style: cleanup use of noexcept, explicit and = default (googlea…

…pis/google-cloud-cpp-spanner#1209)

Our current rule-of-thumb is to only specify `noexcept` on hand-written
move constructors and assignments.  So, remove two current specifications
on defaulted move operators, and one on a normal constructor.  Fixes googleapis/google-cloud-cpp-spanner#228.

Reserve `explicit` for single-argument constructors.  Some ctor-arity
has changed over time, so some `explicit` annotations were missing or
outdated.

Also rewrite any `T() {}` constructor as `T() = default`.

google/cloud/spanner/bytes.h

@@ -44,7 +44,7 @@ std::string BytesToBase64(Bytes b);
 class Bytes {
  public:
   /// An empty sequence.
-  Bytes() {}
+  Bytes() = default;
 
   /// Construction from a sequence of octets.
   ///@{
@@ -82,7 +82,7 @@ class Bytes {
   friend std::string internal::BytesToBase64(Bytes b);
 
   struct Encoder {
-    Encoder(std::string& rep) : rep_(rep), len_(0) {}
+    explicit Encoder(std::string& rep) : rep_(rep), len_(0) {}
     void Flush();
     void FlushAndPad();
 
@@ -135,7 +135,7 @@ class Bytes {
       std::array<value_type, 1 + 3> buf_;
     };
 
-    Decoder(std::string const& rep) : rep_(rep) {}
+    explicit Decoder(std::string const& rep) : rep_(rep) {}
     iterator begin() { return iterator(rep_.begin(), rep_.end()); }
     iterator end() { return iterator(rep_.end(), rep_.end()); }
 

google/cloud/spanner/create_instance_request_builder.h

@@ -49,8 +49,7 @@ class CreateInstanceRequestBuilder {
    * Constructor requires Instance and Cloud Spanner instance config name. It
    * sets node_count = 1, and display_name = instance_id as the default values.
    */
-  explicit CreateInstanceRequestBuilder(Instance const& in,
-                                        std::string config) {
+  CreateInstanceRequestBuilder(Instance const& in, std::string config) {
     request_.set_parent("projects/" + in.project_id());
     request_.set_instance_id(in.instance_id());
     request_.mutable_instance()->set_name(in.FullName());

google/cloud/spanner/internal/database_admin_logging.h

@@ -30,8 +30,8 @@ namespace internal {
  */
 class DatabaseAdminLogging : public DatabaseAdminStub {
  public:
-  explicit DatabaseAdminLogging(std::shared_ptr<DatabaseAdminStub> child,
-                                TracingOptions tracing_options)
+  DatabaseAdminLogging(std::shared_ptr<DatabaseAdminStub> child,
+                       TracingOptions tracing_options)
       : child_(std::move(child)),
         tracing_options_(std::move(tracing_options)) {}
 

google/cloud/spanner/internal/instance_admin_logging.h

@@ -32,8 +32,8 @@ namespace gcsa = ::google::spanner::admin::instance::v1;
  */
 class InstanceAdminLogging : public InstanceAdminStub {
  public:
-  explicit InstanceAdminLogging(std::shared_ptr<InstanceAdminStub> child,
-                                TracingOptions tracing_options)
+  InstanceAdminLogging(std::shared_ptr<InstanceAdminStub> child,
+                       TracingOptions tracing_options)
       : child_(std::move(child)),
         tracing_options_(std::move(tracing_options)) {}
 

google/cloud/spanner/internal/logging_result_set_reader.h

@@ -27,8 +27,8 @@ namespace internal {
 
 class LoggingResultSetReader : public PartialResultSetReader {
  public:
-  explicit LoggingResultSetReader(std::unique_ptr<PartialResultSetReader> impl,
-                                  TracingOptions tracing_options)
+  LoggingResultSetReader(std::unique_ptr<PartialResultSetReader> impl,
+                         TracingOptions tracing_options)
       : impl_(std::move(impl)), tracing_options_(std::move(tracing_options)) {}
   ~LoggingResultSetReader() override = default;
 

google/cloud/spanner/internal/logging_spanner_stub.h

@@ -30,8 +30,8 @@ namespace internal {
  */
 class LoggingSpannerStub : public SpannerStub {
  public:
-  explicit LoggingSpannerStub(std::shared_ptr<SpannerStub> child,
-                              TracingOptions tracing_options)
+  LoggingSpannerStub(std::shared_ptr<SpannerStub> child,
+                     TracingOptions tracing_options)
       : child_(std::move(child)),
         tracing_options_(std::move(tracing_options)) {}
   ~LoggingSpannerStub() override = default;

google/cloud/spanner/internal/range_from_pagination.h

@@ -98,7 +98,7 @@ class PaginationIterator {
     return std::rel_ops::operator!=(lhs, rhs);
   }
 
-  explicit PaginationIterator(Range* owner, value_type value)
+  PaginationIterator(Range* owner, value_type value)
       : owner_(owner), value_(std::move(value)) {}
 
   Range* owner_;
@@ -108,10 +108,9 @@ class PaginationIterator {
 template <typename T, typename Request, typename Response>
 class PaginationRange {
  public:
-  explicit PaginationRange(
-      Request request,
-      std::function<StatusOr<Response>(Request const& r)> loader,
-      std::function<std::vector<T>(Response r)> get_items)
+  PaginationRange(Request request,
+                  std::function<StatusOr<Response>(Request const& r)> loader,
+                  std::function<std::vector<T>(Response r)> get_items)
       : request_(std::move(request)),
         next_page_loader_(std::move(loader)),
         get_items_(std::move(get_items)),

google/cloud/spanner/internal/session.h

@@ -35,7 +35,7 @@ namespace internal {
  */
 class Session {
  public:
-  Session(std::string session_name, std::shared_ptr<SpannerStub> stub) noexcept
+  Session(std::string session_name, std::shared_ptr<SpannerStub> stub)
       : session_name_(std::move(session_name)),
         stub_(std::move(stub)),
         is_bad_(false) {}

google/cloud/spanner/internal/transaction_impl.h

@@ -41,7 +41,7 @@ using VisitInvokeResult = ::google::cloud::internal::invoke_result_t<
  */
 class TransactionImpl {
  public:
-  TransactionImpl(google::spanner::v1::TransactionSelector selector)
+  explicit TransactionImpl(google::spanner::v1::TransactionSelector selector)
       : TransactionImpl(/*session=*/{}, std::move(selector)) {}
 
   TransactionImpl(TransactionImpl const& impl,

google/cloud/spanner/mutations.h

@@ -54,8 +54,8 @@ class Mutation {
    */
   Mutation() = default;
 
-  Mutation(Mutation&&) noexcept = default;
-  Mutation& operator=(Mutation&&) noexcept = default;
+  Mutation(Mutation&&) = default;
+  Mutation& operator=(Mutation&&) = default;
   Mutation(Mutation const&) = default;
   Mutation& operator=(Mutation const&) = default;
 

google/cloud/spanner/query_partition.h

@@ -128,9 +128,8 @@ class QueryPartition {
   friend StatusOr<QueryPartition> DeserializeQueryPartition(
       std::string const& serialized_query_partition);
 
-  explicit QueryPartition(std::string transaction_id, std::string session_id,
-                          std::string partition_token,
-                          SqlStatement sql_statement);
+  QueryPartition(std::string transaction_id, std::string session_id,
+                 std::string partition_token, SqlStatement sql_statement);
 
   // Accessor methods for use by friends.
   std::string const& partition_token() const { return partition_token_; }

google/cloud/spanner/row.h

@@ -187,8 +187,8 @@ class Row {
    * @note columns must not be nullptr
    * @note columns.size() must equal values.size()
    */
-  explicit Row(std::vector<Value> values,
-               std::shared_ptr<const std::vector<std::string>> columns);
+  Row(std::vector<Value> values,
+      std::shared_ptr<const std::vector<std::string>> columns);
 
   std::vector<Value> values_;
   std::shared_ptr<const std::vector<std::string>> columns_;

google/cloud/spanner/value.h

@@ -588,10 +588,10 @@ class Value {
   // this constructor matched their arguments best.
   struct PrivateConstructor {};
   template <typename T>
-  explicit Value(PrivateConstructor, T&& t)
+  Value(PrivateConstructor, T&& t)
       : type_(MakeTypeProto(t)), value_(MakeValueProto(std::forward<T>(t))) {}
 
-  explicit Value(google::spanner::v1::Type t, google::protobuf::Value v)
+  Value(google::spanner::v1::Type t, google::protobuf::Value v)
       : type_(std::move(t)), value_(std::move(v)) {}
 
   friend Value internal::FromProto(google::spanner::v1::Type,