如何从多重地图打印

Question

嘿即时尝试从多重地图打印出来。我的multimap是：multimap<int,Questions*> map; 我想要发生的是当我调用一个问题时：questions.printQuestion（1）;它以随机顺序打印出3个问题。但是，在调用printQuestion时，所有即时消息都是运行时错误。

Run time error: 
Debug Assertion Failed! 
Expression: map/set iterator may not be dereferencable 

这里是我的代码：

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

Questions :: Questions() 
{ 
} 

Questions::Questions(string question,string correctAnswer, string wrongAnswer1,string wrongAnswer2,string wrongAnswer3) 
{ 
    this->question = question; 
    this->pAnswers = new string[4]; 
    this->pAnswers[0]=wrongAnswer1; 
    this->pAnswers[1]=wrongAnswer2; 
    this->pAnswers[2]=wrongAnswer3; 
    this->pAnswers[3] =correctAnswer; 
    this->shuffle(this->pAnswers,4); 
    this->correctAnswer = correctAnswer; 
} 

void Questions::shuffle(string *array, int n) 
{ 
    random_shuffle(&this->pAnswers[0],&this->pAnswers[4]); 
} 

string Questions::getQuestion() 
{ 
    return this->question; 
} 

string Questions::getCorrectAnswer() 
{ 
    return this->correctAnswer; 
} 

string* Questions::getAnswers() 
{ 
    return this->pAnswers; 
} 

bool Questions::checkAnswer(string answer) 
{ 
    if(this->correctAnswer.compare(answer)==0) 
    { 
     return true; 
    } 
    return false; 
} 

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

    addQuestion(1,q1); 
    addQuestion(1,q2); 
    addQuestion(1,q3); 
    addQuestion(2,q4); 
    addQuestion(2,q5); 
    addQuestion(2,q6); 
    addQuestion(3,q7); 
    addQuestion(3,q8); 
    addQuestion(3,q9); 
} 

void Questions::addQuestion(int level, Questions *question) 
{ 
    map.insert(pair<int,Questions*>(level,question)); 
} 


Questions* Questions::printQuestion(int level) 
{ 
    multimap<int, Questions*>::iterator iter; 
    pair<multimap<int, Questions*>::iterator,multimap<int, Questions*>::iterator> constIter; 
    for (multimap< int, Questions*, less<int> >::const_iterator iter =map.begin(); 
    iter != map.end(); ++iter) 
     cout << iter->first << '\t' << iter->second << '\n'; 



     /*constIter = map.equal_range(level); 
    size_t sz = distance(constIter.first, ret.second); 
    size_t idx = rand(); 
    if(ret.first != ret.second) 
    advance(ret.first, idx); 
    it =ret.first; 
    Questions* question = (*it).second; 
    return (*it).second; 
     cout << question->getQuestion() << std::endl;*/ 
     return iter->second; 
} 

任何人可以帮助我。

Answer 1

当你说你得到一个“运行时错误”，我假设函数打印和整数和指针的地址。你可能意味着取消引用指针问题，在打印前：

*it->second 

另一个问题是，取消引用过去的最末端迭代器，当您返回iter->second：在return声明前的循环被退出时iter == map.end()。解引用过去的末端迭代器是非法的，并导致未定义的行为（这实际上是错误的错误）。

当然，您应该首先存储Question对象，并且而不是将它们分配在堆上：它们很可能会泄漏。在“问题”对象中有一个“问题”商店似乎也不太合适：您可能想要分开课程：一个针对单个问题，另一个针对多个问题。