::size_type len)
 {
   return DependentString(str, len);