来自函数模板的显式模板特化不起作用

Question

我正在尝试对从另一个模板函数调用的模板函数进行显式特化。以下是一个最小的非工作示例，我正在尝试实现以下想法：

CInt、CDouble 和 CStr 相当于我需要执行的操作。但是，CStr 构造函数需要一些不同的格式。 MyClass 相当于一个工厂，当被请求时，它将返回 CInt、CDouble 或 CStr 的实例之一。

这种结构的动机：假设 GetCClass 函数是从大约 100 行的函数中调用的，只有一个区别：类的类型。从GetCClass 返回的值具有相同的 API。

#include <iostream>
#include <memory>
#include <string>

using namespace std;

class CStrArg {
 public:
  const char* a;
  int size;
};

class MyClass {
 public:
  class CStr;
  class CInt;
  class CDouble;

  template <typename T>
  typename T::Ptr GetCClass(typename T::ArgType arg);

  template <typename T>
  typename T::Ptr GetCClassInternal(typename T::ArgType arg);
};

class MyClass::CInt {
 public:
  typedef int ArgType;
  typedef shared_ptr<CInt> Ptr;

  static Ptr CreatePtr(ArgType i) { return Ptr(new CInt(i)); }

 private:
  CInt(ArgType i) : i_(i) {}
  ArgType i_;
};

class MyClass::CDouble {
 public:
  typedef double ArgType;
  typedef shared_ptr<CDouble> Ptr;

  static Ptr CreatePtr(ArgType d) { return Ptr(new CDouble(d)); }

 private:
  CDouble(ArgType i) : i_(i) {}
  ArgType i_;
};

class MyClass::CStr {
 public:
  typedef CStrArg ArgType;
  typedef shared_ptr<CStr> Ptr;

  static Ptr CreatePtr(string s) { return Ptr(new CStr(s)); }

 private:
  CStr(string i) : i_(i) {}
  string i_;
};


//template definition
template <typename T>
typename T::Ptr MyClass::GetCClass(typename T::ArgType arg) {
  return GetCClassInternal(arg);
}

template <typename T>
typename T::Ptr MyClass::GetCClassInternal(typename T::ArgType arg) {
  cout << "GetCClass for all types but one" << endl;
  return T::CreatePtr(arg);
}

template <>
MyClass::CStr::Ptr MyClass::GetCClassInternal<MyClass::CStr>(CStrArg arg) {
  return CStr::CreatePtr(arg.a);
}

int main() {
  MyClass test;
  int i = 5;
  double d = 1.2;
  CStrArg s;
  s.a = "why me";
  s.size = 6;

  auto iptr = test.GetCClass(i);
  auto dptr = test.GetCClass(d);
  auto sptr = test.GetCClass(s);

  return 0;
}

我收到以下错误：

experimental/amandeep/proto_test/fn_template_sp.cc:88:31: note: candidate is:
experimental/amandeep/proto_test/fn_template_sp.cc:20:19: note: template<class T> typename T::Ptr MyClass::GetCClass(typename T::ArgType)
   typename T::Ptr GetCClass(typename T::ArgType arg);
                   ^
experimental/amandeep/proto_test/fn_template_sp.cc:20:19: note:   template argument deduction/substitution failed:
experimental/amandeep/proto_test/fn_template_sp.cc:88:31: note:   couldn't deduce template parameter ‘T’
   auto iptr = test.GetCClass(i);
                               ^
experimental/amandeep/proto_test/fn_template_sp.cc:89:31: error: no matching function for call to ‘MyClass::GetCClass(double&)’
   auto dptr = test.GetCClass(d);
                               ^
experimental/amandeep/proto_test/fn_template_sp.cc:89:31: note: candidate is:
experimental/amandeep/proto_test/fn_template_sp.cc:20:19: note: template<class T> typename T::Ptr MyClass::GetCClass(typename T::ArgType)
   typename T::Ptr GetCClass(typename T::ArgType arg);
                   ^
experimental/amandeep/proto_test/fn_template_sp.cc:20:19: note:   template argument deduction/substitution failed:
experimental/amandeep/proto_test/fn_template_sp.cc:89:31: note:   couldn't deduce template parameter ‘T’
   auto dptr = test.GetCClass(d);
                               ^
experimental/amandeep/proto_test/fn_template_sp.cc:90:31: error: no matching function for call to ‘MyClass::GetCClass(CStrArg&)’
   auto sptr = test.GetCClass(s);
                               ^
experimental/amandeep/proto_test/fn_template_sp.cc:90:31: note: candidate is:
experimental/amandeep/proto_test/fn_template_sp.cc:20:19: note: template<class T> typename T::Ptr MyClass::GetCClass(typename T::ArgType)
   typename T::Ptr GetCClass(typename T::ArgType arg);
                   ^
experimental/amandeep/proto_test/fn_template_sp.cc:20:19: note:   template argument deduction/substitution failed:
experimental/amandeep/proto_test/fn_template_sp.cc:90:31: note:   couldn't deduce template parameter ‘T’
   auto sptr = test.GetCClass(s);

我已阅读多个答案，但我不明白为什么这不起作用。任何帮助表示赞赏。

编辑： 我无法理解，但在我的实际代码中，我在本地得到以下信息：

/home/workspace/main/util/storage/smb2_proxy/smb2_proxy.cc:239:29: error: template-id ‘CreateOp<storage::smb2_proxy::Smb2Proxy::PurgeTaskOp>’ for ‘storage::smb2_proxy::Smb2Proxy::PurgeTaskOp::Ptr storage::smb2_proxy::Smb2Proxy::CreateOp(std::shared_ptr<storage::smb2_proxy::Smb2Proxy::TaskState>,storage::smb2_proxy::Smb2Proxy::PurgeTaskOp::ArgType&,storage::smb2_proxy::Smb2Proxy::PurgeTaskOp::ResultType*,storage::smb2_proxy::Smb2Proxy::DoneCb)’ does not match any template declaration
 Smb2Proxy::PurgeTaskOp::Ptr Smb2Proxy::CreateOp<Smb2Proxy::PurgeTaskOp>(
                             ^~~~~~~~~
In file included from /home/workspace/main/util/storage/smb2_proxy/smb2_proxy.cc:5:0:
/home/workspace/main/util/storage/smb2_proxy/smb2_proxy.h:160:20: note: candidate is: template<class Op> typename Op::Ptr storage::smb2_proxy::Smb2Proxy::CreateOp(std::shared_ptr<storage::smb2_proxy::Smb2Proxy::TaskState>, const typename Op::ArgType&, typename Op::ResultType*,storage::smb2_proxy::Smb2Proxy::DoneCb)
   typename Op::Ptr CreateOp(std::shared_ptr<TaskState> task_state,

CreateOp -> GetCClassInternal（两者是等价的） 编译器无法对 CreateOp 进行专门化，并抱怨它与任何声明都不匹配。

PS：我还有一个问题，我在发布代码时犯了一个错误。我已删除问题并重新发布。

Answer 1

问题是GetCClass（和GetCClassInternal）的模板参数T在non-deduced contexts中使用，无法推导出来。

如果模板参数仅用于非推导上下文且未明确指定，则模板参数推导失败。

1) 使用限定 ID 指定的类型的嵌套名称说明符（范围解析运算符 :: 左侧的所有内容）：

您可以明确指定模板参数。例如

auto iptr = test.GetCClass<MyClass::CInt>(i);
auto dptr = test.GetCClass<MyClass::CDouble>(d);
auto sptr = test.GetCClass<MyClass::CStr>(s);

LIVE