Question

I’m working my way through Accelerated C++ and have decided to mess around with the one of structs that were defined in there. While doing so, I’ve come across a problem: creating a vector of these structs and modifying the elements in each one seems to modify the elements in all of them.

I realize that this probably means I’ve initialized all the structs in the vector to a struct at a single memory address, but I used the .push_back() method to insert “dummy” structs in to the vector. I was under the impression that .push_back() pushes a copy of its argument, effectively creating a new struct.

Here is the header for the struct:

#ifndef _STUDENT_INFO__CHAPTER_9_H
#define _STUDENT_INFO__CHAPTER_9_H

#include <string>
#include <iostream>
#include <vector>

class Student_info9{
public:
    Student_info9(){homework = new std::vector<double>;};
    Student_info9(std::istream& is);

    std::string getName() const {return name;};
    double getMidterm() const {return midterm;};
    double getFinal() const {return final;};
    char getPassFail() const {return passFail;};

    std::vector<double> *getHw(){return homework;};

    void setName(std::string n) {name = n;};
    void setMidterm(double m) {midterm = m;};
    void setFinal(double f) {final = f;};


private:
    std::string name;
    double midterm;
    double final;
    char passFail;

    std::vector<double> *homework;
};


#endif  /* _STUDENT_INFO__CHAPTER_9_H */

And here is the code that i’m fooling around with (excuse the excessive print statements… the result of some time trying to debug 🙂 ):

vector<Student_info9> did9, didnt9;

bool did_all_hw9(Student_info9& s)
{
    vector<double>::const_iterator beginCpy = s.getHw()->begin();
    vector<double>::const_iterator endCpy = s.getHw()->end();
    return(find(beginCpy, endCpy, 0) == s.getHw()->end());
}

void fill_did_and_didnt9(vector<Student_info9> allRecords)
{
    vector<Student_info9>::iterator firstDidnt = partition(allRecords.begin(), allRecords.end(), did_all_hw9);


    vector<Student_info9> didcpy(allRecords.begin(), firstDidnt);


    did9 = didcpy;

    vector<Student_info9> didntcpy(firstDidnt, allRecords.end());
    didnt9 = didntcpy;


}

int main(int argc, char** argv) {

    vector<Student_info9> students;

    Student_info9 record;

    for(int i = 0; i < 5; i++)
    {
        students.push_back(record);
    }

    for(int i = 0; i < students.size(); i++)
    {
        students[i].setMidterm(85);
        students[i].setFinal(90);

        students[i].getHw()->push_back(90);
        std::cout << "student[" << i << "]'s homework vector size is " << students[i].getHw()->size() << std::endl;
        students[i].getHw()->push_back(80);
        std::cout << "student[" << i << "]'s homework vector size is " << students[i].getHw()->size() << std::endl;
        students[i].getHw()->push_back(70);
        std::cout << "student[" << i << "]'s homework vector size is " << students[i].getHw()->size() << std::endl;

        std::cout << "Just pushed back students[" << i << "]'s homework grades" << std::endl;

        if(i == 3)
            students[i].getHw()->push_back(0);
    }

    std::cout << "student[3]'s homework vector size is " << students[3].getHw()->size() << std::endl;

    for(vector<double>::const_iterator it = students[3].getHw()->begin(); it != students[3].getHw()->end(); it++)
        std::cout << *it << " ";

    std::cout << std::endl;

    std::cout << "students[3] has " << ( ( find(students[3].getHw()->begin(),students[3].getHw()->end(), 0) != students[3].getHw()->end()) ? "atleast one " : "no " )
            << "homework with a grade of 0" << std::endl;

    fill_did_and_didnt9(students);


    std::cout << "did9's size is: " << did9.size() << std::endl;
    std::cout << "didnt9's size is: " << didnt9.size() << std::endl;

}

As you can see by the print statements, it seems that the homework grades are being added only to one Student_info9 object, copies of which seem to be populating the entire vector. I was under the impression that if you were to use consecutive copies of .push_back() on a single object, it would create copies of that object, each with different memory addresses.

I’m not sure if that’s the source of the problem, but hopefully someone could point me in the right direction.

Thanks.

Answer 1

When you push a StudentInfo onto the vector, it is indeed copied, so that’s not the problem. The problem is the vector containing the homework grades. Since you only store a pointer to that vector in StudentInfo, only the pointer, not the vector, is copied when you copy a StudentInfo. In other words you have many different StudentInfos that all have a pointer to the same homework vector.

To fix this you should define a copy constructor which takes care of copying the homework vector.