'using' 声明为 SFINAE

Question

当从模板 class 派生私有时，我可以使用 SFINAE（或其他技术）进行 using 声明吗？为了更好地理解，请参阅下面的代码：

#include <iostream>

struct S1 {
    void f() { std::cout << "S1::f\n"; }
};

struct S2 {
    void f() { std::cout << "S2::f\n"; }
    void g() { std::cout << "S2::g\n"; }
};

template <class T>
struct D : private T {
    using T::f;
    // using T::g; // need this only if T provides g() function
};

int main() {
    D<S1>().f(); // ok. Prints 'S1::f'
    D<S2>().f(); // ok. Prints 'S2::f' 
    D<S2>().g(); // fail. But wants to be ok and prints 'S2::g'
    return 0;
}

我怎样才能达到期望的行为（如果可能的话）？

Answer 1

C++ 部分模板特化和使用 decltype(void(&T::g)) SFINAE

#include <iostream>
#include <type_traits>

struct S1 {
    void f() { std::cout << "S1::f\n"; }
};

struct S2 {
    void f() { std::cout << "S2::f\n"; }
    void g() { std::cout << "S2::g\n"; }
};

template <class T, class V = void>
struct D : private T {
    using T::f;
};

template <class T>
struct D<T, decltype(void(&T::g))> : private T {
    using T::f;
    using T::g; // need this only if T provides g() function
};

int main() {
    D<S1>().f(); // ok. Prints 'S1::f'
    D<S2>().f(); // ok. Prints 'S2::f' 
    D<S2>().g(); // ok. Prints 'S2::g'
    return 0;
}

Live Demo

编辑：

这是另一种更灵活的方法，但我不知道 private virtual 继承如何处理实际用例。如果它可能会导致任何问题（例如 UB），请告诉我。

#include <iostream>
#include <type_traits>

struct S1 {
    void f() { std::cout << "S1::f\n"; }
};

struct S2 {
    void f() { std::cout << "S2::f\n"; }
    void g() { std::cout << "S2::g\n"; }
};

struct S3 {
    void g() { std::cout << "S3::g\n"; }
};

template <class T, class = void>
struct D_f {};

template <class T>
struct D_f<T, decltype(void(&T::f))> : private virtual T {
    using T::f;
};

template <class T, class = void>
struct D_g {};

template <class T>
struct D_g<T, decltype(void(&T::g))> : private virtual T {
    using T::g;
};

template <class T>
struct D : D_f<T>, D_g<T> {
};


int main() {
    D<S1>().f();
    D<S2>().f();
    D<S2>().g();
    D<S3>().g();
    return 0;
}

Live Demo

Answer 2

Bryan Chen 的答案变体看起来更丑陋，但更容易扩展到多个检查，并且不需要复制 D<type-with-f> 和 D<type-without-f> 之间共享的代码，是为了使用继承链，其中每个步骤检查一个额外的成员。唯一需要的复制是构造函数的继承（如果适用）。

struct A {
  void f() { }
  void g() { }
  void i() { }
};

// The generic case. D<T, char[N]> simply provides what D<T, char[N+1]> provides.
template <typename T, typename U = char[1]>
struct D : D<T, char[sizeof(U) + 1]> {
  using D<T, char[sizeof(U) + 1]>::D;
};

// The end of the chain. This is where T gets inherited. It declares all of its own
// specialisations as its friends, so that they can access other members of T.
template <typename T>
struct D<T, char[6]> : private T {
  template <typename, typename>
  friend struct D;

  D(int) { }
  void fun() { }
};

// Check for T::f.
template <typename T>
struct D<T, char[2 + !sizeof(&T::f)]> : D<T, char[3]> {
  using D<T, char[3]>::D;
  using T::f;
};

// Check for T::g.
template <typename T>
struct D<T, char[3 + !sizeof(&T::g)]> : D<T, char[4]> {
  using D<T, char[4]>::D;
  using T::g;
};

// Check for T::h.
template <typename T>
struct D<T, char[4 + !sizeof(&T::h)]> : D<T, char[5]> {
  using D<T, char[5]>::D;
  using T::h;
};

// Check for T::i.
template <typename T>
struct D<T, char[5 + !sizeof(&T::i)]> : D<T, char[6]> {
  using D<T, char[6]>::D;
  using T::i;
};

int main() {
  D<A> d = 4; // ok: verify that constructors got inherited
  // A &a = d; // error: verify that inheritance of A is private
  d.f(); // ok: verify that f got inherited
  d.g(); // ok: verify that g got inherited
  // d.h(); // error: verify that h is not available
  d.i(); // ok: verify that i got inherited
  d.fun(); // ok: verify that the inheritance chain didn't get broken
}

注意：您可能不想检查 &T::f，而是想用 std::declval<T>().f() 做一些事情。前者无法处理重载函数。

'using' 声明为 SFINAE

'using' declaration as SFINAE

c++

sfinae

using-declaration

private-inheritance