std :: bind分配给std :: function

Question

我想要一个类型的变量std::function<void(char**)>.这是一个未能做到这一点的简单例子.

我想要了解的是:

• 当我打电话时,编译器不会抱怨什么是自动执行jj_2a(5, 6)？该函数的所有参数都绑定了.
• 但是,如果我不使用auto,我会得到我期望的行为(使用参数编译错误).所以显然function<void(void)>根本不是汽车决定的.
• 如果我绑定第一个参数而不是第二个(jj_3),那么使用两个参数调用(但根据我的心智模型删除错误的参数),而使用一个参数调用(我认为应该工作)不会编译.
• 使用std::functionalfor jj_3_f表示"没有可行的转换",虽然错误信息对我没有帮助.

请参阅下面的编译器和特定错误.这是linux,clang 3.8.0,ubuntu 16.04.1.

#include <functional>
#include <iostream>

void jj_1(int x, int y) { std::cout << x << ' ' << y << std::endl; }

int main() {
  using namespace std::placeholders; // for _1, _2, _3...

  auto jj_2a = std::bind(jj_1, 3, 2);
  jj_2a(5, 6);  // This works, prints "3 2", no compiler warning, is auto drunk?
  jj_2a();      // This also works, prints "3 2".
  std::function<void(void)> jj_2a_f = std::bind(jj_1, 30, 20);
  //jj_2a_f(50, 60);  // Compile error, good!
  jj_2a_f();  // This works, prints "30 20", good!

  auto jj_2b = std::bind(jj_1, _2, _1);
  jj_2b(5, 6);  // This works, prints "6 5", good.

  auto jj_3 = std::bind(jj_1, 3, _2);
  jj_3(5, 6);  // This works, prints "3 6", so it's the first arg that is dropped!
  //jj_3(7);     // Compile error!

  //std::function<void(int)> jj_3_f = std::bind(jj_1, 3, _2);  // Compile error, no viable conversion!
  //jj_4(11);
}

我用它编译

clang++  -std=c++14 -Wall -Wextra /tmp/foo.cc -o /tmp/foo

与之关联的编译器警告jj_3(7)是:

/tmp/foo.cc:21:7: error: no matching function for call to object of type 'std::_Bind<void (*(int,
  std::_Placeholder<2>))(int, int)>'
  jj_3(7);     // Compile error!
  ^~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/functional:1129:2: note: 
  candidate template ignored: substitution failure [with _Args = <int>]: no viable conversion from
  'std::_No_tuple_element' to 'int'
    operator()(_Args&&... __args)
    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/functional:1143:2: note: 
  candidate template ignored: substitution failure [with _Args = <int>]: no viable conversion from
  'std::_No_tuple_element' to 'int'
    operator()(_Args&&... __args) const
    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/functional:1157:2: note: 
  candidate template ignored: substitution failure [with _Args = <int>]: no viable conversion from
  'std::_No_tuple_element' to 'int'
    operator()(_Args&&... __args) volatile
    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/functional:1171:2: note: 
  candidate template ignored: substitution failure [with _Args = <int>]: no viable conversion from
  'std::_No_tuple_element' to 'int'
    operator()(_Args&&... __args) const volatile
    ^
1 error generated.

与之关联的编译器警告jj_3_f是:

/tmp/foo.cc:23:32: error: no viable conversion from 'typename _Bind_helper<__is_socketlike<void (&)(int,
  int)>::value, void (&)(int, int), int, const _Placeholder<2> &>::type' (aka '_Bind<void (*(int,
  std::_Placeholder<2>))(int, int)>') to 'std::function<void (int)>'
  std::function<void(int)> jj_3_f = std::bind(jj_1, 3, _2);  // Compile error, no viable conversion!
               ^        ~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/functional:2008:7: note: 
  candidate constructor not viable: no known conversion from 'typename
  _Bind_helper<__is_socketlike<void (&)(int, int)>::value, void (&)(int, int), int, const
  _Placeholder<2> &>::type' (aka '_Bind<void (*(int, std::_Placeholder<2>))(int, int)>') to
  'nullptr_t' for 1st argument
  function(nullptr_t) noexcept
  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/functional:2019:7: note: 
  candidate constructor not viable: no known conversion from 'typename
  _Bind_helper<__is_socketlike<void (&)(int, int)>::value, void (&)(int, int), int, const
  _Placeholder<2> &>::type' (aka '_Bind<void (*(int, std::_Placeholder<2>))(int, int)>') to 'const
  std::function<void (int)> &' for 1st argument
  function(const function& __x);
  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/functional:2028:7: note: 
  candidate constructor not viable: no known conversion from 'typename
  _Bind_helper<__is_socketlike<void (&)(int, int)>::value, void (&)(int, int), int, const
  _Placeholder<2> &>::type' (aka '_Bind<void (*(int, std::_Placeholder<2>))(int, int)>') to
  'std::function<void (int)> &&' for 1st argument
  function(function&& __x) : _Function_base()
  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/functional:2054:2: note: 
  candidate template ignored: substitution failure [with _Functor = std::_Bind<void (*(int,
  std::_Placeholder<2>))(int, int)>, $1 = void]: no type named 'type' in
  'std::result_of<std::_Bind<void (*(int, std::_Placeholder<2>))(int, int)> (int)>'
    function(_Functor);
    ^
1 error generated.

Fwiw,我真正想要的是平行.我有一个功能

void MyFunction(A& a, B& b, const char** thing);

在哪里A和B是类名.我有另一个期望回调的函数:

C DoStuff(const std::string& s, std::function<void(const char** thing)> f);

然后我试着打电话

DoStuff("Hello!", std::bind(MyFunction, an_a, a_b, _3));

并且我得到关于没有可行转换的错误.

Answer 1

注意:这个答案假定你已经读过为什么从bind返回的对象会忽略额外的参数？

返回的对象std::bind不是std::function; 它是一种未指明类型的对象,它std::function在许多方面都有所不同; 最重要的是,它会忽略传递给它的任何额外参数.

如果_2在绑定表达式中使用占位符(例如),则只要该数字至少为2,就可以使用任意数量的参数调用返回的对象.

• 当我调用jj_2a(5,6)时,编译器不会抱怨什么是自动执行？该函数的所有参数都绑定了.

你正在传递额外的论点; 那些额外的论点被忽略了.

• 但是,如果我不使用auto,我会得到我期望的行为(使用参数编译错误).所以显然功能根本不是汽车决定的.

正确; bind不返回a std::function,它返回一个未指定类型的可调用对象,可以(取决于签名)用于构造一个std::function.

• 如果我绑定第一个参数而不是第二个参数(jj_3),那么使用两个参数调用(但根据我的心理模型删除错误的参数),而使用一个参数调用(我认为应该工作)不会编译.

如果使用std::placeholders::_2,则必须至少传递2个参数.std::placeholders::_2选择传递给返回的对象的第二个参数bind.

• 使用std :: functional for jj_3_f说"没有可行的转换",虽然错误信息对我没有帮助.

如果使用std::placeholders::_2,则必须传递至少2个参数; std::function检查这个的构造函数.