我正在尝试实现一个简单的序列化器类,该类具有一个Serialize函数,该函数将实际的序列化调度到不同的重载函数模板,这些模板是在编译时使用带有decltype的expression-SFINAE选择的:
#ifndef SERIALIZER_H
#define SERIALIZER_H
#include <string>
#include <utility>
/**** base/primary template for partial/explicit specialization on serializable classes ****/
template <typename T>
struct SerializeHelper;
/**** base abstract serialize class ****/
class Serializer
{
public:
// main serializer member function template
template <typename T>
void Serialize(const std::string &name, const T &value) const;
private:
virtual void Prepare(const std::string &name) const = 0;
virtual void Finalize(const std::string &name) const = 0;
// natively supported types
template <typename T>
auto Serialize(const T &value) const -> decltype(SerializeNative(value), std::declval<void>());
// use static function in class template specialized for type (partial or explicit specialization)
template <typename T>
auto Serialize(const T &value) const -> decltype(SerializeHelper<T>::Serialize(*this, value), std::declval<void>());
// use serializable type interface
template <typename T>
auto Serialize(const T &value) const -> decltype(value.Serialize(*this), std::declval<void>());
private:
// virtual functions for natively supported types
virtual void SerializeNative(int value) const = 0;
virtual void SerializeNative(float value) const = 0;
virtual void SerializeNative(double value) const = 0;
virtual void SerializeNative(const std::string &value) const = 0;
protected:
Serializer() = default;
};
template <typename T>
void Serializer::Serialize(const std::string &name, const T &value) const
{
Prepare(name);
Serialize(value);
Finalize(name);
}
// natively supported types
template <typename T>
auto Serializer::Serialize(const T &value) const -> decltype(SerializeNative(value), std::declval<void>()) // COMPILER ERROR
{
SerializeNative(value);
}
// use serialize function specialized for type
template <typename T>
auto Serializer::Serialize(const T &value) const -> decltype(SerializeHelper<T>::Serialize(*this, value), std::declval<void>())
{
SerializeHelper<T>::Serialize(*this, value);
}
// use serializable type interface
template <typename T>
auto Serializer::Serialize(const T &value) const -> decltype(value.Serialize(*this), std::declval<void>())
{
value.Serialize(*this);
}
#endif // SERIALIZER_H
问题是,此代码无法编译,因为编译器抱怨本机类型的Serialize的类外定义在类内没有相应的声明:
In file included from main.cpp:1:
serializer.hpp:53:6: error: no declaration matches 'decltype ((((const Serializer*)this)->Serializer::SerializeNative(value), declval<void>())) Serializer::Serialize(const T&) const'
auto Serializer::Serialize(const T &value) const -> decltype(SerializeNative(value), std::declval<void>())
如果将内联定义放在类中,则可以正常编译。在GCC和VC ++中会发生这种情况。
编辑
如果我在成员函数模板声明之前声明SerializeNative成员函数,则代码运行良好,这似乎是因为对SerializeNative函数的调用位于Serialize函数头(以decltype表示)内,因此需要查看该声明。