Why is this very simple constructor causing a seg fault?

Question

This is embarrassing, but I'm a little rusty on my C++ and for the life of me I can't see why this code is causing a segmentation fault. The weird this is that several iterations ago it seemed to work fine. I don't even know what I'm doing differently now.

It's basically a constructor for a template class object that is meant to hold three of the objects. I am getting a seg fault on the first line of the constructor's implementation (where I assign a to x*):

VecXd.hpp:

#ifndef JF_VecXd
#define JF_VecXd

#include <iostream>

template <class T>
class VecXd{
    public: 
        VecXd(T a, T b, T c);   

        VecXd(const VecXd & vector);

        T getElement(int n)const;
        void setElements(T a, T b, T c);
        void display();
        void VecXd<T>::clearElements();
        VecXd<T>& VecXd<T>::operator=(const VecXd& vector);

/*
        VecXd& VecXd::operator<<(const VecXd & vector);
        VecXd& VecXd::operator>>(const VecXd & vector);
        VecXd&  VecXd::operator+(const VecXd & vector);
        VecXd& VecXd::operator+=(const VecXd & vector);

        ~VecXd();
*/              

        private:
            T * x, T * y, T * z; 

};

template <class T> 
VecXd<T>::VecXd(T a, T b, T c){
    x = new T(a); 
    y = new T(b);
    z = new T(c);
}
//CVector& CVector::operator= (const CVector& param)


template <class T> 
VecXd<T>::VecXd(const VecXd & vector){
    x = new T(vector.getElement(0));
    y = new T(vector.getElement(1));
    z = new T(vector.getElement(2));
}

template <class T> 
VecXd<T>& VecXd<T>::operator=(const VecXd& vector){
    if(this != &vector){
        *x = vector.getElement(0);
        *y = vector.getElement(1);
        *z = vector.getElement(2);
    }
    return *this;
}

template <class T> 
T VecXd<T>::getElement(int n) const{
    n = n%3;
    T result;  
    switch(n){
        case 0: 
            result = *x;
            break;
        case 1:
            result = *y;
            break;
        case 2:
            result = *z;
            break;  
    }    
    return result;    
}

template <class T> 
void VecXd<T>::clearElements(){
    delete x; 
    delete y;
    delete z;    
}

template <class T>
void VecXd<T>::setElements(T a, T b, T c){
    clearElements(); 
    *x = a;
    *y = b;
    *z = c;    
}

template <class T>
void VecXd<T>::display(){
    std::cout << "x: " << x << "\n";
    std::cout << "y: " << y << "\n";
    std::cout << "z: " << z << "\n\n\n";
}

#endif

test.cpp:

#include "vecxd.hpp"
#include <iostream>

int main(){
    std::cout << "Creating vector 1..." << std::endl;
    VecXd<int> v1(1,2,3);
    std::cout << "Vector 1:" << std::endl; 
    v1.display();

    std::cout << "Vector 2 (copy-constructed from v1):" << std::endl;
    VecXd<int> v2(v1);
    v2.display();

    std::cout << "V1 set to 3,4,5:" << std::endl;
    v1.setElements(3,4,5);
    v1.display();

    std::cout << "V2 = V1, display V2" << std::endl;
 //   v2 = v1; 
    v2.display();


    system("pause");

    return 0;
}

I've tried a few variations on this including

x* = a;

x = new(a);

And I've tried having the function work like so:

VecXd(T & a, T & b, T & c);

But then it wouldn't let me call it by using:

VecXd<int>(1,2,3);

Thank you very much for your time, it is much appreciated!

This code doesn't compile in the first place, so obviously it can't segfault either. Please show us the real code. — syam, Sep 22 '13 at 16:56
`new` returns a pointer, and *x is asking for a value, for one thing. Can you just use `x = new T(a)`? — BrainSteel, Sep 22 '13 at 16:57
Why are you trying to assign an object to a de-referenced pointer? — OldProgrammer, Sep 22 '13 at 16:57
This isn't even *compilable*. `T * x, T * y, T * z;` is not a legal declaration. Post the *real* code please. And even if you fix that, `*x = new T(a);` dereferences whatever indeterminate value is in `x` (it is uninitalized) and attempts to store an allocation return address within it. — WhozCraig, Sep 22 '13 at 16:57

Mark Tolonen · Accepted Answer · 2013-09-22T17:03:08.307

2

You are dereferencing your member variables before assignment. Directly assign the new pointers to them:

template <class T> 
VecXd<T>::VecXd(T a, T b, T c){
    x = new T(a); 
    y = new T(b);
    z = new T(c);
}

Of course, you are going to leak this memory unless you have a destructor as well.

edited Sep 22 '13 at 17:03

answered Sep 22 '13 at 16:57

Mark Tolonen

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Thank you very much! I will post the full code next time. I naively thought I was being polite by trying to isolate the problem first; I didn't realize that that made it harder to solve. Thanks again. – SemperCallide Sep 22 '13 at 17:47

Why is this very simple constructor causing a seg fault?

1 Answers1