Mojo: more idiomatic C++ bindings

mojo/public/cpp/bindings/string.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MOJO_PUBLIC_CPP_BINDINGS_STRING_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_STRING_H_
+
+#include <assert.h>
+
+#include <string>
+
+#include "mojo/public/cpp/bindings/type_converter.h"
+#include "mojo/public/cpp/system/macros.h"
+
+namespace mojo {
+
+class String {

[yzshen1, 2014/05/27 22:16:59]

I think you mentioned that you would like to add operator=(const std::string&)?

Without it I think the following code still works, but does an extra copy,
right?
(construct a String object from a, and then do default assignment)
=======================
std::string a;
String mojo_a;
mojo_a = a;
=======================

(And it might make sense to add assignment operator for const char* / const
char[N], too.)

+ public:
+  String() : is_null_(true) {}
+  String(const std::string& str) : value_(str), is_null_(false) {}
+  String(const char* chars) : value_(chars), is_null_(false) {}
+  String(const char* chars, size_t num_chars)
+      : value_(chars, num_chars),
+        is_null_(false) {
+  }
+  template <size_t N>
+  String(const char chars[N]) : value_(chars, N-1), is_null_(false) {}
+
+  template <typename U>
+  static String From(const U& other) {
+    return TypeConverter<String, U>::ConvertFrom(other);
+  }
+
+  template <typename U>
+  U To() const {
+    return TypeConverter<String, U>::ConvertTo(*this);
+  }
+
+  void reset() {
+    value_.clear();
+    is_null_ = true;
+  }
+
+  bool is_null() const { return is_null_; }
+
+  size_t size() const { return value_.size(); }
+
+  const char* data() const { return value_.data(); }
+
+  const char& at(size_t offset) const { return value_.at(offset); }
+  const char& operator[](size_t offset) const { return value_[offset]; }
+
+  const std::string& get() const { return value_; }
+  operator const std::string&() const { return value_; }
+
+  void Swap(String* other) {
+    std::swap(is_null_, other->is_null_);
+    value_.swap(other->value_);
+  }
+
+  void Swap(std::string* other) {
+    is_null_ = false;
+    value_.swap(*other);
+  }
+
+ private:
+  typedef std::string String::*Testable;
+
+ public:
+  operator Testable() const { return is_null_ ? 0 : &String::value_; }
+
+ private:
+  std::string value_;
+  bool is_null_;
+};
+
+inline bool operator==(const String& a, const String& b) {
+  return a.is_null() == b.is_null() && a.get() == b.get();
+}
+inline bool operator==(const char* a, const String& b) {
+  return !b.is_null() && a == b.get();
+}
+inline bool operator==(const String& a, const char* b) {
+  return !a.is_null() && a.get() == b;
+}
+inline bool operator!=(const String& a, const String& b) { return !(a == b); }
+inline bool operator!=(const char* a, const String& b) { return !(a == b); }
+inline bool operator!=(const String& a, const char* b) { return !(a == b); }
+
+// TODO(darin): Add similar variants of operator<,<=,>,>=
+
+template <>
+class TypeConverter<String, std::string> {
+ public:
+  static String ConvertFrom(const std::string& input) {
+    return String(input);
+  }
+  static std::string ConvertTo(const String& input) {
+    return input;
+  }
+};
+
+template <size_t N>
+class TypeConverter<String, char[N]> {
+ public:
+  static String ConvertFrom(const char input[N]) {
+    assert(input);
+    return String(input, N-1);
+  }
+};
+
+// Appease MSVC.
+template <size_t N>
+class TypeConverter<String, const char[N]> {
+ public:
+  static String ConvertFrom(const char input[N]) {
+    assert(input);
+    return String(input, N-1);
+  }
+};
+
+template <>
+class TypeConverter<String, const char*> {
+ public:
+  static String ConvertFrom(const char* input) {
+    return input ? String(input) : String();

[yzshen1, 2014/05/27 22:16:59]

Do we want the null-handling behavior of String(const char* input) to be
different from String::From(const char* input)?
That seems a little confusing to me.

+  }
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_STRING_H_