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

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 
#include 
#include 

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 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;
}