How to copy vector of struct pointer to another vector of struct pointer?

Question

Hi,

How to copy vector of struct pointer to another vector of struct pointer?

Thanks

Answer 1

By way of illustrating the differences, study this example.

Note that when using a shallow copy via assignment of one
vector to another, changes to structs pointed to in one
vector will appear in the other vector as well. Both
vectors have pointers to the same structs.

By contrast, when using a deep copy which allocates new
structs and copies the contents from the structs in the
original vector changes to structs in one vector do
not affect the contents of the structs in the other vector.

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

using namespace std;

struct S {
    int id;
    string name;
};

void ListVect(vector<S*>& v)
{
    for (size_t n = 0; n < v.size(); ++n)
    {
        cout << v[n]->id << "  " << v[n]->name << endl;
    }
}

int main()
{
    vector<S*> vsp1;
    vector<S*> vsp2;
    S *s1 = new S{1, "Name1"};
    S *s2 = new S{2, "Name2"};
    vsp1.push_back(s1);
    vsp1.push_back(s2);

    cout << "Shallow copy:\n";
    ListVect(vsp1);
    vsp2 = vsp1;
    ListVect(vsp2);

    cout << endl;

    vsp1[0]->id = 3;
    vsp1[0]->name = "Name3";
    ListVect(vsp1);
    ListVect(vsp2);

    cout << "\nDeep copy:\n";
    vsp2.clear();
    for (size_t n = 0; n < vsp1.size(); ++n)
    {
        S *s = new S{ vsp1[n]->id, vsp1[n]->name };
        vsp2.push_back(s);
    }
    ListVect(vsp1);
    ListVect(vsp2);
    cout << endl;

    vsp1[0]->id = 4;
    vsp1[0]->name = "Name4";
    ListVect(vsp1);
    ListVect(vsp2);

    std::cout << "Press any key and Enter to end.";
    char c;
    cin >> c;
}

Output:

Shallow copy:
1 Name1
2 Name2
1 Name1
2 Name2

3 Name3
2 Name2
3 Name3
2 Name2

Deep copy:
3 Name3
2 Name2
3 Name3
2 Name2

4 Name4
2 Name2
3 Name3
2 Name2
Press any key and Enter to end.

• Wayne

Answer 2

Taken literally, the answer to your question is that you just assign the object:

vector2 = vector1;

However, your question may be a loaded one since you refer to a "vector of struct pointer", which I presume means you have a vector of pointers to struct?

If so, the question then needs to specify whether you want to copy each structure or not.

Answer 3

As David noted, you need to be more specific about what you want to
see in the second vector. Do you want to do a shallow copy - in which
the second vector will hold the same pointers as the first? Or do you
want to do a deep copy - in which case the second vector will hold
pointers to new structs which will have initially the same contents as
the first vector?

If you do a shallow copy as in David's example using a simple
assignment, then the pointers in both vectors will point to
the exact same structs. If you later alter the contents of one of
those structs in one of the vectors, the change will be reflected
in the other vector as well.

If you do a deep copy then the second vector will have pointers
to different structs than the first vector, but those new structs
will be initialized with the contents of the structs in the first
vector. However, as the pointers in each vector will point to
different structs, altering a struct pointed to in one vector
will not affect the contents of the structs pointed to in the
other vector.

• Wayne