小编Shi*_*izu的帖子

我使用 mongoose 将我的数据库连接到我的应用程序。

我收到了这个意外错误。

我的 app.js 看起来像这样

const express = require('express');
const mongoose = require('mongoose');
const authRoutes = require('./routes/authRoutes');
const cookieParser = require('cookie-parser');
const { requireAuth, checkUser } = require('./middleware/authMiddleware');
const run = require('./admin/connection');

const app = express();

// middleware
app.use(express.static('public'));
app.use(express.json());
app.use(cookieParser());

const {default : AdminBro} = require('admin-bro');
const buildAdminRouter = require('./admin/admin.router');
const options = require('./admin/admin.options');
const port = 3000;


const url = 'mongodb://localhost:27017/dbName';
let mongooseDb;
const databaseConnect = async () => {
  mongooseDb = await mongoose.connect(url, { useNewUrlParser: …

我无法理解GeeksforGeeks的这段代码：

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
  
// Comparison function for sorting the
// set by increasing order of its pair's
// second value
struct comp {
    template <typename T>
  
    // Comparator function
    bool operator()(const T& l,
                    const T& r) const
    {
        if (l.second != r.second) {
            return l.second < r.second;
        }
        return l.first < r.first;
    }
};

我在StackOverflow中找到了类似的文章。但它们都略有不同。

bool operator ()我还想问一下，写和写有什么区别operator bool()？