Using for_each to modify std containers (even though you shouldn't)

Question

I'm taking a self-study course for C++, learning how the Standard Library works, and I want to understand how this code that uses for_each works, particularly regarding mutating objects (as opposed to native data types). I realize that you shouldn't use for_each this way, but this is for the purpose of learning.

I had thought this code would mutate all the elements in the set, but it doesn't.

My question is: 1. Why doesn't this code mutate the set? 2. How can the code be altered so that it will modify the set? To clarify: is there a way to keep the for_each and have it manipulate the set, or is this not possible and some other method (such as transform) has to be used?

Code

#include <iostream>
#include <algorithm>
#include <set>
using namespace std;

class A {
    int a;
public:
    A(int a) : a(a) {}
    int getA() const { return a; }
    void setA(int a) { this->a = a; }
    bool operator<(const A & b) const { return a<b.a; }
};

struct myprinter { 
    void operator()(const A & a) { cout << a.getA() << ", "; }  
};

struct doubler {
    void operator()(A a) { a.setA(a.getA()*2); }
};

int main() {
    int mynumbers[] = {8, 9, 7, 6, 4, 1};
    set<A> s1(mynumbers, mynumbers+6);
    for_each(s1.begin(), s1.end(), doubler()); //<-- Line in question
    for_each(s1.begin(), s1.end(), myprinter());
    return 0;
}

//Expected output: 2, 8, 12, 14, 16, 18
//Actual output: 1, 4, 6, 7, 8, 9,

What I've tried so far

• At first I thought the problem was that doubler was taking the parameter by value and not by reference, so it wasn't saving the changes to the set. But when I change the signature to be void operator()(A & a), I get the error of:

  error: no match for call to '(doubler) (const A&)
  ' __f(*__first);
    ~~~^~~~~~~~~~
  error: binding 'const A' to reference of type 'A&' discards qualifiers
  

  I deducted that that line being pointed out is from the internal implementation of for_each. I cannot make the parameter a const ref, because I am trying to change the a value using the setA() method, so it cannot be const.

• If I change the set to be a vector instead, then when I changed the signature of doubler to take a reference, it successfully doubled the elements in the container. Why would it not work if the container is a set instead?

Edit: moooeeeep linked to another question that shows how to edit each element of a set. This is a practical solution to my problem, but my question is more theoretical - why can you not edit sets using for_each, where you can edit vectors and other stl containers?

Answer 1

Because a std::set manages the order of it's elements, it prohibits the user to change it's elements through it's iterators. Which means it's begin() and end() methods return a const_iterator. You're only allowed to read the element pointed to by that iterator, not modify it (it's const) which is what doubler() is trying to do.

A solution would be to just use std::vector and std::sort to order it yourself:

#include <iostream>
#include <algorithm>
#include <vector>

class A {
    int a;
public:
    A(int a) : a(a) {}
    int getA() const { return a; }
    void setA(int a) { this->a = a; }
    bool operator<(const A & b) const { return a<b.a; }
};

struct myprinter { 
    void operator()(const A & a) { cout << a.getA() << ", "; }  
};

struct doubler {
    void operator()(A& a) { a.setA(a.getA()*2); } // by reference!
};

int main() {
    int mynumbers[] = {8, 9, 7, 6, 4, 1};
    std::vector<A> s1(mynumbers, mynumbers+6);
    std::sort(s1.begin(), s1.end());
    std::for_each(s1.begin(), s1.end(), doubler());
    std::for_each(s1.begin(), s1.end(), myprinter());
    return 0;
}

Answer 2

The problem is that you are not allowed to modify elements in a std::set. If it were possible, then how would it handle something like this:

std::set<int> my_set { 1, 2, 3 };
int& foo = *(my_set.begin());
foo = 2;

Now there is two elements with value 2. That doesn't make sense in a set due to

std::set is an associative container that contains a sorted set of unique objects of type Key.

(emphasis mine)

http://en.cppreference.com/w/cpp/container/set

Answer 3

Because the contents of an object in an std::set determines its position. That's why std::set iterators are always const.

Of course, it is often the case that not all members of an object have an effect on its position. A possible workaround is then to declare those members mutable. Then you can modify them using const references.

To modify a member that does affect the object's position in a set, remove the object from the set, modify it, and add it back in.