C++ MovieList 数组和指针

Question

我仍然有点卡在作业的另一部分上。

这里是提示询问的内容:

Now you can modify the LoadMovies function to create a MovieList object and add each of the Movie objects to it. The function LoadMovies should return a pointer to the MovieList object. That means you need to create the MovieList object dynamically and on the heap.

Change the main function and store the returned MovieList pointer in a variable. To test if everything works as expected, you can use the PrintAll function of the MovieList object.

到目前为止，这是我的代码:

class MovieList {

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

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

~MovieList() {
    delete [] movies;
}

int Length() {
    return movie_count;
}

bool IsFull() {
    return movie_count == movies_size;
}

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

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

void PrintAll() {
    for (int i = 0; i < movie_count; i++) {
        movies[last_movie_index].PrintMovie();
    }
}

};

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

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

int main()
{
    LoadMovies();

    // TODO:
    // You need to implement the Movie and MovieList classes and
    // the methods below so that the program will produce
    // the output described in the assignment.
    //
    // Once you have implemented everything, you should be able
    // to simply uncomment the code below and run the program.

    MovieList *movies = LoadMovies();

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

        //delete movies;
    return 0;
}

void LoadMovies()
{
    vector<string> movies;
    ReadMovieFile(movies);

    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);
        movie.PrintMovie();
    }
}

现在我卡在了教授让我修改提示中的 LoadMovies 并将其变成指针的地方。我在画空白。同样出于某种原因，如果我尝试编译它会说:

C:\Users\Andy\Documents\C++ Homework\MovieStatisticsProgram\MovieStatsProgram.cpp:163: error: void value not ignored as it ought to be
     MovieList *movies = LoadMovies();
                                    ^

Answer 1

你的构造函数的顺序是错误的

MovieList(int size) {
    movies = new int[movies_size];  // movies_size hasn't been initialized yet!
    movies_size = size;
    last_movie_index = -1;
}

应该是

MovieList(int size)
:  movies_size{size}, movies{new int[size]}, last_movie_index{0}
{}

虽然@ScottMcP-MVP 指出你的数组应该是

Movie* movie;

所以你的构造函数是

MovieList(int size)
:  movies_size{size}, movies{new Movie[size]}, last_movie_index{0}
{}

开始使用其余功能的一些建议

Length 函数将从 last_movie_index 返回当前使用的数量。

IsFull 将检查 last_movie_index == movies_size - 1

Add 将需要使用 last_movie_index 来确定数组中的哪个元素来存储电影。

PrintAll 必须从 [0] 迭代到 [movie_count] 并打印出每个元素。

您的Add 函数看起来像

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

    movies[last_movie_index] = m;   // assigns a copy to your array
    ++last_movie_index;             // Increment the movie index
}