Standard containers encapsulation and range-based for loops - c++

Question

I'm designing a class which has two standard vectors as members. I would like to be able to use range-based for loops on the vector elements and I came up with this solution
#include <iostream>
#include <vector>
using namespace std;
class MyClass {
public:
void addValue1(int val){data1_.push_back(val);}
void addValue2(int val){data2_.push_back(val);}
vector<int> const & data1() const {return data1_;}
vector<int> const & data2() const {return data2_;}
// ...
private:
vector<int> data1_;
vector<int> data2_;
// ...
};
void print1(MyClass const & mc) {
for (auto val : mc.data1()){
cout << val << endl;
}
}
void print2(MyClass const & mc) {
for (auto val : mc.data2()){
cout << val << endl;
}
}
int main(){
MyClass mc;
mc.addValue1(1);
mc.addValue1(2);
mc.addValue1(3);
print1(mc);
}
Clearly, the alternative of defining begin() and end() functions doesn't make sense since I have two distinct vectors.
I would like to ask the following questions:
A shortcoming of the proposed solution is that the contents of the two vectors cannot be changed (due to the const qualifier). In the case I need to modify the vector elements how can I modify the code?
EDIT: the modification should preserve encapsulation
Considering data encapsulation, do you think it is bad practice to return a (const) reference to the two vectors?

Answer 1

Use something like gsl::span<int> and gsl::span<const int>.
Here is a minimal one:
template<class T>
struct span {
T* b = 0; T* e = 0;
T* begin() const { return b; }
T* end() const { return e; }
span( T* s, T* f ):b(s),e(f) {}
span( T* s, std::size_t len ):span(s, s+len) {}
template<std::size_t N>
span( T(&arr)[N] ):span(arr, N) {}
// todo: ctor from containers with .data() and .size()
// useful helpers:
std::size_t size() const { return end()-begin(); }
bool empty() const { return size()==0; }
T& operator[](std::size_t i) const { return begin()[i]; }
T& front() const { return *begin(); }
T& back() const { return *(std::prev(end())); }
// I like explicit defaults of these:
span() = default;
span(span const&) = default;
span& operator=(span const&) = default;
~span() = default;
};
now you can write:
span<int const> data1() const {return {data1_.data(), data1_.size()};}
span<int const> data2() const {data2_.data(), data2_.size()};}
span<int> data1() {return {data1_.data(), data1_.size()};}
span<int> data2() {data2_.data(), data2_.size()};}

Answer 2

A shortcoming of the proposed solution is that the contents of the two vectors cannot be changed (due to the const qualifier). In the case I need to modify the vector elements how can I modify the code?
First of all, you should add a data1() and a data2() not-const versions that return a reference to the data1_ and data2_ members
vector<int> const & data1() const {return data1_;}
vector<int> const & data2() const {return data2_;}
vector<int> & data1() {return data1_;}
vector<int> & data2() {return data2_;}
Second: if you want modify the element in print1() (by example) you have to receive mc as not const reference
// ..........vvvvvvvvv no more const
void print1 (MyClass & mc) {
so you can change mc.
Third: in the range based loop you have to define val as reference so you can modify it modifying also the referenced value inside the vector
// ........V by reference
for ( auto & val : mc.data1() ) {
++val ; // this modify the value in the vector inside mc
cout << val << endl;
}
Considering data encapsulation, do you think it is bad practice to return a (const) reference to the two vectors?
IMHO: if the reference is const, not at all: it's a good practice because permit the safe use of the member without the need to duplicate it.
If the reference isn't const, I don't see big difference with declaring the member public.