class A { int a; };

template<typename, typename = void>
class test {};

template<typename T>
class test<T,decltype(T::a)> {};

int main() { test<A> a; }

main.cpp: In function ‘int main()’:
main.cpp:9:22: error: ‘int A::a’ is private within this context
 int main() { test<A> a; }
                      ^
main.cpp:1:15: note: declared private here
 class A { int a; };

If a substitution results in an invalid type or expression, type deduction fails. An invalid type or expression is one that would be ill-formed, with a diagnostic required, if written using the substituted arguments. [ Note: If no diagnostic is required, the program is still ill-formed. Access checking is done as part of the substitution process. — end note ]

class A { void a() {}; };

template<typename, typename = void>
class test {};

template<typename T>
class test<T,decltype(&T::a)> {};

int main() { test<A> a; }

最佳答案

这很有趣! 我想 这是一个 g++ 编译器错误和 我想 这就是发生的事情。我已经尝试使用 g++ 4.9.3 和 clang 3.7.0 对您的代码进行了几次更改。

尽管函数与类模板实例化的规则有些不同，但我相信这些是模板实例化的一般步骤:

class A {};

template<typename, typename = void>
struct test
{ test(){std::cout<< "Using None" <<std::endl;} };

template<typename T>
struct test<T, decltype(T::a)>
{ test(){std::cout<< "Using T::a" <<std::endl;} };

int main()
{ test<A> a; }

Using None

test<A , decltype(A::a)> a;

error: no member named 'a' in 'A'

class A { int a; };

template<typename, typename = void>
struct test
{ test(){std::cout<< "Using None" <<std::endl;} };

template<typename T>
struct test<T, decltype(T::a)>
{ test(){std::cout<< "Using T::a" <<std::endl;} };

int main()
{ test<A> a; }

Using None

error: ‘int A::a’ is private

class A{ void a() {}; };

template<typename, typename = void>
struct test
{ test(){std::cout<< "Using None" <<std::endl;} };

template<typename T>
struct test<T,decltype(&T::a)>
{ test(){std::cout<< "Using A::a" <<std::endl;} };

int main()
{ test<A> a; }

Using None

int main()
{ test<A , decltype(&A::a)> b; }

error: ‘void A::a()’ is private

#include <iostream>
#include <boost/type_index.hpp>

class Golf
{
    public:
        int v;

        void f()
        {};
};


template<typename T>
using var_t = decltype(T::v);

template<typename T>
using func_t = decltype(&T::f);
//using func_t = decltype(((T*)nullptr)->f()); //void


template<typename, typename = void>
struct test
{
    test(){std::cout<< "Using None" <<std::endl;}
};

template<typename T>
struct test<T,var_t<T> >
{
    test(){std::cout<< "Using Golf::v" <<std::endl;}
};

template<typename T>
struct test<T,func_t<T> >
{
    test(){std::cout<< "Using Golf::f" <<std::endl;}
};


int main()
{
    test<Golf> a;
    test<Golf,var_t<Golf> > b;
    test<Golf,func_t<Golf> > c;

    using boost::typeindex::type_id_with_cvr;
    std::cout<< typeid(func_t<Golf>).name() << " -> " << type_id_with_cvr<func_t<Golf>>().pretty_name() <<std::endl;
}

Using None
Using Golf::v
Using Golf::f
M4GolfFvvE -> void (Golf::*)()

Using Golf::f
Using Golf::v
Using Golf::f
v -> void

#include <utility>

class A
{
    int a;
};

template<typename, typename = void>
class test
{};

template<typename T>
class test<T,decltype(std::declval<A>().a)>
{};

int main()
{
    test<A> a;
}

关于c++ - 模板替换和 SFINAE 中的私有(private)成员访问，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/39758335/