C++ Abstract Classes and Inheritance - c++

Question

I have this problem I'm trying to solve. Basically the base class has the function map, which takes a vector as input and outputs the final vector after some mapping function, in this case - f, has been performed. However, I'm really lost as to why when I print out 2*testVector - test1 in the main function, I get proper output, i.e. 6, -182 etc... but when I print out 2*testVector - test 2, it's still the same vector.
This happens both when I create "DoubleElements" twice or just call the same "DoubleElements" pointer twice (it only ever performs 1 map). Am I fundamentally missing some understanding? Any help is appreciated!
#include <iostream>
#include <vector>
using namespace std;
class RecursiveBase {
public:
vector<int> map(vector<int> baseVector) {
static int iter = 0;
// Base case, return the final vector.
if (iter == 5) {
return baseVector;
// Replace the element with the old element mapped to the function.
} else {
baseVector[iter] = this->f(baseVector[iter]);
iter++;
return map(baseVector);
}
}
private:
virtual int f(int value) = 0;
};
class DoubleElements: public RecursiveBase {
private:
int f(int value) {
return 3*value;
}
};
int main() {
vector<int> testVector, o1, o2;
testVector.push_back(3);
testVector.push_back(-91);
testVector.push_back(-42);
testVector.push_back(-16);
testVector.push_back(13);
DoubleElements de;
DoubleElements de1;
RecursiveBase *test1 = &de;
RecursiveBase *test2 = &de1;
o1 = test1->map(testVector);
o2 = test2->map(testVector);
std::cout << "2*testVector - test1" << std::endl;
for (unsigned int iter = 0; iter < o1.size(); iter++) {
std::cout << o1[iter] << std::endl;
}
std::cout << "2*testVector - test2" << std::endl;
for (unsigned int iter = 0; iter < o2.size(); iter++) {
std::cout << o2[iter] << std::endl;
}
}

Answer 1

static int iter = 0;
You should avoid declaring local static variables in methods unless 100% necessary.
The first call will increment iter to 5, but on the next call, iter, since it's static, will not reset it's value to 0.
As an example, a simple program like:
void test()
{
static int x = 0;
++x;
cout << x << endl;
}
int main()
{
test();
test();
return 0;
}
Will output
1
2

Answer 2

From class.static.data/1:
A static data member is not part of the subobjects of a class.
For iter is static. It is part of the class RecursiveBase NOT part of the RecursiveBase objects.
To fix it, reset iter to 0:
if (iter == 5) {
iter = 0; // reset iter
return baseVector;
}
OUTPUT
2*testVector - test1
9
-273
-126
-48
39
2*testVector - test2
9
-273
-126
-48
39

Answer 3

You can only ever call RecursiveBase::map once as it stands, because the iter is static. You also assume that you will only ever call it with a 5 element std::vector<int>, at which point std::array<int, 5> is a better choice.
If you want a recursive solution, instead pass the index as an additional parameter
public:
std::vector<int> map(std::vector<int> vec) {
return do_map(vec, 0);
}
private:
std::vector<int> do_map(std::vector<int> & vec, std::size_t index) {
if (index == vec.size()) { return vec; }
vec[index] = f(vec[index]);
return do_map(vec, ++index);
}
But that's still a gratuitous use of recursion. A much better solution is
public:
std::vector<int> map(std::vector<int> vec) {
std::transform(vec.begin(), vec.end(), vec.begin(), [this](int i) { return f(i); });
return vec;
}
You also have superfluous RecursiveBase * in your main
int main() {
std::vector<int> testVector{3, -91, -42, -16, 13};
DoubleElements de;
DoubleElements de1;
// declare at point of initialisation
// don't need ->
auto o1 = de.map(testVector);
auto o2 = de1.map(testVector);
std::cout << "2*testVector - test1" << std::endl;
for (unsigned int iter = 0; iter < o1.size(); iter++) {
std::cout << o1[iter] << std::endl;
}
std::cout << "2*testVector - test2" << std::endl;
for (unsigned int iter = 0; iter < o2.size(); iter++) {
std::cout << o2[iter] << std::endl;
}
return 0;
}