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

src/code_generators.cpp

@@ -0,0 +1,158 @@
+/*
+ * Copyright 2016 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "flatbuffers/code_generators.h"
+#include <assert.h>
+#include "flatbuffers/util.h"
+
+namespace flatbuffers {
+
+void CodeWriter::operator+=(std::string text) {
+
+  while (true) {
+    auto begin = text.find("{{");
+    if (begin == std::string::npos) {
+      break;
+    }
+
+    auto end = text.find("}}");
+    if (end == std::string::npos || end < begin) {
+      break;
+    }
+
+    // Write all the text before the first {{ into the stream.
+    stream_.write(text.c_str(), begin);
+
+    // The key is between the {{ and }}.
+    const std::string key = text.substr(begin + 2, end - begin - 2);
+
+    // Find the value associated with the key.  If it exists, write the
+    // value into the stream, otherwise write the key itself into the stream.
+    auto iter = value_map_.find(key);
+    if (iter != value_map_.end()) {
+      const std::string &value = iter->second;
+      stream_ << value;
+    } else {
+      assert(false && "could not find key");
+      stream_ << key;
+    }
+
+    // Update the text to everything after the }}.
+    text = text.substr(end + 2);
+  }
+  if (!text.empty() && text.back() == '\\') {
+    text.pop_back();
+    stream_ << text;
+  } else {
+    stream_ << text << std::endl;
+  }
+}
+
+const char *BaseGenerator::FlatBuffersGeneratedWarning() {
+  return "automatically generated by the FlatBuffers compiler,"
+         " do not modify\n\n";
+}
+
+std::string BaseGenerator::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;
+}
+
+std::string BaseGenerator::NamespaceDir(const Namespace &ns) const {
+  return BaseGenerator::NamespaceDir(parser_, path_, ns);
+}
+
+bool BaseGenerator::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;
+  }
+
+std::string BaseGenerator::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;
+}
+
+std::string BaseGenerator::LastNamespacePart(const Namespace &ns) {
+  if (!ns.components.empty())
+    return ns.components.back();
+  else
+    return std::string("");
+}
+
+// Ensure that a type is prefixed with its namespace whenever it is used
+// outside of its namespace.
+std::string BaseGenerator::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;
+}
+
+
+std::string BaseGenerator::WrapInNameSpace(const Definition &def) const {
+  return WrapInNameSpace(def.defined_namespace, def.name);
+}
+
+// Generate a documentation comment, if available.
+void GenComment(const std::vector<std::string> &dc, std::string *code_ptr,
+                const CommentConfig *config, const char *prefix) {
+  if (dc.begin() == dc.end()) {
+    // Don't output empty comment blocks with 0 lines of comment content.
+    return;
+  }
+
+  std::string &code = *code_ptr;
+  if (config != nullptr && config->first_line != nullptr) {
+    code += std::string(prefix) + std::string(config->first_line) + "\n";
+  }
+  std::string line_prefix = std::string(prefix) +
+      ((config != nullptr && config->content_line_prefix != nullptr) ?
+       config->content_line_prefix : "///");
+  for (auto it = dc.begin();
+       it != dc.end();
+       ++it) {
+    code += line_prefix + *it + "\n";
+  }
+  if (config != nullptr && config->last_line != nullptr) {
+    code += std::string(prefix) + std::string(config->last_line) + "\n";
+  }
+}
+
+}  // namespace flatbuffers