c++ - 函数模板的显式模板特化不起作用
问题描述
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
并且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,它在请求时将返回CInt
,CDouble
或的实例之一CStr
。
这种结构的动机:假设GetCClass
从一个大约 100 行的函数调用该函数,并且只有一个区别:类的类型。从返回的值GetCClass
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);
我已经阅读了多个答案,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) 编译器不能对 CreateOp 进行专门化,并抱怨它与任何声明都不匹配。
PS:我有另一个问题,我在发布代码时犯了一个错误。我已删除问题并重新发布。
解决方案
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