file: protobuf/src/google/protobuf/compiler/cpp/cpp_message.cc

void MessageGenerator::GenerateClassDefinition(io::Printer* printer)
{
....
uses_string_ = false;
for (int i = 0; i < descriptors.size(); i++) {
const FieldDescriptor* field = descriptors[i];
if (field->cpp_type() == FieldDescriptor::CPPTYPE_STRING) {
switch (field->options().ctype()) {
default: uses_string_ = true; break;
}
}
}
....
}