C++函数导致应用程序崩溃并且无法正常工作

Question

我的应用程序出现问题，我的 PrintAll 函数无法正常工作，最终只会使我的应用程序崩溃。我的应用程序应该从文件中读取字符串并将它们插入到数组中。问题是它读取不正确，最终会使我的应用程序崩溃。这是我认为问题所在:

int main()
{
    LoadMovies();

    MovieList *movies = LoadMovies();
    //movies->MovieList::PrintAll();

    //    // test methods for the Movie and MovieList classes
        //PrintAllMoviesMadeInYear(movies, 1984);
        //PrintAllMoviesWithStartLetter(movies, 'B');
        //PrintAllTopNMovies(movies, 5);

    //delete movies;
    return 0;
}

MovieList* LoadMovies()
{
    vector<string> movies;
    ReadMovieFile(movies);
    MovieList ml = MovieList(movies.size());

    string name;
    int year;
    double rating;
    int votes;

    for (int i = 0; i < movies.size(); i++)
    {
        istringstream input_string(movies[i]);
        getline(input_string, name, '\t');
        input_string >> year >> rating >> votes;
        Movie movie (name, year, votes, rating);
        ml.Add(movie);
    }
    ml.PrintAll();
}

完整示例:

/*
 * File: MovieStatsProgram.cpp
 * Author:
 * Date:
 * ===============================================================
 * This is a console app to test the Movie and MovieList classes.
 *
 * TODO:
 *
 * You need to finish the implementation of the loadMovies method
 * to create and initialize the MovieList object.
 *
 * You also need to create three static methods:
 *
 * PrintAllMoviesMadeInYear - it will print all the movies made in a
 * given year once sort in alphabetical order and once sorted by the number
 * of votes with the movie with the most number of votes printed first.
 *
 * PrintAllMoviesWithStartLetter - it will print all the movies started with
 * a given letter sorted in alphabetical order
 *
 * PrintAllTopNMovies - it will display the top N movies based on the number of
 * votes
 */

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

using namespace std;

class Movie {
public:
    Movie();
    Movie(string n, int y, int v, double r);
    string get_name();
    void set_name(string n);
    int get_year();
    void set_year(int y);
    int get_votes();
    void set_votes(int v);
    double get_rating();
    void set_rating(double r);
    string PrintMovie();

private:
    string name;
    int year_made;
    int votes;
    double rating;

};

Movie::Movie() {
    name = "null";
    year_made = 0;
    votes = 0;
    rating = 0.0;
}

Movie::Movie(string n, int y, int v, double r) {
    name = n;
    year_made = y;
    votes = v;
    rating = r;
}

string Movie::get_name() {
    return name;
}

void Movie::set_name(string n) {
    name = n;
}

int Movie::get_year() {
    return year_made;
}

void Movie::set_year(int y) {
    year_made = y;
}

int Movie::get_votes() {
    return votes;
}

void Movie::set_votes(int v) {
    votes = v;
}

double Movie::get_rating() {
    return rating;
}

void Movie::set_rating(double r) {
    rating = r;
}

string Movie::PrintMovie() {
    cout << fixed << setprecision(1) << rating << "\t\t" << votes << "\t\t" << "(" <<
            year_made << ")" << "\t" << name << endl;
}

class MovieList {
public:
    MovieList(int size);
    ~MovieList();
    int Length();
    bool IsFull();
    void Add(Movie const& m);
    string PrintAll();

private:
    Movie* movies;
    int last_movie_index;
    int movies_size;
    int movie_count = 0;

};

MovieList::MovieList(int size) {
    movies_size = size;
    movies = new Movie[movies_size];
    last_movie_index = -1;
}

MovieList::~MovieList() {
    delete [] movies;
}

int MovieList::Length() {
    return last_movie_index;
}

bool MovieList::IsFull() {
    return last_movie_index == movies_size;
}

void MovieList::Add(Movie const& m)
{
    if (IsFull()) {
        cout << "Cannot add movie, list is full" << endl;
        return;
    }

    ++last_movie_index;
    movies[last_movie_index] = m;
}

string MovieList::PrintAll() {
    for (int i = 0; i < last_movie_index; i++) {
        movies[last_movie_index].Movie::PrintMovie();
        //cout << movies[last_movie_index] << endl;
    }
}

void ReadMovieFile(vector<string> &movies);
MovieList* LoadMovies();

enum MovieSortOrder
{
    BY_YEAR = 0,
    BY_NAME = 1,
    BY_VOTES = 2
};

int main()
{
    LoadMovies();

    MovieList *movies = LoadMovies();
    //movies->MovieList::PrintAll();

    //    // test methods for the Movie and MovieList classes
        //PrintAllMoviesMadeInYear(movies, 1984);
        //PrintAllMoviesWithStartLetter(movies, 'B');
        //PrintAllTopNMovies(movies, 5);

    //delete movies;
    return 0;
}

MovieList* LoadMovies()
{
    vector<string> movies;
    ReadMovieFile(movies);
    MovieList ml = MovieList(movies.size());

    string name;
    int year;
    double rating;
    int votes;

    for (int i = 0; i < movies.size(); i++)
    {
        istringstream input_string(movies[i]);
        getline(input_string, name, '\t');
        input_string >> year >> rating >> votes;
        Movie movie (name, year, votes, rating);
        ml.Add(movie);
    }
    ml.PrintAll();
}

void ReadMovieFile(vector<string> &movies)
{
    ifstream instream;
    instream.open("imdbtop250.txt");
    if (instream.fail())
    {
        cout << "Error opening imdbtop250.txt" << endl;
        exit(1);
    }


    while (!instream.eof())
    {
        string movie;
        getline(instream, movie);
        movies.push_back(movie);
    }

    instream.close();
}

当我在主函数中使用 MovieList::PrintAll 时，我的函数就崩溃了，而当我将它放在 LoadMovies 函数中时，它会在崩溃前错误地读取和添加数据。列表的大小为 251，应用程序将只读取相同的数据 251 次。

Answer 1

你有一个两部分的问题:

1:正如 Brad S 所说，您的函数不返回任何内容。这是一个禁忌。

MovieList* LoadMovies()
{
    MovieList ml = MovieList(movies.size());
    // Your function returns a pointer to a MovieList, so...
    return &ml;
}

因此，问题 #2 是您要返回一个指针，指向您在函数的堆栈上创建的内容。当您尝试在您的函数之外访问它时，您将遇到未定义的行为。

选项 1:

 MovieList* ml = new MovieList( movies.size() );
 return ml;

您现在需要在完成后删除 ml。

选项 2:更改您的函数以返回非指针...然后您就没有管理内存的麻烦。

编辑:试试这个

int main()
{
    // Don't need this
    // LoadMovies();

    MovieList *movies = LoadMovies();

    // Uncommented this
    delete movies;
return 0;
}

MovieList* LoadMovies()
{
    vector<string> movies;
    ReadMovieFile(movies);
    // CHANGE
    MovieList* ml = new MovieList(movies.size());
    // CHANGE

    string name;
    int year;
    double rating;
    int votes;

    for (int i = 0; i < movies.size(); i++)    
    {
        istringstream input_string(movies[i]);
        getline(input_string, name, '\t');    
        input_string >> year >> rating >> votes;
        Movie movie (name, year, votes, rating);
        ml.Add(movie);
    }
    ml.PrintAll();
    // CHANGE
    return ml;
}