如何使用bind1st和bind2nd？

Question

我想学习如何使用绑定函数。这就是我的想法:我有这个函数，它接受参数：

void print_i(int t, std::string separator)
{
        std::cout << t << separator;
}

我想做的是：

std::vector<int> elements;
// ...
for_each(elements.begin(), elements.end(), std::bind2nd(print_i, '\n'));

但是它不起作用！

下面是我得到的结果：

/usr/include/c++/4.3/backward/binders.h: In instantiation of ‘std::binder2nd<void ()(int, std::string)>’:
main.cpp:72:   instantiated from here
/usr/include/c++/4.3/backward/binders.h:138: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:141: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:145: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:149: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:155: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:140: error: field ‘std::binder2nd<void ()(int, std::string)>::op’ invalidly declared function type
/usr/include/c++/4.3/backward/binders.h: In function ‘std::binder2nd<_Operation> std::bind2nd(const _Operation&, const _Tp&) [with _Operation = void ()(int, std::string), _Tp = char]’:
main.cpp:72:   instantiated from here
/usr/include/c++/4.3/backward/binders.h:164: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/bits/stl_algo.h: In function ‘_Funct std::for_each(_IIter, _IIter, _Funct) [with _IIter = __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >, _Funct = std::binder2nd<void ()(int, std::string)>]’:
main.cpp:72:   instantiated from here
/usr/include/c++/4.3/bits/stl_algo.h:3791: error: no match for call to ‘(std::binder2nd<void ()(int, std::string)>) (int&)’
make: *** [all] Error 1

我可以使用functor，但使用绑定会更快。

谢谢!

Answer 1

bind2nd的参数必须是AdaptableBinaryFunction。普通的二进制函数不能满足这一要求(自适应函数需要typedefs作为其返回和参数类型，普通函数类型不提供任何typedefs)。您可以使用std::bind，这可能是更好的选择。

Answer 2

这些函数从C++11开始就被弃用了，并且在C++17中被删除了。正如上面的一条评论所提到的，现在更好的解决方案是使用std::bind和占位符：

void callable(int a, int b);
auto f = std::bind1st(&callable, 42); // returns a 1-arg function

变成：

// returns a 1-arg function
auto f = std::bind(&callable, 42, std::placeholders::_1);