c ++在结构向量中找到var

Question

I'm trying to make a c++ program where I can dynamically add points to a list, and then search for if those points are in the list.

I was trying originally to use sets, but am now trying to use vectors. My code below will run, but gets an error involving the search if statement.

#include <stdio.h>       /* printf */
#include <bits/stdc++.h> /* vector of strings */
#include <algorithm>
#include <vector>
using namespace std;

void solve_point(vector<vector<char>> &board)
{
    printf("solve_point\n");
    board[2][2] = 'c';
}

struct point
{
    int x;
    int y;
};

int main()
{

    vector<point> vec; 
    point point1{4, 1};
    point point1{8, 3};
    vec.push_back(point1);

    if (find(vec.begin(), vec.end(), point1) != vec.end())
    {   
        printf("point1 found in vec\n");
    }
    else
    {
        printf("point1 not found in vec\n");
    }

   }

This leads to some errors saying:

/usr/include/c++/7/bits/predefined_ops.h:241:17: error: no match for ‘operator==’ (operand types are ‘point’ and ‘const point’)
  { return *__it == _M_value; }

  ..and..

  /usr/include/c++/7/bits/predefined_ops.h:241:17: note:   ‘point’ is not derived from ‘const __gnu_cxx::new_allocator<_Tp>’
  { return *__it == _M_value; }

I encountered an error similar to this earlier when I was trying to insert a point into a set, is there something wrong with my struct point initialization? Thanks

Answer 1

You are experiencing two problems:

1. std::find() needs a way of comparing two struct point instances. For this, it defaults to using point::operator==() which is not defined. You have to add this manually:

struct point
{
    int x;
    int y;

    bool operator==(const point& rhs) const {
        return x == rhs.x and y == rhs.y;   
    }
};

Alternatively you can use std::find_if() which allows you to supply a lambda. However, instances of a class like point will most likely need to be compared in a lot of places so defining operator==() is definitely a good idea.

2. You're redeclaring point1 . Rename it to for example point2 .

Working example:

#include <bits/stdc++.h> /* vector of strings */
#include <algorithm>
#include <vector>
using namespace std;

void solve_point(vector<vector<char>> &board)
{
    printf("solve_point\n");
    board[2][2] = 'c';
}

struct point
{
    int x;
    int y;

    bool operator==(const point& rhs) const {
        return x == rhs.x and y == rhs.y;   
    }
};

int main()
{

    vector<point> vec; 
    point point1{4, 1};
    point point2{8, 3};
    vec.push_back(point1);

    if (find(vec.begin(), vec.end(), point1) != vec.end())
    {   
        printf("point1 found in vec\n");
    }
    else
    {
        printf("point1 not found in vec\n");
    }

   }