Add base::FeatureParam<> struct

base/metrics/field_trial_params.cc

Index: base/metrics/field_trial_params.cc
diff --git a/base/metrics/field_trial_params.cc b/base/metrics/field_trial_params.cc
index bcefa8944fb7bd79aca79d90b916bcd0e796e3f2..7195f4a813e18611fb5bca23f2e3c6e79fa8c24c 100644
--- a/base/metrics/field_trial_params.cc
+++ b/base/metrics/field_trial_params.cc
@@ -118,4 +118,32 @@ bool GetFieldTrialParamByFeatureAsBool(const base::Feature& feature,
   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