將 arguments 傳遞到 lambda 比較器時出現轉換錯誤

Question

我有一個排序的結構向量，我試圖找到在其成員屬性中具有查詢值的元素的索引。 為此，我嘗試使用 lower_bound，但比較器 lambda function 出現問題。 這是我的代碼的簡化版本：

#include <iostream>
#include <vector>
#include <algorithm>

struct Student
{
    int id;
};

std::vector<Student> Students;

int main()
{
    Student Ann = {1};
    Student Bob = {3};
    Student Alice = {4};
    Students = {Ann, Bob, Alice};
    int id_query = 3;
    auto index_ptr = std::lower_bound(Students.begin(), Students.end(), id_query, [](const int a, const Student s) -> bool {return a < s.id;});
}

g++ with std=17 給了我以下

Starting build...
/bin/g++ -g /home/aram/dev/cpp/coverages/stack_overflow.cpp -std=c++17 -o /home/aram/dev/cpp/coverages/stack_overflow
In file included from /usr/include/c++/9/bits/stl_algobase.h:71,
                 from /usr/include/c++/9/bits/char_traits.h:39,
                 from /usr/include/c++/9/ios:40,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from /home/aram/dev/cpp/coverages/stack_overflow.cpp:1:
/usr/include/c++/9/bits/predefined_ops.h: In instantiation of ‘bool __gnu_cxx::__ops::_Iter_comp_val<_Compare>::operator()(_Iterator, _Value&) [with _Iterator = __gnu_cxx::__normal_iterator<Student*, std::vector<Student> >; _Value = const int; _Compare = main()::<lambda(int, Student)>]’:
/usr/include/c++/9/bits/stl_algobase.h:979:14:   required from ‘_ForwardIterator std::__lower_bound(_ForwardIterator, _ForwardIterator, const _Tp&, _Compare) [with _ForwardIterator = __gnu_cxx::__normal_iterator<Student*, std::vector<Student> >; _Tp = int; _Compare = __gnu_cxx::__ops::_Iter_comp_val<main()::<lambda(int, Student)> >]’
/usr/include/c++/9/bits/stl_algo.h:2032:32:   required from ‘_FIter std::lower_bound(_FIter, _FIter, const _Tp&, _Compare) [with _FIter = __gnu_cxx::__normal_iterator<Student*, std::vector<Student> >; _Tp = int; _Compare = main()::<lambda(int, Student)>]’
/home/aram/dev/cpp/coverages/stack_overflow.cpp:21:142:   required from here
/usr/include/c++/9/bits/predefined_ops.h:177:11: error: no match for call to ‘(main()::<lambda(int, Student)>) (Student&, const int&)’
  177 |  { return bool(_M_comp(*__it, __val)); }
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/9/bits/predefined_ops.h:177:11: note: candidate: ‘bool (*)(int, Student)’ <conversion>
/usr/include/c++/9/bits/predefined_ops.h:177:11: note:   candidate expects 3 arguments, 3 provided
/home/aram/dev/cpp/coverages/stack_overflow.cpp:21:83: note: candidate: ‘main()::<lambda(int, Student)>’
   21 |     auto index_ptr = std::lower_bound(Students.begin(), Students.end(), id_query, [](const int a, const Student s) -> bool {return a < s.id;});
      |                                                                                   ^
/home/aram/dev/cpp/coverages/stack_overflow.cpp:21:83: note:   no known conversion for argument 1 from ‘Student’ to ‘int’

Build finished with error(s).
The terminal process failed to launch (exit code: -1).

Answer 1

std::lower_bound的比較器應該采用從迭代器中解引用的 object，即元素作為第一個參數，要比較的值作為第二個參數。

類型Type1必須使得 ForwardIt 類型的ForwardIt可以被取消引用，然后隱式轉換為Type1 。 Type2類型必須使得T類型的 object 可以隱式轉換為Type2 。

您需要更改參數的順序，例如

auto index_ptr = std::lower_bound(Students.begin(), Students.end(), id_query, [](const Student s, const int a) -> bool {return s.id < a;});

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