Using Bind to produce a parameterless function results in error

Question

I am trying to figure out the proper use of std::bind with a boost::signal2 signal.

The error set I am getting with clang++ (from Xcode 4.2.1) is:

~/Projects/Myron/Myron/main.cpp:29:59: error: reference to '_1' is ambiguous [3]
     main.resize.connect(std::bind(resize, std::ref(main), _1, _2));
                                                           ^
/Developer/usr/bin/../lib/c++/v1/functional:1496:18: note: candidate found by name lookup is 'std::__1::placeholders::_1' [3]
 extern __ph<1>   _1;
                  ^
/Developer/SDKs/MacOSX10.7.sdk/usr/local/include/boost/bind/placeholders.hpp:55:15: note: candidate found by name lookup is '<anonymous namespace>::_1' [3]
 boost::arg<1> _1;
               ^
~/Projects/Myron/Myron/main.cpp:30:24:{30:24-30:33}: error: no matching function for call to 'bind' [3]
     main.close.connect(std::bind(close, std::ref(main)));
                        ^~~~~~~~~
/Developer/usr/bin/../lib/c++/v1/functional:1789:1: note: candidate template ignored: couldn't infer template argument '_F' [3]
 bind(_F&& __f, _BoundArgs&&... __bound_args)
 ^
/Developer/usr/bin/../lib/c++/v1/functional:1798:1: note: candidate template ignored: couldn't infer template argument '_R' [3]
 bind(_F&& __f, _BoundArgs&&... __bound_args)
 ^
2 errors generated.

---

They are primarily focused on two lines:

main.resize.connect(std::bind(resize, std::ref(main), _1, _2));
main.close.connect(std::bind(close, std::ref(main)));

Which use the function prototypes:

bool resize(Myron::Window &win, int &width, int &height);
bool close(Myron::Window &win);

and main is a Myron::Window&, and resize and close are respectfully:

bs2::signal<bool(int&,int&)> resize;
bs2::signal<bool(void)> close;

Everything I have read and have been told leads me to believe I am using these fairly correctly, so I am unsure what is the problem is.

I realize the first error is a problem with boost::bind making its way in here probably through the boost/signals2.hpp in the Myron.h file. The second error, is more of a mystery.

The full main source file is here:

#include <iostream>
#include <functional>
#include <string>
#include <type_traits>

#include "Myron.h"

bool setup();
bool resize(Myron::Window &win, int &width, int &height);
bool close(Myron::Window &win);


bool setup()
{
    using namespace std::placeholders;

    std::cout << "setup()" << std::endl;
    Myron::Window &main = Myron::createWindow(640, 480);

    main.resize.connect(std::bind(resize, std::ref(main), _1, _2));
    main.close.connect(std::bind(close, std::ref(main)));
    return true;
}

bool close(Myron::Window &win)
{
    std::cout << "Window Closed" << std::endl;
    return true;
}

bool resize(Myron::Window &win, int &width, int &height)
{
    std::cout << "Resize: " << width << ", " << height << std::endl;
    return true;
}



int main(int argc, char**argv)
{
    std::cout << "Initialize..." << std::endl;

    Myron::Init(setup);

    std::cout << "Done Initialize." << std::endl;
}

Where the main source code repository showing everything is here: https://github.com/iaefai/Myron/tree/master/Myron

Any recommendations to try would be appreciated. I have tried making smaller test cases without success to bring out the issue.

Answer 1

The second issue is probably caused by overload ambiguity. Specifically, I suspect that the culprit is the system call close() that sneaks into your scope via <iostream>. Try enclosing your callbacks in a namespace and see if that helps. I'm surprised that clang++ isn't more helpful in this case, showing the overloads that cause ambiguity, it's usually praised for its very clear diagnostics.