如何在参数和返回类型上使用匹配的const修饰符编写函数？

分类：编程问答

如何解决如何在参数和返回类型上使用匹配的const修饰符编写函数？

我想编写一个从结构中提取指针字段的函数。要求是，如果我将struct作为const参数传递，则返回的类型应为const。如果不是，则返回的类型不应为const。

例如，

<link href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.14.0/css/all.min.css" rel="stylesheet"/>
<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/css/bootstrap.min.css" rel="stylesheet"/>
<div class="table-responsive">
  <div style="max-height: 200px; max-width: 300px; margin-bottom: 0;">
    <div class="cal-sectionDiv">
      <table class="table table-striped table-bordered">
        <thead class="cal-thead">
          <tr>
            <th class="cal-viewmonth" id="changemonth">juli 2020</th>
            <th class="cal-toprow">Wednesday 1</th>
            <th class="cal-toprow">Thursday 2</th>
            <th class="cal-toprow">Friday 3</th>
            <th class="cal-toprow weekend">Saturday 4</th>
            <th class="cal-toprow weekend">Sunday 5</th>
             <th class="cal-toprow">Monday 6</th>
            <th class="cal-toprow weekend">Thuesday 7</th>
            <th class="cal-toprow weekend">Wednesday 8</th>
          </tr>

        </thead>
        <tbody class="cal-tbody">
          <tr id="u1">
            <td class="cal-userinfo">
              <span><b style="text-decoration: underline">Van Els</b> Numan<span></span></span>
              <div class="cal-usercounter">
                <span class="cal-userbadge badge badge-light">140:13</span><span class="cal-userbadge badge badge-warning">134:36</span>
              </div>
              <div class="cal-userarrows">
                <i class="up mdi mdi-arrow-up-bold"></i><i class="down mdi mdi-arrow-down-bold"></i>
              </div>
            </td>
            <td class="ui-droppable" data-date1="1/7/2020" data-userid="1">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="13956" data-userid="1" style="border-left: 5px solid rgb(81,255,0); position: relative;">
                <h3 class="details-task" style=" background: #51FF00; color: #000000">Training</h3>
                <div class="details-uren">
                  15:00 - 16:30
                </div>
              </div>
            </td>
            <td class="ui-droppable" data-date1="2/7/2020" data-userid="1">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="13957" data-userid="1" style="border-left: 5px solid rgb(121,32,32); position: relative;">
                <h3 class="details-task" style=" background: #792020; color: #FFFFFF">Day Shift</h3>
                <div class="details-uren">
                  00:00 - 00:00
                </div>
              </div>
            </td>
            <td class="ui-droppable" data-date1="3/7/2020" data-userid="1">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="13959" data-userid="1" style="border-left: 5px solid rgb(175,0); position: relative;">
                <h3 class="details-task" style=" background: #AF0000; color: #FFFFFF">Sick</h3>
                <div class="details-uren">
                  00:00 - 00:00
                </div>
              </div>
            </td>
            <td class="weekend ui-droppable" data-date1="4/7/2020" data-userid="1">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="13958" data-userid="1" style="border-left: 5px solid rgb(36,115,171); position: relative;">
                <h3 class="details-task" style=" background: #2473AB; color: #FFFFFF">Late shift</h3>
                <div class="details-uren">
                  07:30 - 16:30
                </div>
              </div>
            </td>
            <td class="weekend ui-droppable" data-date1="5/7/2020" data-userid="1">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="12179" data-userid="1" style="border-left: 5px solid rgb(30,137,90); position: relative;">
                <h3 class="details-task" style=" background: #1E895A; color: #FFFFFF">Vacation</h3>
                <div class="details-uren">
                  00:00 - 00:00
                </div>
              </div>
            </td>
            <td class="ui-droppable" data-date1="6/7/2020" data-userid="1"></td>
            <td class="ui-droppable" data-date1="7/7/2020" data-userid="1"></td>
            <td class="ui-droppable" data-date1="8/7/2020" data-userid="1"></td>
          </tr>
          <tr id="u2">
            <td class="cal-userinfo">
              <span><b style="text-decoration: underline">Henzen</b> Susanna<span></span></span>
              <div class="cal-usercounter">
                <span class="cal-userbadge badge badge-light">140:13</span><span class="cal-userbadge badge badge-warning">134:36</span>
              </div>
              <div class="cal-userarrows">
                <i class="up mdi mdi-arrow-up-bold"></i><i class="down mdi mdi-arrow-down-bold"></i>
              </div>
            </td>
            <td class="ui-droppable" data-date1="1/7/2020" data-userid="2"></td>
            <td class="ui-droppable" data-date1="2/7/2020" data-userid="2">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="12326" data-userid="2" style="border-left: 5px solid rgb(36,171); position: relative;">
                <h3 class="details-task" style=" background: #2473AB; color: #FFFFFF">Late shift</h3>
                <div class="details-uren">
                  11:00 - 18:00
                </div>
              </div>
            </td>
            <td class="ui-droppable" data-date1="3/7/2020" data-userid="2">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="11978" data-userid="2" style="border-left: 5px solid rgb(255,184,72); position: relative;">
                <h3 class="details-task" style=" background: #FFB848; color: #000000">Late shift</h3>
                <div class="details-uren">
                  17:00 - 00:00
                </div>
              </div>
            </td>
            <td class="weekend ui-droppable" data-date1="4/7/2020" data-userid="2">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="12189" data-userid="2" style="border-left: 5px solid rgb(189,0); position: relative;">
                <h3 class="details-task" style=" background: #BD0000; color: #FFFFFF">Extra shift</h3>
                <div class="details-uren">
                  21:00 - 04:00
                </div>
              </div>
            </td>
            <td class="weekend ui-droppable" data-date1="5/7/2020" data-userid="2">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="12169" data-userid="2" style="border-left: 5px solid rgb(30,90); position: relative;">
                <h3 class="details-task" style=" background: #1E895A; color: #FFFFFF">Vacation</h3>
                <div class="details-uren">
                  00:00 - 00:00
                </div>
              </div>
            </td>
            <td class="ui-droppable" data-date1="6/7/2020" data-userid="1"></td>
            <td class="ui-droppable" data-date1="7/7/2020" data-userid="1"></td>
            <td class="ui-droppable" data-date1="8/7/2020" data-userid="1"></td>
          </tr>
          <tr id="u3">
            <td class="cal-userinfo">
              <span><b style="text-decoration: underline">Mak</b> Lokman<span></span></span>
              <div class="cal-usercounter">
                <span class="cal-userbadge badge badge-light">140:13</span><span class="cal-userbadge badge badge-warning">134:36</span>
              </div>
              <div class="cal-userarrows">
                <i class="up mdi mdi-arrow-up-bold"></i><i class="down mdi mdi-arrow-down-bold"></i>
              </div>
            </td>
            <td class="ui-droppable" data-date1="1/7/2020" data-userid="3">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="12938" data-userid="3" style="border-left: 5px solid rgb(36,171); position: relative;">
                <h3 class="details-task" style=" background: #2473AB; color: #FFFFFF">Late shift</h3>
                <div class="details-uren">
                  12:00 - 19:00
                </div>
              </div>
            </td>
            <td class="ui-droppable" data-date1="2/7/2020" data-userid="3">
            </td>
            <td class="ui-droppable" data-date1="3/7/2020" data-userid="3">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="12266" data-userid="3" style="border-left: 5px solid rgb(36,171); position: relative;">
                <h3 class="details-task" style=" background: #2473AB; color: #FFFFFF">Late shift</h3>
                <div class="details-uren">
                  05:00 - 12:00
                </div>
              </div>
            </td>
            <td class="weekend ui-droppable" data-date1="4/7/2020" data-userid="3">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="12286" data-userid="3" style="border-left: 5px solid rgb(36,171); position: relative;">
                <h3 class="details-task" style=" background: #2473AB; color: #FFFFFF">Late shift</h3>
                <div class="details-uren">
                  07:00 - 14:00
                </div>
              </div>
            </td>
            <td class="weekend ui-droppable" data-date1="5/7/2020" data-userid="3">
              <div class="drag details ui-draggable ui-draggable-handle" data-taskid="12107" data-userid="3" style="border-left: 5px solid rgb(126,126,126); position: relative;">
                <h3 class="details-task" style=" background: #7E7E7E; color: #FFFFFF">Rest</h3>
                <div class="details-uren">
                  00:00 - 00:00
                </div>
              </div>
            </td>
            <td class="ui-droppable" data-date1="6/7/2020" data-userid="1"></td>
            <td class="ui-droppable" data-date1="7/7/2020" data-userid="1"></td>
            <td class="ui-droppable" data-date1="8/7/2020" data-userid="1"></td>
          </tr>
        </tbody>
      </table>
    </div>
  </div>
</div>


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模板或最新的C ++标准中是否有任何技巧可以编写强类型函数而无需复制主体？

编辑： 对该示例进行了一些更改，以更准确地反映实际问题。

解决方法

这是一个具有单个功能模板的解决方案：

template<typename T,typename = std::enable_if_t<
                      std::is_same_v<
                        std::remove_cv_t<
                          std::remove_reference_t<T>>,S>>>
auto extract(T&& input) 
  -> std::conditional_t<
       std::is_const_v<
         std::remove_reference_t<T>>,int const *,int *> 
{
    return input.p;
}

这里是demo。

我认为不用多说，过载设置会更好。如果函数体很大，您仍然可以从const重载中调用非const版本，然后在其中添加const。

,

int[] numbers = {1,2,3}; System.out.println(lcmm(1,3)); // LCM(1,3) = 8,which is wrong private static int lcmm(int... numbers) { int sum = 0; for (int i = 0; i<numbers.length -1; i++) { int curr = numbers[i]; int next = numbers [i+1]; sum += lcm(curr,next); } return sum; } private static int lcm(int p,int q) { // Return lowest common multiple. return p * q / gcd(p,q); } private static int gcd(int p,int q) { //Return greatest common divisor using Euclid's Algorithm. int temp; while (q != 0) { temp = q; q = p % q; p = temp; } return p; } 和if constexpr作为返回类型解决方案：

auto
,

PLZ查看ISO提案：

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4388.html

还有std::experimental::propagate_const规范：

https://en.cppreference.com/w/cpp/experimental/propagate_const

或者可以实现自己的propagate_const版本。

玩得开心， 调频

,

SFINAE应该能够做到这一点。大致格式为：

  notequal.c:8:48: error: character too large for enclosing character literal type
  printf(" and '≠' has a value of 0x%X\n",'≠');

基本上使用通用转发引用使其适用于任何内容：S，S＆，const S＆，S &&，易失性S＆等。

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