Why is the destructor called?

Why is the destructor called?

I wrote a simple vector class which overloads assignment and addition
operators. The code adds two vectors and prints vectors before and after
the addition. Surprisingly destructor ~MyVector() is being called after
the line c=a+b. I do not see the reason why is it so, as neither of
objects a,b and c gets out of scope. Do you understand the reason for
invocation of the destructor?
#include<iostream>
#include<cstdlib>
using namespace std;
template<typename T>
class MyVector
{
public:
MyVector(int s)
{
size=s;
a=new T[s];
}
~MyVector()
{
delete[] a;
size=0;
};
void freeVec()
{
cout<<"Calling destructor. a[0]= "<<a[0]<<endl;
if(a!=NULL)
{
free(a);
a=NULL;
}
}
int getSize(void)
{
return size;
}
void setSize(int ss)
{
size=ss;
}
T &operator[](int i)
{
return a[i];
}
MyVector &operator=(MyVector mv)
{
if(this==&mv)
return *this;
for(int i=0;i<mv.getSize();i++)
this->a[i]=mv[i];
this->size=mv.getSize();
return *this;
}
MyVector operator+(MyVector &mv)
{
for(int i=0;i<size;i++)
{
this->a[i]+=mv[i];
}
cout<<endl;
return *this;
}
private:
int size;
T *a;
};
int main(void)
{
MyVector<int> a(3),b(3),c(3);
a[0]=1;a[1]=2;a[2]=3;
b[0]=4;b[1]=5;b[2]=6;
cout<<"initial vector"<<endl;
for(int i=0;i<3;i++)
cout<<a[i]<<" ";
cout<<endl;
for(int i=0;i<3;i++)
cout<<b[i]<<" ";
cout<<endl;
c=a+b;
cout<<"final vectors"<<endl;
for(int i=0;i<3;i++)
cout<<a[i]<<" ";
cout<<endl;
for(int i=0;i<3;i++)
cout<<b[i]<<" ";
cout<<endl;
for(int i=0;i<3;i++)
cout<<c[i]<<" ";
cout<<endl;
cout<<endl;
return 0;
}