Add CodeWriter utility class.

Helps simplify code generation code.  Instead of this:
  code += "inline const " + cpp_qualified_name + " *Get";
  code += name;
  code += "(const void *buf) {\n  return flatbuffers::GetRoot<";
  code += cpp_qualified_name + ">(buf);\n}\n\n";

You do this:
  code.SetValue("NAME", struct_def.name);
  code.SetValue("CPP_NAME", cpp_qualified_name);
  code += "inline const {{CPP_NAME}} *Get{{NAME}}(const void *buf) {";
  code += "  return flatbuffers::GetRoot<{{CPP_NAME}}>(buf);";
  code += "}";
  code += "";

Updated the CPP code generator to use the CodeWriter class.  Most of the
changes in the generated code are white-space changes, esp. around new
lines (since the code generator class automatically appends new lines
when appending a string).  Actual code changes include:

* Renamed "rehasher" to "_rehasher" for consistency with other args in
  Pack function.

* Renamed "union_obj" to "obj: in UnPack function.

* Always do "(void)_o;" to prevent unused variable warning in Create
  function (instead of only doing it if there are no fields) in order
  to avoid two-passes.

* Renamed padding variables from __paddingX to paddingX__.
  "Each name that contains a double underscore (_ _) [...] is reserved
   to the implementation for any use."  C++ standards 17.4.3.1.2.

* Add braces around switch cases.

* Calculate index as a separate statement in EnumName function, eg.
    const size_t index = ...;
    return EnumNamesX()[index];
  vs.
    return EnumNamesX()[...];

* Stored end table offset in variable in Finish() functions, eg.
    const auto end = fbb_.EndTable(start_, ...);
    auto o = flatbuffers::Offset<T>(end);
  vs.
    auto o = flatbuffers::Offset<T>(fbb_.EndTable(start, ...));

* Separate reinterpret_cast calls from function calls in Union
  functions, eg.
    auto ptr = reinterpret_cast<const T *>(obj);
    return ptr->UnPack(resolver);
  vs.
    return reinterpret_cast<const T *>(obj)->UnPack(resolver);

* Removed unecessary (void)(padding__X) no-ops from constructors, eg.
    Test(int16_t a, int8_t b) : ... {
      (void)__padding0;  // <-- Removed this line.
    }

In the idl_gen_cpp.cpp file itself, I refactored some code generation into
new functions: GenParam, GenNativeTable, GenVerifyCall, GenBuilders,
GenUnpackFieldStatement, and GenCreateParam.

Change-Id: I727b1bd8719d05b7ce33cbce00eb58fda817b25d

include/flatbuffers/code_generators.h

@@ -17,25 +17,63 @@
 #ifndef FLATBUFFERS_CODE_GENERATORS_H_
 #define FLATBUFFERS_CODE_GENERATORS_H_
 
+#include <map>
+#include <sstream>
+#include "flatbuffers/idl.h"
+
 namespace flatbuffers {
 
+// Utility class to assist in generating code through use of text templates.
+//
+// Example code:
+//   CodeWriter code;
+//   code.SetValue("NAME", "Foo");
+//   code += "void {{NAME}}() { printf("%s", "{{NAME}}"); }";
+//   code.SetValue("NAME", "Bar");
+//   code += "void {{NAME}}() { printf("%s", "{{NAME}}"); }";
+//   std::cout << code.ToString() << std::endl;
+//
+// Output:
+//  void Foo() { printf("%s", "Foo"); }
+//  void Bar() { printf("%s", "Bar"); }
+class CodeWriter {
+ public:
+  CodeWriter() {}
+
+  // Clears the current "written" code.
+  void Clear() {
+    stream_.str("");
+    stream_.clear();
+  }
+
+  // Associates a key with a value.  All subsequent calls to operator+=, where
+  // the specified key is contained in {{ and }} delimiters will be replaced by
+  // the given value.
+  void SetValue(const std::string& key, const std::string& value) {
+    value_map_[key] = value;
+  }
+
+  // Appends the given text to the generated code as well as a newline
+  // character.  Any text within {{ and }} delimeters is replaced by values
+  // previously stored in the CodeWriter by calling SetValue above.  The newline
+  // will be suppressed if the text ends with the \\ character.
+  void operator+=(std::string text);
+
+  // Returns the current contents of the CodeWriter as a std::string.
+  std::string ToString() const { return stream_.str(); }
+
+ private:
+  std::map<std::string, std::string> value_map_;
+  std::stringstream stream_;
+};
+
 class BaseGenerator {
  public:
   virtual bool generate() = 0;
 
   static std::string NamespaceDir(const Parser &parser,
                                   const std::string &path,
-                                  const Namespace &ns) {
-    EnsureDirExists(path.c_str());
-    if (parser.opts.one_file) return path;
-    std::string namespace_dir = path;  // Either empty or ends in separator.
-    auto &namespaces = ns.components;
-    for (auto it = namespaces.begin(); it != namespaces.end(); ++it) {
-      namespace_dir += *it + kPathSeparator;
-      EnsureDirExists(namespace_dir.c_str());
-    }
-    return namespace_dir;
-  }
+                                  const Namespace &ns);
 
  protected:
   BaseGenerator(const Parser &parser, const std::string &path,
@@ -53,43 +91,15 @@ class BaseGenerator {
   BaseGenerator &operator=(const BaseGenerator &);
   BaseGenerator(const BaseGenerator &);
 
-  std::string NamespaceDir(const Namespace &ns) const {
-    return BaseGenerator::NamespaceDir(parser_, path_, ns);
-  }
+  std::string NamespaceDir(const Namespace &ns) const;
 
-  const char *FlatBuffersGeneratedWarning() {
-    return "automatically generated by the FlatBuffers compiler,"
-           " do not modify\n\n";
-  }
+  static const char *FlatBuffersGeneratedWarning();
 
-  bool IsEverythingGenerated() const {
-    for (auto it = parser_.enums_.vec.begin(); it != parser_.enums_.vec.end();
-         ++it) {
-      if (!(*it)->generated) return false;
-    }
-    for (auto it = parser_.structs_.vec.begin();
-         it != parser_.structs_.vec.end(); ++it) {
-      if (!(*it)->generated) return false;
-    }
-    return true;
-  }
+  bool IsEverythingGenerated() const;
 
-  static std::string FullNamespace(const char *separator, const Namespace &ns) {
-    std::string namespace_name;
-    auto &namespaces = ns.components;
-    for (auto it = namespaces.begin(); it != namespaces.end(); ++it) {
-      if (namespace_name.length()) namespace_name += separator;
-      namespace_name += *it;
-    }
-    return namespace_name;
-  }
+  static std::string FullNamespace(const char *separator, const Namespace &ns);
 
-  static std::string LastNamespacePart(const Namespace &ns) {
-    if (!ns.components.empty())
-      return ns.components.back();
-    else
-      return std::string("");
-  }
+  static std::string LastNamespacePart(const Namespace &ns);
 
   // tracks the current namespace for early exit in WrapInNameSpace
   // c++, java and csharp returns a different namespace from
@@ -100,17 +110,9 @@ class BaseGenerator {
   // Ensure that a type is prefixed with its namespace whenever it is used
   // outside of its namespace.
   std::string WrapInNameSpace(const Namespace *ns,
-                              const std::string &name) const {
-    if (CurrentNameSpace() == ns) return name;
-    std::string qualified_name = qualifying_start_;
-    for (auto it = ns->components.begin(); it != ns->components.end(); ++it)
-      qualified_name += *it + qualifying_separator_;
-    return qualified_name + name;
-  }
+                              const std::string &name) const;
 
-  std::string WrapInNameSpace(const Definition &def) const {
-    return WrapInNameSpace(def.defined_namespace, def.name);
-  }
+  std::string WrapInNameSpace(const Definition &def) const;
 
   const Parser &parser_;
   const std::string &path_;
@@ -119,6 +121,17 @@ class BaseGenerator {
   const std::string qualifying_separator_;
 };
 
+struct CommentConfig {
+  const char *first_line;
+  const char *content_line_prefix;
+  const char *last_line;
+};
+
+extern void GenComment(const std::vector<std::string> &dc,
+                       std::string *code_ptr,
+                       const CommentConfig *config,
+                       const char *prefix = "");
+
 }  // namespace flatbuffers
 
 #endif  // FLATBUFFERS_CODE_GENERATORS_H_