std::bind assigned to std::function

Question

I want a variable of type std::function<void(char**)>. Here's a simple example of failing to do that.

What I'm trying to understand is this:

• What is auto doing that the compiler doesn't complain when I call jj_2a(5, 6)? That function has all its parameters bound.
• But if I don't use auto, I get the behaviour I expect (compile error with arguments). So clearly function<void(void)> is not at all what auto decided.
• If I bind the first argument and not the second (jj_3), then calling with two arguments works (but drops the wrong argument, according to my mental model) while calling with one argument (which I think should work) doesn't compile.
• Using std::functional for jj_3_f says "no viable conversion", though the error message isn't so far helping me.

See below for compiler and specific errors. This is 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);
}

I compile this with

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

The compiler warnings associated with jj_3(7) are these:

/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.

The compiler warnings associated with jj_3_f are these:

/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, the thing I really want to do is parallel. I have a function

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

where A and B are class names. I have another function that expects a callback thus:

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

which I then try to call

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

and I get errors about no viable conversion.

Answer 1

Note: this answer assumes that you have read Why do objects returned from bind ignore extra arguments?

The object returned by std::bind is not a std::function; it is an object of an unspecified type that differs from std::function in a number of ways; most importantly for your case, it ignores any extra arguments passed to it.

If you use placeholders up to _2 (for example) in your bind expression, the returned object can be called with any number of arguments as long as that number is at least 2.

• What is auto doing that the compiler doesn't complain when I call jj_2a(5, 6)? That function has all its parameters bound.

You are passing extra arguments; those extra arguments are ignored.

• But if I don't use auto, I get the behaviour I expect (compile error with arguments). So clearly function is not at all what auto decided.

Correct; bind does not return a std::function, it returns a callable object of unspecified type that can (depending on signature) be used to construct a std::function.

• If I bind the first argument and not the second (jj_3), then calling with two arguments works (but drops the wrong argument, according to my mental model) while calling with one argument (which I think should work) doesn't compile.

If you use std::placeholders::_2, you must pass at least 2 arguments. std::placeholders::_2 picks the second argument passed to the object returned from bind.

• Using std::functional for jj_3_f says "no viable conversion", though the error message isn't so far helping me.

If you use std::placeholders::_2, you must pass at least 2 arguments; the constructor of std::function checks this.

Answer 2

This wont compile:

 jj_3(7);     // Compile error!

because you said earlier that jj_3 binds second parameter to the second argument at the call place. So without chaning it, you would have to call:

 jj_3(0, 7); // 0 is notused

or rather change your jj_3 declaration to:

 auto jj_3 = std::bind(jj_1, 3, _1);

This:

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

does not compile because you should have:

 std::function<void(int, int)> jj_3_f

as I said above, if you use place holder _2 then you want your jj_3_f to ba called with two parameters.