C ++：从txt文件读取和写入的成员函数

Question

I apologize in advance for how poorly this question is asked, I'm really struggling here. I am writing a class named Point in C++ with private members x and y, and member functions getX, getY, setX, setY, read and write. I have been able to do everything except read and write, as I am awful with input/output files. I have the following declaration for read and write:

void read(istream& ins);
void write(ostream& outs);

The RME is as follows for read:

* Requires: ins is in good state.
* Modifies: ins, x, y.
* Effects:  Reads point in form (x,y)

and for write:

* Requires: outs is in good state.
* Modifies: outs.
* Effects:  Writes point in form (x,y).

'read' takes ordered points like (1, 5), (2, 7), etc. from a given file "data1.txt" and extracts the x and y components (at least, I believe this is what it should do). I was provided with a test suite for read:

void test_point() {
Point pt1;

pt1.setX(15);

cout << "pt1 is: " << pt1 << endl;

ifstream input_file;
input_file.open("data1.txt");
pt1.read(input_file);
cout << "pt1 is: " << pt1 << endl;

return;}

I really have no idea how to write the read function. I have tried defining characters a, b, c, and integers u, v, and executing:

ins >> a >> u >> b >> v >> c;

but that didn't work. Could someone please help me see how to implement this?

Answer 1

Quite a few things missing from your question, that you will need such that the use of this class to be viable. For one, reading a file of ordered points should not be implemented as a member function. If anything, you could use a loop as such:

    while(input_file) {
        // set a point to have members x, y that were read from file
        // store this point in a vector of points
    }

Otherwise there is no reasonable way to store a bunch of points that you have read from a file.

Your idea for this solution within the read function should definitely work (assuming chars a, b, c, and ints u, v):

    input_stream >> a >> u >> b >> v >> c;

In fact, with the format that you have given us (taking an ifstream as an argument), there really is no need for be much else in the read function, as it mutates the object and doesn't need to return anything. Your implementation is the simplest way to parse such "records" in a structured file.

After you read all these, set the x coordinate of the caller object to u, and the y to v. This should be a void function, does not need to return anything but should alter the Point object that it is being called for. This member function should be called on a temporary point object in the loop that I mentioned, then add that object to a vector of points as such:

    vector<Point> points;
    while(...) {
        // declare Point
        // initialize with values from read
        points.push_back(//the point you just created);
    }

If in fact you need to be able to read multiple points.

In summary, your read function needs:

1. To check that the ifstream is actually good first of all, which I didn't mention yet (but this is one of your requirements):

    if (!input_file) { //however you want to handle this error } 

2. The temporary char and int variables (serving as buffers) to read into (by the way, you can even read straight into x and y instead of reading into u and v then copying, just saying).

3. If you choose to not read straight into x and y, you must assign the u and v values to x and y. Now your object is complete.

As for the write function, instead of std::cout you will use the name of the ofstream that was passed as an argument to the write function, and write the records with the format that you have shown. This is (in essence) no different from printing to a console, except the output is on a text file.

Note: Make sure you understand the difference between iostream objects (istream, ostream) and fstream (ifstream, ofstream, fstream) objects, which are preferable in this case.