尝试向结构向量内的结构成员输入会导致分段错误

Question

我正在做一个项目，我正在为一家假餐馆编写自动计费系统。该程序应该获取包含菜单的文本文件，将其放入结构数组或向量中，显示菜单，让客户订购并打印收据。 我正在为菜单使用结构的全局向量。

这段代码就是与问题相关的所有内容。

`

#include <iostream>
#include <fstream>
#include <vector>
//there is more code to this program, but the fault occurs very soon in the program
//and none of the rest of the code has any relevance.
//also, I don't really think that the problem is with trying to input, but I don't have enough experience to rule it out.
using namespace std;

struct menuItemType
{
  string menuItem; //this is the name of the item
  double menuPrice; // this is the price of the item
  int menuCount;
};

vector<menuItemType> menuList; //the menu can be any size so I don't know how big it will be at this point. I'm using a vector to avoid having to declare a size
// I also have 2 other functions and some extra code in main that all need to access this vector. That is why I made it global

void getData() //this function opens the text file containing the menu and tries to read in each line.
{
    ifstream input;
    input.open("inData.txt");

    input.peek();
    int i = 0;
    string item;
    double price;

    while(!input.eof())
    {
        getline(input,menuList[i].menuItem); //This is the line creating the fault.
        input >> menuList[i].menuPrice;

        i++;
        input.peek();
    }
}
int main()
{
    getData();
    return 0;
}

`

我已经尝试调试并确定分段错误不是特定于代码 sn-p 中注释的行。每当我尝试输入向量内的结构成员时，似乎都会发生错误。我也尝试过使用cin，所以我不相信文本文件流是问题所在。 文本文件如下所示：

Bacon and eggs 1.00 Muffin 0.50 Coffee 0.90

具体来说，我的问题是：为什么尝试输入向量内的结构成员会导致分段错误，我该如何解决。

对于冗长的解释和尴尬的格式，我们深表歉意。我对堆栈溢出和 c++ 都很陌生。

Answer 1

从文件中检索数据时；我倾向于检索单行的内容并将其存储到某个字符串、流或缓冲区并稍后解析它，或者我将检索文件的全部内容并执行相同的操作。从文件中提取数据并关闭其句柄后，我发现解析字符串更容易。我不喜欢使用非 CONST 的全局变量。此外，从文件while( file.eof() ) 或while ( !file.eof() ) 读取文件时使用 for 循环的方式也是不好的做法，并且可能会导致以后出现许多错误、崩溃和头痛。如果您在下面查看我的函数，它所做的就是接受一个文件名并尝试打开它（如果存在）。一旦它打开，它将得到一行将其保存到一个字符串并将该字符串推入一个向量，直到没有其他内容可读取。然后它关闭文件句柄并返回。这符合具有单一职责的功能的概念。

如果您有打开文件、读取一行、解析数据、读取一行、解析数据等功能，然后关闭它；这种功能被认为承担多项任务，这可能是一件坏事。首先是性能原因。可以说，打开和读取文件本身是一项计算成本很高的任务。您还尝试动态创建对象，如果您从未检查过以验证从文件中收到的值，这可能会很糟糕。看看我下面的代码，你会看到我所指的设计模式，其中每个函数都有自己的职责。这也有助于防止file corruption。

#include <vector>
#include <string>
#include <sstream>
#include <iostream>
#include <fstream>
#include <exception>

struct MenuItem {
  string menuItem; 
  double menuPrice; 
  int menuCount;
};

// This function is not used in this case but is a very helpful function
// for splitting a string into a vector of strings based on a common delimiter
// This is handy when parsing CSV files {Comma Separated Values}.
std::vector<std::string> splitString( const std::string& s, char delimiter ) {
    std::vector<std::string> tokens;
    std::string token;
    std::istringstream tokenStream( s );
    while( std::getline( tokenStream, token, delimiter ) ) {
        tokens.push_back( token );
    }

    return tokens;
}

void getDataFromFile( const char* filename, std::vector<std::string>& output ) {
    std::ifstream file( filename );
    if( !file ) {
        std::stringstream stream;
        stream << "failed to open file " << filename << '\n';
        throw std::runtime_error( stream.str() );
    }

    std::string line;
    while( std::getline( file, line ) ) {
        if ( line.size() > 0 ) 
            output.push_back( line );
    }
    file.close();
}

void parseFileData( const std::vector<std::string>& fileContents, std::vector<MenuItem> menuItems ) {
    // The first param is the contents of the file where each line
    // from the file is stored as a string and pushed into a vector.

    // Here you need to parse this data. The second parameter is the
    // vector of menu items that is being passed by reference.

    // You can not modify the fileContents directly as it is const and read only
    // however the menuItems is passed by reference only so you can update that

    // This is where you will need to go through some kind of loop and get string
    // of text that will stored in your MenuItem::menuItem variable.
    // then the next string will have your price. Here you showed that your
    // text file has `$` in front of the value. You will then have to strip this out 
    // leaving you with just the value itself. 
    // Then you can use `std::stod( stringValue ) to convert to value, 
    // then you can save that to MenuTiem::menuPrice variable.

    // After you have the values you need then you can push back this temp MenuItem
    // Into the vector of MenuItems that was passed in. This is one iteration of
    // your loop. You continue this until you are done traversing through the fileContents vector.


    // This function I'll leave for you to try and write.        
}

int main() {
    try {
        std::vector<std::string> fileConents;
        getDataFromFile( "test.txt", fileConents );
        std::vector<MenuItem> data; // here is the menu list from your example
        generateVectors( fileConents, data );

        // test to see if info is correct
        for( auto& d : data ) {
            std::cout << data.menuItem << " " << data.menuPrice << '\n';
        }

    } catch( const std::runtime_error& e ) {
        std::cerr << e.what() << '\n';
        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;
}

至于您的错误或崩溃，您可能正在访问超出向量末尾的索引，或者您试图使用向量中包含无效数据的内容。

Answer 2

如果您查看operator[] of a vector，然后检查异常部分，它会告诉您如果 n 大于向量的大小，它实际上是未定义的行为。您可能想推回您之前创建的项目。

我通常更喜欢vector::at，因为它经过边界检查，如果请求的位置超出范围，则通过抛出 out_of_range 异常发出信号。

Answer 3

首先从 inData.txt 中删除 "$" 然后我建议像这样使用 while(getline(input, item)) ：

#include <iostream>
#include <fstream>
#include <vector>
#include <math.h>
//there is more code to this program, but the fault occurs very soon in the program
//and none of the rest of the code has any relevance.
//also, I don't really think that the problem is with trying to input, but I don't have enough experience to rule it out.
using namespace std;

struct menuItemType
{
  string menuItem; //this is the name of the item
  double menuPrice; // this is the price of the item
  int menuCount;
};

vector<menuItemType*> menuList; //the menu can be any size so I don't know how big it will be at this point. I'm using a vector to avoid having to declare a size
// I also have 2 other functions and some extra code in main that all need to access this vector. That is why I made it global

void getData() //this function opens the text file containing the menu and tries to read in each line.
{
    ifstream input;
    input.open("inData.txt");

    int i = 0;
    string item;
    double price;

    while(getline(input, item))
    {
        menuList.push_back(new menuItemType);
        menuList[i]->menuItem = item;
        getline(input,item);
        menuList[i]->menuPrice = atof((char*)item.c_str()); //math.h
        i++;
    }
}
int main()
{
    getData();
    for(menu : menuList)
    {
        cout << menu->menuItem << ": " << menu->menuPrice << endl;
        delete menu; //cleaning memory
    }
    return 0;
}