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#include <iostream>
#include <functional>
 
class Foo
{
public:
    explicit Foo(int n) : n_{n} {}
 
    int getN() const
    {
    	return n_;
    }
 
private:
    int n_;
};
 
class Bar
{
public:
    void bar(const std::function<int(std::function<int()>)>& callback,
            const std::function<int()>& callback2) const
    {
    	std::cout << "The value of n+1 is: " << callback(callback2) << '\n';
    }
};
 
class FooBar
{
public:
    int increment(const std::function<int()>& callback) const
    {
    	auto n = callback();
    	return ++n;
    }
};
 
int main()
{
	Bar b;
	b.bar(std::bind(&FooBar::increment, FooBar{},
	std::bind(&Foo::getN, Foo{2018})),
	std::bind(&Foo::getN, Foo{2018}));
 
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:43:34: error: no matching function for call to ‘Bar::bar(std::_Bind_helper<false, int (FooBar::*)(const std::function<int()>&) const, FooBar, std::_Bind<std::_Mem_fn<int (Foo::*)() const>(Foo)> >::type, std::_Bind_helper<false, int (Foo::*)() const, Foo>::type)’
  std::bind(&Foo::getN, Foo{2018}));
                                  ^
prog.cpp:21:10: note: candidate: void Bar::bar(const std::function<int(std::function<int()>)>&, const std::function<int()>&) const
     void bar(const std::function<int(std::function<int()>)>& callback,
          ^~~
prog.cpp:21:10: note:   no known conversion for argument 1 from ‘std::_Bind_helper<false, int (FooBar::*)(const std::function<int()>&) const, FooBar, std::_Bind<std::_Mem_fn<int (Foo::*)() const>(Foo)> >::type {aka std::_Bind<std::_Mem_fn<int (FooBar::*)(const std::function<int()>&) const>(FooBar, std::_Bind<std::_Mem_fn<int (Foo::*)() const>(Foo)>)>}’ to ‘const std::function<int(std::function<int()>)>&’

Standard output is empty