标签: unary-function

我发现它binary_function已从C++ 11中删除.我想知道为什么.

C++ 98:

template <class T> struct less : binary_function <T,T,bool> {
  bool operator() (const T& x, const T& y) const {return x<y;}
};

C++ 11:

template <class T> struct less {
  bool operator() (const T& x, const T& y) const {return x<y;}
  typedef T first_argument_type;
  typedef T second_argument_type;
  typedef bool result_type;
};

MODIFIED ------------------------------------------------- ---------------------------

template<class arg,class result>
struct unary_function
{
       typedef arg argument_type;
       typedef result result_type;
};

例如,如果我们想在C++ 98中为函数编写适配器,

template <class T> struct even : …

我正在寻找一个一元函子,它将取消引用它的参数并返回结果.当然我可以写一个,它似乎应该已经存在.

所以给出代码:

const auto vals = { 0, 1, 2, 3 };
vector<const int*> test(size(vals), nullptr);

iota(begin(test), end(test), data(vals));

transform(cbegin(test), cend(test), ostream_iterator<int>(cout, " "), [](const auto& i){ return *i; });

Live Example

我希望有一个我可以用而不是lambda的仿函数.这样的事情存在,还是我需要使用lambda？

我通过 vcpkg 安装了 boost-program-options 版本 1.78。当我编译时clang++，-std=c++20出现以下错误。当我用 编译时不会发生这种情况g++。据此，从 C++11 开始，这已被弃用。 std::unary_function

In file included from /home/david/C/vcpkg/installed/x64-linux/include/boost/program_options/variables_map.hpp:12:
In file included from /home/david/C/vcpkg/installed/x64-linux/include/boost/any.hpp:20:
In file included from /home/david/C/vcpkg/installed/x64-linux/include/boost/type_index.hpp:29:
In file included from /home/david/C/vcpkg/installed/x64-linux/include/boost/type_index/stl_type_index.hpp:47:
/home/david/C/vcpkg/installed/x64-linux/include/boost/container_hash/hash.hpp:132:33: warning: 'unary_function<const std::error_category *, unsigned long>' is deprecated [-Wdeprecated-declarations]
        struct hash_base : std::unary_function<T, std::size_t> {};
                                ^
/home/david/C/vcpkg/installed/x64-linux/include/boost/container_hash/hash.hpp:692:18: note: in instantiation of template class 'boost::hash_detail::hash_base<const std::error_category *>' requested here
        : public boost::hash_detail::hash_base<T*>
                 ^
/home/david/C/vcpkg/installed/x64-linux/include/boost/container_hash/hash.hpp:420:24: note: in instantiation of template class 'boost::hash<const std::error_category *>' …

传递函数对象的以下小程序有什么问题？

#include <iostream>
#include <functional>

void foo(const std::unary_function<const std::string&, void>& fct) {
  const std::string str = "test";
  fct(str); // error
}

class MyFct : public std::unary_function<const std::string&, void> {
public:
  void operator()(const std::string& str) const {
    std::cout << str << std::endl;
  }
};

int main(int argc, char** argv){
  MyFct f;
  foo(f);
  return 0;
}

我在第6行收到以下错误:

 no match for call to 
`(const std::unary_function<const std::string&, void>) (const std::string&)'

int bar(int *arr, size_t n)
{
    int sum = 0, i;
    for (i = n; i > 0; i--)
    {
        sum += !arr[i - 1];
    }
    return ~sum + 1;
}

我看过这段代码，但不太明白sum += !arr[i - 1];：！（NOT）应用于数组指针的作用是什么？另外，〜之前有sum什么作用？

我想找出在调用括在括号中的SQL函数之前包含连字符时，SQL语法背后的逻辑是什么。

这是SQL：

IF (@StartDate > @EndDate)
BEGIN
    SET @EndDate = @StartDate
    SET @StartDate = @EndDate
END

DECLARE @nonworkingweekdays int

--now deal with public holidays
SELECT @nonworkingweekdays = count("Date") from 
(
    select distinct
    (
    CASE datepart(weekday,date)
            WHEN 1 THEN null --ignore sundays   
            WHEN 7 THEN null --ignore saturdays
            else "Date"
    END
    ) AS "date" 
    from publicholidays
) nonworkingweekdays 
WHERE 
"Date" is not null and 
"Date" between @StartDate and DATEADD(day, -1, @EndDate)

RETURN
    CASE WHEN @StartDate <= @EndDate
    THEN
        dbo.FullWeekDays(@StartDate, @EndDate) - …