Pointer in C++ copy constructor [duplicate] - c++

Question

This question already has an answer here:
C++ Deep copy of dynamic array through assignment operator
(1 answer)
Closed last year.
I try to learn copy constructors principle.
I have this code and I need to COPY data to testB data type.
Right now, I only copy pointers to previous memory but I need to copy all data.
Can you help me?
#include <cstdio>
#include <stdio.h>
struct Test {
int i, *p;
Test(int i): i(i), p(new int[i]){
p[0] = 1;
p[1] = 2;
p[2] = 3;
}
~Test() {
}
};
struct TestB : Test {
TestB(const Test &test) : Test(test){
p = new int[3];
p[0] = test.p[0];
p[1] = test.p[1];
p[2] = test.p[2];
i = test.i;
}
};
int main(){
Test test(3);
TestB b = test;
return 0;
}

Answer 1

First off, what you are trying to implement is a converting constructor, not a copy constructor, since you are trying to convert a Test object into a TestB object, rather than create a copy of a TestB object from another TestB object.
Since the array exists in Test, you should implement an actual copy constructor (and a copy assignment operator) in Test, and then TestB's converting constructor can call Test's copy constructor. Test's copy constructor can then allocate a new array that has the same number of items as the array being copied, and copy the values from the original array into the new array, eg:
#include <utility>
struct Test {
int i, *p;
Test(int i): i(i), p(new int[i]) {
for(int j = 0; j < i; ++j)
p[j] = j+1;
}
Test(const Test &src): i(src.i), p(new int[src.i]) {
for(int j = 0; j < i; ++j)
p[j] = src.p[j];
}
~Test() {
delete[] p;
}
// don't forget an assignment operator too, per the Rule of 3:
// https://en.cppreference.com/w/cpp/language/rule_of_three
Test& operator=(const Test &rhs) {
if (this != &rhs) {
Test tmp(rhs);
std::swap(i, tmp.i);
std::swap(p, tmp.p);
}
return *this;
}
};
struct TestB : Test {
TestB(const Test &test) : Test(test) {}
};
int main(){
Test test(3);
TestB b = test;
return 0;
}
That being said, you should use std::vector instead of new[], then you get all of the array management for free since std::vector already implements everything for you, eg:
#include <vector>
struct Test {
std::vector<int> p;
Test(int i): p(i) {
for(int j = 0; j < i; ++j)
p[j] = j+1;
}
};
struct TestB : Test {
TestB(const Test &test) : Test(test) {}
};
int main(){
Test test(3);
TestB b = test;
return 0;
}