我不能在调用 istream 作为参数的函数中使用 ifstream 吗？

Question

这是C++编程原理与实践第10章的练习。 该程序使用积分：

1. 它提示用户输入 (x,y) 对。然后将它们存储在称为 original_points 的点向量中。
2. 原始点被打印到文件中
3. 然后程序读取文件以检索相同的点，这些点存储在名为processed_points 的点向量中
4. 比较两个向量，如果不相同，程序会报错。

问题是processed_points 的size() 为0，只有当我使用get_vector_points() 来获得这些分数时才会发生这种情况。函数有什么问题？

//put any vector points from ist into points
void get_vector_points(istream& ist, vector<Point>& points)
{
    //this function is bad, it doesn't do anything the second time you use it
    while(true)
    {
        Point p;
        if( ist >> p)
            points.push_back(p);
        else return;
    }
}
//
void write_to_file(string& path, vector<Point>& v_of_points)
{
    //open output file
    ofstream ofile(path);

    //output results to file
    for(Point p : v_of_points)
    {
        ofile << p <<endl;
    }

    //close output stream
    ofile.close();
}
void read_from_file(string& path, vector<Point>& v_of_points)
{
    //open input file
    ifstream ifile(path);

    //read from file : Here comes trouble
    get_vector_points(ifile, v_of_points);

    //But no problem if you do the following instead
    //    while(true)
    //    {
    //        Point p;
    //        if( ifile >> p)
    //            v_of_points.push_back(p);
    //        else break;
    //    }
}
//read point from specified istream ist, format:(0,0)
//output points from specified ostream ost, format:(0,0)
void points_drill(string path)
{
    //read from console a vector of points
    vector<Point> original_points;
    get_vector_points(cin, original_points);

    //write the same vector to file
    write_to_file(path,original_points);

    //read the same vector from file, but call it processed
    vector<Point> processed_points;
    read_from_file(path,original_points);

    //they should be the same
    for (int i= 0; i < original_points.size(); i++)
    {
        if (original_points.size()!=processed_points.size())
            throw runtime_error("Size Mismatch"); //error here and proccesed_point has size of 0
        if (original_points[i]!=processed_points[i])
            throw runtime_error("something is wrong!");
    }
}

请注意，我使用的是自定义点。完整代码在http://pastebin.com/ZvfcZxf3

对编码风格/可读性的任何评论表示赞赏。

Answer 1

电话

read_from_file(path,original_points);

正在读入向量original_points，而不是processed_points

Answer 2

您应该尝试不同的方式来读取文件：

void get_vector_points(istream& ist, vector<Point>& points)
{
   std::string line;
   std::string first, second;
   while(std::getline(ist,line)) 
   // this should get every line in the file as a string.
   // if the formatting of the file is 2 doubles per line.
   // You will need to do the parsing yourself by grabbing each value
   {
   // example first line:    123   321
    first = line.split[0]; // this is not a real function this is just Psudo code
    //first = "123"
    second = line.split[1];// for parsing text and splitting into 2 doubles
    // second = "321"
    Point p;
    p.x = atoi(first); // atoi is = ascii to int.  Makes a string intager into a 
    p.y = atoi(second);// actual int data type
    // p.x = 123   p.y = 321
    points.push_back(p)
    else return;
   }
}

Answer 3

此外，您的函数get_vector_points 会将输入流置于失败状态。您可以检查 EOF 并读取分隔符（空格）以避免它。

这里是整数：

#include <iostream>
#include <sstream>
#include <vector>

std::istream& get_integers(std::istream& stream, std::vector<int>& integers)
{
    int n;
    // Read integers and trailing white spaces.
    while( ! stream.eof() && stream >> n) {
        integers.push_back(n);
        stream >> std::ws;
    }
    return stream;
}

int main()
{
    std::istringstream input(" 1 2 3 ");
    std::vector<int> integers;
    // Read and test the result:
    if( ! get_integers(input, integers)) std::cout << "Failure\n";
    else {
        for(int i : integers)
            std::cout << i;
        std::cout << '\n';
    }
}