c++ - std::length_error 尝试从类数组输出字符串时

Question

我的程序由三个文件组成。 arithcard.hpp(头文件)、assign2.cpp(主程序)和arithcard.cpp(方法)。

在数组 cards 中，存储从文本文件读取的字符串。例如，cards[0].fileLine 的字符串为 2 + 2 = 4。(fileLine 是包含所有字符串的私有(private)成员变量)，cards[1].fileLine 的字符串为 2 + 4 = 3，等等...

我知道 cards[i].fileLine 包含所有这些字符串，因为我通过在 readCards 方法中将它们全部打印出来进行测试，以查看它们是否真的在其中。

这是我的问题，当我尝试在主程序中调用 cards[i].getQuestion(或 .getAnswer())时。有时，它会向我显示问题(或答案)或错误

“在抛出‘std::length_error’实例后调用终止 what(): basic_string::_S_create"

arithcard.hpp

#include <iostream>
#include <string>

/*
  Defines the class ArithCard, for handling arithmetic flash cards.

  You may not modify the public method declarations provided below. The main
  program used to test your solution will assume that these methods exist.

  You can add private attributes and methods as you need them.

  You may find it convenient to add additional public methods for testing.
  Keep in mind that your code must work with the unmodified main program
  or it will fail automated testing.
*/
class ArithCard {

  public:

  // Default constructor
  ArithCard() ;

  static bool readCards(const std::string &fileName,
            int &cardCnt,ArithCard *&cards) ;

  void displayQuestion(std::ostream &out) ;

  bool checkAnswer(std::istream &in) ;

  // Return the question as a string.
  std::string getQuestion() ;

  // Return the answer as a string.
  std::string getAnswer() ;

  // Return the answer as an integer.
  int getAnswerValue() ;

  private:

  // Add your private methods and attributes here.
  std::string fileLine;
  std::string question;
  std::string answer;
} ;

arithcard.cpp

#include <iostream>
#include <string>
#include <fstream>
#include "arithcard.hpp"
using namespace std;

ArithCard::ArithCard()
{
    //body intentionally empty
}

bool ArithCard::readCards(const string &fileName, int &cardCnt, ArithCard *&cards)
{
    //read the file line by line and store it in cards
    return (true);
}

void displayQuestion(std::ostream &out)
{
   //display output
}

 bool checkAnswer(std::istream &in)
 {
     return (true);
 }

// Return the question as a string.
 string ArithCard::getQuestion()
 {
     size_t pos = 0;
     pos = fileLine.find("=");
     question = fileLine.substr(0, pos);
     return question;
 }

 // Return the answer as a string.
  string ArithCard::getAnswer()
  {
      size_t pos = 0;
      pos = fileLine.find("=") + 1;
      answer = fileLine.substr(pos);
      return answer;
  }

    // Return the answer as an integer.
   int getAnswerValue()
   {
       return answer;

   }

assign2.cpp

#include <iostream>
#include <sstream>
#include <string>
#include <limits>
#include <random>
#include "arithcard.hpp"

// File-local anonymous namespace

namespace {

int verbosity = 1 ;

}

/*
  Read in the files of cards, then loop: query the user to specify a
  question, print the question, check the answer. Terminate when the user
  types an empty line instead of specifying a question.
*/
int main (int argc, const char *argv[])
{
  if (argc != 2) {
    std::cout << "Usage: " << argv[0] << " <datafile>" << std::endl ;
    return (1) ;
  }
  std::string dataFile(argv[1]) ;

  bool result = false ;
  int cardCnt = -1 ;
  ArithCard *cards = nullptr ;

/*
  Parse the file of questions & answers. A false return value indicates an
  absent file or other i/o error. It's also possible the file is present but
  contains no valid questions.
*/
result = ArithCard::readCards(dataFile,cardCnt,cards) ;
std::cout << cards[0].getQuestion() << std:: endl;

  return (0) ;
}

Answer 1

来自 §21.4.2/1 [string.require]，

If any operation would cause size() to exceed max_size(), that operation shall throw an exception object of type length_error.

因此，在您的代码中某处，您试图创建一个长度超过 string::max_size() 的字符串，这通常是一个巨大的数字，所以您应该为自己感到自豪 :-)

开个玩笑，很难说出错误在哪里，因为看起来你已经删除了相关代码部分，尤其是 ArithCard::readCards 的实现。我不明白你为什么选择让这个函数成为static，然后将它传递给一个指向 ArithCard 实例的指针，而不是仅仅让它成为一个非静态成员函数.

您的代码将传递给 ArithCard::readCards 的指针初始化为 nullptr，因此我假设您正在为函数内的对象分配内存。如果不是，那么您很可能在该函数中发生了未定义的行为。

我会将该函数更改为

class ArithCard {
  public:
  ...
  bool readCards(const std::string &fileName, int &cardCnt) ;
};

并将main()中的代码修改为

ArithCard cards;

result = cards.readCards(dataFile, cardCnt);

此外，在 getQuestion() 和 getAnswer() 中，您都没有检查 string::find 的返回值，所以尝试使用无效结果提取子字符串可能是导致此错误的原因。这是最可能的解释，因为 string::find 将返回 string::npos , 如果找不到搜索词，这是一个巨大的数字。