解决方法，允许tr1 :: function接收返回值 [英] Workaround to allow tr1::function to swallow return values

问题描述

作为可以使用tr1 :: function swallow返回值的后续动作 ，如何解决 tr1 :: function 不能接收返回值的限制？

As a follow-up to Can tr1::function swallow return values?, how can one work around the limitation that tr1::function cannot swallow return values?

没有参数的可调用对象的返回值的具体情况：

This should work in the specific case of swallowing the return value of a callable object taking no arguments:

template<typename FuncT>
struct swallow_return_t
{
  explicit swallow_return_t(FuncT i_Func):m_Func(i_Func){}
  void operator()(){ m_Func(); }
  FuncT m_Func;
};

template<typename FuncT>
swallow_return_t<FuncT>
swallow_return(FuncT f)
{
  return swallow_return_t<FuncT>(f);
}

然后使用类似：

int Foo();
std::tr1::function<void()> Bar = swallow_return(Foo);

我假设可变参数模板和完美转发允许将此技术泛化到任意参数列表。有没有更好的方法？

I assume variadic templates and perfect forwarding would permit generalization of this technique to arbitrary parameter lists. Is there a better way?

推荐答案

以下适用于我在GCC 4.6.1：

The following works for me in GCC 4.6.1:

#include <functional>

int foo() { return 5; }
int goo(double, char) { return 5; }

int main()
{
  std::function<void()> f = foo;
  std::function<void(float, int)> g = goo;
  (void)f();
  (void)g(1.0f, 'a');
}

---

这里是一个使用lambdas ，但不是自动的

---

Here's a wrapper using lambdas, but it's not automagic yet

template <typename T, typename ...Args>
struct strip_return
{
  static inline std::function<void(Args...)> make_function(std::function<T(Args...)> f)
  {
    return [&f](Args... args) -> void { f(args...); };
  }
};

int main()
{
  auto q = strip_return<int>::make_function(std::bind(foo));
  q();
}

---

忘记中间部分。 OK，因为 std :: function 是类型擦除，很难得到底层类型。然而，如果你直接去函数引用，你可以完全避免这些问题：

---

Forget the middle part. OK, since std::function is type-erasing, it's hard to get at the underlying types. However, if you go for the function reference directly, you can avoid these problems entirely:

template <typename T, typename ...Args>
static inline std::function<void(Args...)> make_direct(T (&f)(Args...))
{
  return [&f](Args... args) -> void { f(args...); };
}

int main()
{
  auto p = make_direct(foo);
  q();
}

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