尝试制作对象时出错答案

【问题标题】：Error when trying to make an object尝试制作对象时出错
【发布时间】：2012-12-19 08:47:53
【问题描述】：

我正在尝试创建一个 Question 对象。 Question 是班级，但我收到一个错误：

错误 1 错误 C2440：“正在初始化”：无法从 Questions * 转换为 Questions

我正在尝试制作一个对象，以便可以将其放入 multimap 类型的 <int, Questions>

这是我的代码：

#include <iostream>
#include "Questions.h"
using namespace std;

Questions::Questions() {}
Questions::Questions(string question,string correctAnswer, string wrongAnswer1,string wrongAnswer2,string wrongAnswer3) {}

void Questions::questionStore() {
    Questions q1 = new Questions("Whats the oldest known city in the world?", "Sparta", "Tripoli", "Rome", "Demascus");
    string q2 = ("What sport in the olympics are beards dissallowed?", "Judo", "Table Tennis", "Volleyball", "Boxing");
    string q3 = ("What does an entomologist study?", "People", "Rocks", "Plants", "Insects");
    string q4 = ("Where would a cowboy wear his chaps?", "Hat", "Feet", "Arms", "Legs");
    string q5 = ("which of these zodiac signs is represented as an animal that does not grow horns?", "Aries", "Tauris", "Capricorn", "Aquarius");
    string q6 = ("Former Prime Minister Tony Blair was born in which country?", "Northern Ireland", "Wales", "England", "Scotland");
    string q7 = ("Duffle coats are named after a town in which country?", "Austria", "Holland", "Germany", "Belgium");
    string q8 = ("The young of which creature is known as a squab?", "Horse", "Squid", "Octopus", "Pigeon");
    string q9 = ("The main character in the 2000 movie ""Gladiator"" fights what animal in the arena?", "Panther", "Leopard", "Lion", "Tiger");

    map.insert(pair <int, Questions>(1, q1));
    map.insert(pair <int, string>(2, q2));
    // map.insert(pair<int,string>(3, q3));
    for (multimap <int, string, std::less <int> >::const_iterator iter = map.begin(); iter != map.end(); ++iter)
        cout << iter->first << '\t' << iter->second << '\n';
}

【问题讨论】：

q1 应该是一个指针，这样声明它Questions *q1。
你所有的字符串都将是最右边的值。另外，如果您需要指针，请不要使用new。请改用智能指针。
您在 q9 字符串中使用双引号的方式是错误的。将""Gladiator"" 替换为\"Gladiator\"。

标签： c++ stl multimap

【解决方案1】：

Questions q1 = new Questions 语法不正确。

从map.insert(pair <int, Questions>(1, q1)); 我可以看到你的map 值类型是 Questions 对象而不是 Questions 指针，所以应该是

Questions q1 = Questions ("Whats the oldest known city in the world?", "Sparta" , "Tripoli" , "Rome", "Demascus");

另外你的变量 map 与 std::map 同名，std::map 是一个 STL 容器，建议你使用另一个名字，例如：question_map;

编辑

要允许<< iter->second，您需要为问题类型重载operator<<。

std::ostream& operator<<(const std::ostream& out, const Questions& q)
{
    out << q.question;  // I made up this member as I can't see your Questions code
    return out;
}

【讨论】：

当我改成这个时，你能看到这条线 cout first second
我收到此错误：错误 1 错误 C2679：二进制“
其实这是另一个问题。它与`second`有关，但您还没有为问题类型重载运算符
我知道我是个菜鸟，但我该怎么做呢？
我在标题中声明了它，它说我有太多参数。

【解决方案2】：

new 表达式为您提供了一个指向您已动态分配的对象的指针。你需要做Questions* q1 = new Questions(...);。但是，如果您不需要动态分配（最终将对象复制到地图中），请不要打扰。只需Questions q1(...);。

您可能会更改以下几行（q2、q3 等）以匹配，但实际上它们并没有按照您的预期进行。 (..., ..., ...) 将评估为此逗号分隔列表中最右边的项目。所以你的q2 行等同于string q2 = "Boxing";。

【讨论】：

【解决方案3】：

questionScore 方法的第一行是问题所在：

Questions q1 = new Questions ...

new x返回一个指向x对象的指针，因此q1应该被定义为一个指针。

Questions * q1 = new Questions ...

【讨论】：