我不明白为什么 clang 会拒绝这段代码。我从我的 friend 那里得到它并在 VisualStudio 上为他编译...我有 clang 的 TOT。
#include <utility>
#include <iostream>
template< typename Signature >
class Delegate;
template< typename Ret, typename Param >
class Delegate< Ret(Param) >
{
public:
Ret operator()(Param&& p_param)
{
return m_ifunc(m_obj, std::forward< Param >(p_param));
}
template< typename ObjType, typename Ret(ObjType::*Method)(Param) >
friend auto createDelegate(ObjType * const p_obj)
{
Delegate< Ret(Param) > del;
del.m_obj = p_obj;
del.m_ifunc = &ifunction< ObjType, Method >;
return del;
}
private:
void * const m_obj = nullptr;
Ret (*m_ifunc)(void*, Param&&) = nullptr;
template< typename ObjType, typename Ret(ObjType::*Method)(Param) >
static Ret ifunction(void * const p_obj, Param&& p_param)
{
ObjType * const obj = (ObjType * const) p_obj;
return (obj->*Method)(std::forward< Param >(p_param));
}
};
struct Test
{
void test(int x)
{
std::cout << x << std::endl;
}
};
int main()
{
Test t;
Delegate< void(int) > d = Delegate< void(int) >::createDelegate< Test, &Test::test >(&t);
d(5);
}
这是我得到的错误 有人知道发生了什么事吗?我已经看到这种为函数指针指定模板参数的方法,我猜想缺少一些对 clang 的严格要求。
main.cpp:17:41: error: expected a qualified name after 'typename'
template< typename ObjType, typename Ret(ObjType::*Method)(Param) >
^
main.cpp:31:41: error: expected a qualified name after 'typename'
template< typename ObjType, typename Ret(ObjType::*Method)(Param) >
^
main.cpp:52:53: error: no member named 'createDelegate' in 'Delegate<void (int)>'
Delegate< void(int) > d = Delegate< void(int) >::createDelegate< Test, &Test::test >(&t);
~~~~~~~~~~~~~~~~~~~~~~~^
main.cpp:52:69: error: 'Test' does not refer to a value
Delegate< void(int) > d = Delegate< void(int) >::createDelegate< Test, &Test::test >(&t);
^
main.cpp:39:8: note: declared here
struct Test
^
main.cpp:52:82: error: definition or redeclaration of 'test' not allowed inside a function
Delegate< void(int) > d = Delegate< void(int) >::createDelegate< Test, &Test::test >(&t);
~~~~~~^
main.cpp:52:86: error: expected ';' at end of declaration
Delegate< void(int) > d = Delegate< void(int) >::createDelegate< Test, &Test::test >(&t);
^
最佳答案
您的代码有几个问题,我很确定它从未以当前形式在 VisualStudio 上编译过。无论如何,我无法在 VS2013 上编译它。以下是错误:
template< typename ObjType, typename Ret(ObjType::*Method)(Param) >
// ^^^^^^^^
// the member function pointer is a non-type template parameter, remove typename
friend auto createDelegate(ObjType * const p_obj)
// ^^^^
// you've declared this as a friend, but call it within main() as if it is a
// static member function, change 'friend' to 'static'
template< typename ObjType, typename Ret(ObjType::*Method)(Param) >
// ^^^^^^^^
// same as above, remove typename
static Ret ifunction(void * const p_obj, Param&& p_param)
m_obj
数据成员是一个 const
指针,但您试图将它指向 createDelegate
中的一个新对象
void * const m_obj = nullptr;
// ^^^^^
// remove the const
做出这些改变之后
template< typename Signature >
class Delegate;
template< typename Ret, typename Param >
class Delegate< Ret(Param) >
{
public:
Ret operator()(Param&& p_param)
{
return m_ifunc(m_obj, std::forward< Param >(p_param));
}
template< typename ObjType, Ret(ObjType::*Method)(Param) >
static auto createDelegate(ObjType * const p_obj)
{
Delegate< Ret(Param) > del;
del.m_obj = p_obj;
del.m_ifunc = &ifunction< ObjType, Method >;
return del;
}
private:
void * m_obj = nullptr;
Ret (*m_ifunc)(void*, Param&&) = nullptr;
template< typename ObjType, Ret(ObjType::*Method)(Param) >
static Ret ifunction(void * const p_obj, Param&& p_param)
{
ObjType * const obj = (ObjType * const) p_obj;
return (obj->*Method)(std::forward< Param >(p_param));
}
};
现在代码编译并输出5
。 Live demo
要在 VS2013(发行版,不知道 CTP)上编译,需要进行额外更改。由于VS2013没有实现C++14对普通函数的返回类型推导,createDelegate
需要显式指定返回类型
template< typename ObjType, Ret(ObjType::*Method)(Param) >
static Delegate< Ret(Param) > createDelegate(ObjType * const p_obj)
{ /* ... */ }
最后,只是为了确保您了解替代方案:
Test t;
auto d = std::bind(&Test::test, t, std::placeholders::_1);
d(5); // prints 5
std::function<void(int)> d2 = std::bind(&Test::test, t, std::placeholders::_1);
d2(5); // prints 5
关于c++ - 将成员函数指针作为模板类型传递,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/23643878/