question about template class

[breadbread1984]

Why the following code can't be compiled? thanks!

#include <cassert>
#include <iostream>

using namespace std;

template<typename T>
class Vector {
	T * data;
	int sz;
public:
	Vector(int s) {
		data = new T[s]();
		sz = s;
	}
	template<typename N> Vector(const Vector<N> & t) {
		int * tmp = new T[sz]();
		for(int i = 0 ; i < sz ; i++) tmp[sz] = t.data[i];
		delete[] data;
		data = tmp;
	}
	T & operator()(int i) {
		assert(0 <= i && i < sz);
		return data[i];
	}
	template<typename N> Vector<T> & operator=(const Vector<N> & t) {
		int * tmp = new T[sz]();
		for(int i = 0 ; i < sz ; i++) tmp[sz] = t.data[i];
		delete[] data;
		data = tmp;
		return *this;
	}
	template<typename T1,typename T2,typename T3> friend Vector<T3> f(const Vector<T1> & a,const Vector<T2> & b);
};

template<typename T1,typename T2,typename T3>
Vector<T3> f(const Vector<T1> & a,const Vector<T2> & b) {
	assert(a.sz == b.sz);
	Vector<T3> retVal(a.sz);
	for(int i = 0 ; i < a.sz ; i++) retVal.data[i] = a.data[i] + b.data[i];
	return retVal;
}

int main()
{
	Vector<int> t1(3),t2(3),t3(3);
	for(int i = 0 ; i < 3 ; i++) {
		t1(i) = i; t2(i) = i;
	}
	t3 = f(t1,t2);
	return 0;
}

[coder777]

Try this:

Line 32:
template<typename T1,typename T2,typename T3> friend Vector<T1> f(const Vector<T2> & a,const Vector<T3> & b);

Line 35:

template<typename T1,typename T2,typename T3>
Vector<T1> f(const Vector<T2> & a,const Vector<T3> & b) {

Line 49:
t3 = f<int>(t1,t2);

The problem in your code is that the compiler can not resove T3 hence you'd provide all template parameter until T3: t3 = f<int, int, int>(t1,t2);

[breadbread1984]

Thanks for your reply. I restricted the type of the return value. And again met a error when running the program. The error is

*** glibc detected *** ./a.out: double free or corruption (fasttop): 0x0000000000c19050 ***

the code is

#include <cassert>
#include <iostream>

using namespace std;

template<typename T>
class Vector {
	T * data;
	int sz;
public:
	Vector() : data(NULL),sz(0) {
	}
	Vector(int s) {
		data = new T[s]();
		sz = s;
	}
	~Vector() {
		if(data) delete[] data;
	}
	template<typename N> Vector(const Vector<N> & t) {
		int * tmp = new T[sz]();
		for(int i = 0 ; i < sz ; i++) tmp[sz] = t.data[i];
		if(data) delete[] data;
		data = tmp;
	}
	T & operator()(int i) {
		assert(0 <= i && i < sz);
		return data[i];
	}
	template<typename N> Vector<T> & operator=(const Vector<N> & t) {
		int * tmp = new T[sz]();
		for(int i = 0 ; i < sz ; i++) tmp[sz] = t.data[i];
		delete[] data;
		data = tmp;
		return *this;
	}
	template<typename T1,typename T2> friend Vector<T1> f(const Vector<T1> & a,const Vector<T2> & b);
};

template<typename T1,typename T2>
Vector<T1> f(const Vector<T1> & a,const Vector<T2> & b) {
	assert(a.sz == b.sz);
	Vector<T1> retVal(a.sz);
	for(int i = 0 ; i < a.sz ; i++) retVal.data[i] = a.data[i] + b.data[i];
	return retVal;
}

int main()
{
	Vector<int> t1(3),t2(3),t3;
	for(int i = 0 ; i < 3 ; i++) {
		t1(i) = i; t2(i) = i;
	}
	t3 = f(t1,t2);
	return 0;
}

How does the double free happen? Thanks

[coder777]

It works fine for me

[Peter87]

In the copy assignment operator you never update sz so the old value is still used. tmp[sz] is out of bounds.

In the copy constructor you have similar problems. sz and data is never initialized.

[guestgulkan]

    template<typename N>
    Vector(const Vector<N> & t)
    {
        int * tmp = new T[sz]();
        for(int i = 0 ; i < sz ; i++) tmp[sz] = t.data[i];
        if(data) delete[] data;
        data = tmp;
    }
    
    
    template<typename N> 
    Vector<T> & operator=(const Vector<N> & t)
    {
        int * tmp = new T[sz]();
        for(int i = 0 ; i < sz ; i++) tmp[sz] = t.data[i];
        delete[] data;
        data = tmp;
        return *this;
    }

They bug me because the template parameters T and N are not necessarily the same - therefore you could end up could be trying to construct
a Vector<std::string> from a Vector<int> which makes no sense.
Also
int * tmp = new T[sz](); //really???