Add base::FeatureParam<> struct

base/metrics/field_trial_params.cc

 // Copyright 2017 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 "base/metrics/field_trial_params.h"
 
 #include "base/feature_list.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/field_trial_param_associator.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 100 matching lines @@
   if (!value_as_string.empty()) {
     DLOG(WARNING) << "Failed to parse field trial param " << param_name
                   << " with string value " << value_as_string
                   << " under feature " << feature.name
                   << " into a bool. Falling back to default value of "
                   << default_value;
   }
   return default_value;
 }
 
+std::string FeatureParam<std::string>::Get() const {
+  const std::string value = GetFieldTrialParamValueByFeature(*feature, name);
+  return value.empty() ? default_value : value;
+}
+
+double FeatureParam<double>::Get() const {
+  return GetFieldTrialParamByFeatureAsDouble(*feature, name, default_value);
+}
+
+int FeatureParam<int>::Get() const {

[Alexei Svitkine (slow), 2017/04/24 18:35:47]

What happens if someone tries to use a type that we don't have a Get() defined
for - e.g. uint32_t? Will it be a compile error? (If so, that's fine, but we
should document in the .h comment about which types are supported).

[sfiera, 2017/04/28 16:07:04]

Yes, a compile error.

+  return GetFieldTrialParamByFeatureAsInt(*feature, name, default_value);
+}
+
+bool FeatureParam<bool>::Get() const {
+  return GetFieldTrialParamByFeatureAsBool(*feature, name, default_value);
+}
+
+void LogInvalidEnumValue(const base::Feature& feature,
+                         const std::string& param_name,
+                         const std::string& value_as_string,
+                         int default_value_as_int) {
+  DLOG(WARNING) << "Failed to parse field trial param " << param_name
+                << " with string value " << value_as_string << " under feature "
+                << feature.name
+                << " into an enum. Falling back to default value of "
+                << default_value_as_int;
+}
+
 }  // namespace base