Language: C++

#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
using namespace std;
//using ll=long long;
const double EPS = 1e-10;
inline bool equals(double a, double b) { return fabs(a - b) < EPS; }

class Point {
public:
	double x, y;
	Point(double x, double y) :x(x), y(y) {}
	Point() {}
	Point operator +(Point &p) { return Point(x + p.x, y + p.y); };
	Point operator -(Point &p) { return Point(x - p.x, y - p.y); }
	Point operator *(double k) { return Point(x*k, y*k); }
	Point operator /(double k) { return Point(x / k, y / k); }
};
typedef Point Vector;

inline double norm(Vector a) {
	return a.x*a.x + a.y*a.y;
}
double absv(Vector a) {
	return sqrt(norm(a));
}
inline double dot(Vector a, Vector b) {
	return a.x*b.x + a.y*b.y;
}
inline double cross(Vector a, Vector b) {
	return a.x*b.y - a.y*b.x;
}
struct Segment {
	Point p1, p2;
};
typedef Segment Line;

class Circle {
public:
	Point c;
	double r;
	Circle(Point c = Point(), double r = 0.0) :c(c), r(r) {}
};
typedef vector<Point> Polygon;
Point project(Segment s, Point p) {
	Vector base = s.p2 - s.p1;
	double r = dot(p - s.p1, base) / norm(base);
	Point point = s.p1 + base*r;
	return point;
}
int main() {
	Point p[2];
	int q, x, y;
	cin >> p[0].x >> p[0].y >> p[1].x >> p[1].y;
	Segment seg;
	seg.p1 = p[0]; seg.p2 = p[1];
	cin >> q;
	for (int i = 0; i < q; i++) {
		cin >> x >> y;
		//cout << x << " " << y << endl;
		Point po = Point(x, y);
		Point proj = project(seg, po);
		printf("%.10lf %.10lf\n", proj.x, proj.y);
	}
}

Compiler output:
a.cc: In function 'Point project(Segment, Point)':
a.cc:47:28: error: no match for 'operator+' (operand types are 'Point' and 'Point')
   47 |         Point point = s.p1 + base*r;
      |                       ~~~~ ^ ~~~~~~
      |                         |        |
      |                         Point    Point
a.cc:13:15: note: candidate: 'Point Point::operator+(Point&)' (near match)
   13 |         Point operator +(Point &p) { return Point(x + p.x, y + p.y); };
      |               ^~~~~~~~
a.cc:13:15: note:   conversion of argument 1 would be ill-formed:
a.cc:47:34: error: cannot bind non-const lvalue reference of type 'Point&' to an rvalue of type 'Point'
   47 |         Point point = s.p1 + base*r;
      |                              ~~~~^~
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_iterator.h:627:5: note: candidate: 'template<class _Iterator> constexpr std::reverse_iterator<_Iterator> std::operator+(typename reverse_iterator<_Iterator>::difference_type, const reverse_iterator<_Iterator>&)'
  627 |     operator+(typename reverse_iterator<_Iterator>::difference_type __n,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:627:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'const std::reverse_iterator<_Iterator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/stl_iterator.h:1798:5: note: candidate: 'template<class _Iterator> constexpr std::move_iterator<_IteratorL> std::operator+(typename move_iterator<_IteratorL>::difference_type, const move_iterator<_IteratorL>&)'
 1798 |     operator+(typename move_iterator<_Iterator>::difference_type __n,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1798:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'const std::move_iterator<_IteratorL>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
In file included from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/bits/basic_string.h:3598:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3598 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3598:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3616:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3616 |     operator+(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3616:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   mismatched types 'const _CharT*' and 'Point'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3635:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(_CharT, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3635 |     operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Alloc>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3635:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3652:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3652 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3652:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3670:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, _CharT)'
 3670 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs, _CharT __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3670:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3682:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3682 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3682:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3689:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3689 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3689:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3696:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3696 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3696:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3719:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const _CharT*, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3719 |     operator+(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3719:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   mismatched types 'const _CharT*' and 'Point'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3726:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(_CharT, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3726 |     operator+(_CharT __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3726:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3733:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, const _CharT*)'
 3733 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3733:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;
      |                                   ^
/usr/include/c++/14/bits/basic_string.h:3740:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, _CharT)'
 3740 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3740:5: note:   template argument deduction/substitution failed:
a.cc:47:35: note:   'Point' is not derived from 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   47 |         Point point = s.p1 + base*r;