如何正确使用 C++ 中设置的 .find() 函数?

Question

这里是'point'的结构

struct point
{
double x;
double y;
};

这是产生错误的函数， 我使用 .find() 存在问题，如下图所示。 当我将鼠标悬停在错误上时，它说"在模板中：二进制表达式的操作数无效('常量点'和'常量点')">

bool isChecked(point left, point right, set<vector<point>>const& inSet)
{
// Check both possible arrangements of points
vector<point> tmp1 = {left, right};
vector<point> tmp2 = {right, left};
// .find() returns an iterator up to the element found
// If not found, return element after last, .end()
auto first = inSet.find(tmp1);
auto second = inSet.find(tmp2);
// Check if elements were checked already
if (first != inSet.end() || second != inSet.end())
return true;
return false;
}

这是编译器提供的错误：

C:/msys64/mingw64/include/c++/12.2.0/bits/predefined_ops.h:45:23: error: no match for 'operator<' (operand types are 'const point' and 'const point')
45 |       { return *__it1 < *__it2; }
|                ~~~~~~~^~~~~~~~
In file included from C:/msys64/mingw64/include/c++/12.2.0/string:47:
C:/msys64/mingw64/include/c++/12.2.0/bits/stl_iterator.h:1246:5: note: candidate: 'template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator<(const __normal_iterator<_IteratorL, _Container>&, const __normal_iterator<_IteratorR, _Container>&)'
1246 |     operator<(const __normal_iterator<_IteratorL, _Container>& __lhs,
|     ^~~~~~~~
C:/msys64/mingw64/include/c++/12.2.0/bits/stl_iterator.h:1246:5: note:   template argument deduction/substitution failed:
C:/msys64/mingw64/include/c++/12.2.0/bits/predefined_ops.h:45:23: note:   'const point' is not derived from 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>'
45 |       { return *__it1 < *__it2; }
|                ~~~~~~~^~~~~~~~
C:/msys64/mingw64/include/c++/12.2.0/bits/stl_iterator.h:1254:5: note: candidate: 'template<class _Iterator, class _Container> bool __gnu_cxx::operator<(const __normal_iterator<_Iterator, _Container>&, const __normal_iterator<_Iterator, _Container>&)'
1254 |     operator<(const __normal_iterator<_Iterator, _Container>& __lhs,
|     ^~~~~~~~
C:/msys64/mingw64/include/c++/12.2.0/bits/stl_iterator.h:1254:5: note:   template argument deduction/substitution failed:
C:/msys64/mingw64/include/c++/12.2.0/bits/predefined_ops.h:45:23: note:   'const point' is not derived from 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>'
45 |       { return *__it1 < *__it2; }
|                ~~~~~~~^~~~~~~~
ninja: build stopped: subcommand failed.

Answer 1

你需要把它添加到你的代码中：

bool operator<(const point& lhs, const point& rhs)
{
return lhs.x < rhs.x;
}

使<运算符重载。

然后你写你的函数，你可以这样写它：

bool isChecked(point left, point right, set<vector<point>> inSet)
{
// Check both possible arrangements of points
vector<point> tmp1 = {left, right};
vector<point> tmp2 = {right, left};
if (inSet.find(tmp1) != inSet.end() || inSet.find(tmp2) != inSet.end())
{
return true;
}
else
{
return false;
}
}

Answer 2

来自Yksisarvinen的评论：

而且不是因为find，std::set要求它的元素能与<相媲美。std::vector可以与<进行比较，当且仅当其元素也与<进行比较。

以上面的注释作为参考，如果其他读者不清楚，解决方案是重载<运算符函数。