Vector+-OpLoad, C++ (gcc)

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//g++  7.4.0
//Vector : overloading addition and subtraction operator
//code credits for default constructor,copy constructor,stream insertion and extraction operators,destructor,assignment operator go to John R. Hubbard; codes for addition and subtraction operators and test driver are created by Rezaul Hoque on September 08,2021; contact: jewelmrh@yahoo.com

//note: codes shared by Rezaul Hoque on rextester are not for sale; they are created and shared to facilitate the algorithm learning process; many like Hoque use this platform to practice programming ;Rezaul hopes his contribution helps others to fine tune their learning;

#include <iostream>
class Vector {

friend std::ostream& operator<<(std::ostream &, const Vector &);//stream extraction operator
friend std::istream& operator>>(std::istream &, Vector &);//stream insertion operator
friend Vector operator+(const Vector &, const Vector&);//binary addition operator
friend Vector operator-(const Vector &, const Vector&);//binary subtraction operator
public:
Vector (int ,double );//default constructor
Vector (const Vector &);//copy constructor
~Vector();//destructor
const Vector & operator=(const Vector &);//assignment operator
double& operator[](int) const;//subscript operator

int size;
double * data;
};

std::ostream& operator<<(std::ostream& ostr,const Vector& v)
{
int l;
ostr<<"( ";
for(l=0;l<v.size-1;l++){
ostr<<v[l]<<",";
if((l+1)%8 == 0)  std::cout<<"\n"; }
return ostr<<v[l]<<")\n";
}
std::istream& operator>>(std::istream& istr,const Vector& v)
{
for(int l=0;l<=v.size-1;l++)
{
std::cout<<l<<":";
istr>>v[l];
}
return istr;
}

Vector::Vector(int sz=1,double t=0.0): size(sz)
{
data = new double[size];
for(int l=0;l<size;l++)
data[l]=t;
}
Vector:: Vector(const Vector & v): size(v.size)
{
data = new double[v.size];
for(int l=0;l<v.size;l++)
data[l]=v.data[l];
}
Vector::~Vector()
{
delete [] data;
data = NULL;
size=0;
}

const Vector& Vector::operator=(const Vector & v)
{
if(&v != this)
  {
 delete [] data;
size = v.size;
data = new double[size];
for(int l=0;l<size;l++)
data[l]=v.data[l];
}
return *this;
}

double& Vector:: operator[](int I) const
{
return data[I];
}
Vector operator+(const Vector & v,const Vector& w)
{  Vector z(4,0.0);

for(int l=0;l<z.size;l++){
z.data[l] = v.data[l] + w.data[l];}
return z;
}
Vector operator-(const Vector & v,const Vector& w)
{  Vector z(4,0.0);

for(int l=0;l<z.size;l++){
z.data[l] = v.data[l] - w.data[l];}
return z;
}

int main()
{
Vector vec1(4,3.0),vec2(4,2.0),vec3(4),vec4(4);
vec3=vec1+vec2;
std::cout<<" vec3:\n";
std::cout<<vec3;
std::cout<<" vec4:\n";
vec4=vec1-vec2;
std::cout<<vec4;
return 0;
}

Show compiler warnings [ + ] Compiler args [ + ] Show input
Compilation time: 0.76 sec, absolute running time: 0.24 sec, cpu time: 0.02 sec, memory peak: 5 Mb, absolute service time: 1,14 sec