c ++将成员函数的输出写入文本文件

Question

I am trying to write the output of a member function of my class to a text file. I cannot seem to get my output overload operator to behave as I wish. I naively use unidentified parameters in the line

 outStream << myClass.myMemberFunction(x1, x2, results)

because I still haven't found any method that works without changing anything about myMemberFunction.

Here's an example:

header file

proper include guards


class myClass {

public:

bool myMemberFunction( int& x1, int& x2, std::vector<int> results);


friend ostream &operator<< (ostream& out, myClass& Class)


};

then in

classDef source file

proper include files

using namespace std;
using std::vector;

bool myClass::myMemberFunction(int& x1, int& x2, vector<int> results) {

int x3;
x3 = x1 + x2;
results.push_back(x3);

return true;
};

myClass& operator<< (ostream& out, myClass& myClass) {

ofstream outStream;

outStream.open("emptyFile.txt", ios::app);

if (outStream.is_open()) {
    outStream << myClass.myMemberFunction(x1, x2, results);

important part here is that I want to output the values that are stored in the results vector

    outStream.close();
}
else throw "Unable to open file";
}

Is there any way to do this without changing myMemberFunction?

Answer 1

ofstream& operator<< (ofstream& out, const myClass& instance) {
    std::vector<int> results;
    instance.myMemberFunction(x1, x2, results); // x1 and x2 need to be defined
    for(int i : results) {
       out << i;
    }
    return out;
}

You need to create the file etc. somewhere else

myClass classObject; // Some instance of myClass you want to output
ofstream outStream;

outStream.open("emptyFile.txt", ios::app);

if (outStream.is_open()) {
    outStream << classObject; // You can output an instance of your class now
    outStream.close();
}
else throw "Unable to open file";

You also need to update the declaration of operator<< in the header file to return an ostream& instead of myClass& . What you precisely do is overload the stream operator for your class. So when you use it with a stream this method gets called and your implementation determines what happens with the stream when you want to output an instance of your class. So you are not supposed to open a file there. Just output the return value of your member function to the stream and return it.

EDIT: You will also have to change the signature of your member function to pass the vector by reference (otherwise you fill a copy). The ints doesn't need to be passed by reference.