Allow GN configs to have sub-configs.

tools/gn/config_values.cc

 // Copyright (c) 2013 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.
 
 #include "tools/gn/config_values.h"
 
+namespace {
+
+template<typename T>
+void VectorAppend(std::vector<T>* append_to,
+                  const std::vector<T>& append_this) {
+  if (append_this.empty())
+    return;
+  append_to->insert(append_to->end(),append_this.begin(), append_this.end());
+}
+
+}  // namespace
+
 ConfigValues::ConfigValues() {
 }
 
 ConfigValues::~ConfigValues() {
 }
+
+void ConfigValues::AppendValues(const ConfigValues& append) {
+  #define APPEND_VALUES(variable) VectorAppend(&variable, append.variable)
+    APPEND_VALUES(cflags_);
+    APPEND_VALUES(cflags_c_);
+    APPEND_VALUES(cflags_cc_);
+    APPEND_VALUES(cflags_objcc_);
+    APPEND_VALUES(defines_);
+    APPEND_VALUES(include_dirs_);
+    APPEND_VALUES(ldflags_);
+    APPEND_VALUES(lib_dirs_);
+    APPEND_VALUES(libs_);
+  #undef APPEND_VALUES

[Dirk Pranke, 2015/09/15 21:31:31]

I'm not sure the macro enhances readability much here.

+
+  // Only append precompiled header if there isn't one. It might be nice to
+  // throw an error if there are conflicting precompiled headers, but that
+  // requires piping through some context of the actual configs involved, and
+  // conflicts here should be very unusual. Instead, use the first value.
+  if (!append.precompiled_header_.empty() && !precompiled_header_.empty())
+    precompiled_header_ = append.precompiled_header_;
+  if (!append.precompiled_source_.is_null() && !precompiled_source_.is_null())
+    precompiled_source_ = append.precompiled_source_;
+}