如何避免显式转换std :: bind（）临时对象？ [英] How to avoid explicit cast with std::bind() temporary objects?

问题描述

std :: bind的返回类型（有意）未指定。

The return type of std::bind is (intentionally) unspecified. It is storable in a std::function.

下面的示例程序显示了如何显式转换由std返回的临时对象： ：bind（）到std ::函数，以便调用fn1（）。

The example program below shows how I have to explicitly cast the temporary object returned by std::bind() to a std::function in order to call fn1().

如果std :: bind的返回类型是可知的，我可以重载回调构造函数&将不再需要显式转换std :: bind临时对象。

If the return type of std::bind was knowable, I could overload the Callback constructor & would no longer need to explicitly cast std::bind temporary objects.

有没有办法避免显式转换？

Is there any way to avoid the explicit cast?

// g++ -std=c++11 test.cxx
#include <functional>

using std::placeholders::_1;

class A
{
    public:
        void funcA (int x) { }
};

class Callback
{
    public:
        Callback () = default;
        Callback (std::function<void(int)> f) { }
        // Wish we knew the return type of std::bind()
        // Callback (return_type_of_std_bind f) { }
};

void fn0 (std::function<void(int)> f) { }
void fn1 (Callback cb) { }

int main (void)
{
    A a;
    fn0(std::bind(&A::funcA, &a, _1)); // ok
    fn1(std::function<void(int)>(std::bind(&A::funcA, &a, _1))); // ok, but verbose
    fn1(std::bind(&A::funcA, &a, _1)); // concise, but won't compile
}

可能不相关， m在

推荐答案

最好给回调一个通用构造函数：

Best to give Callback a universal constructor:

struct Callback
{
    typedef std::function<void(int)> ftype;
    ftype fn_;

    template <typename T,
              typename = typename std::enable_if<std::is_constructible<ftype, T>::value>::type>
    Callback(T && f) : fn_(std::forward<T>(f))
    { }
};

（我添加了第二个默认的模板参数，只为类型 T ，语句是有意义的，因此不会创建虚假的可转换属性。）注意这种技术是如何从转换链中移动一个隐式的用户定义的转换：调用显示 con­ struc­ tor for fn _ 。

(I added the second, defaulted template argument to only enable this constructor for types T for which the statement makes sense, so as not to create false convertibility properties.) Note how this technique re­moves one implicit user-defined conversion from the conversion chain, by invoking an explicit con­struc­tor for fn_.

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