Assignment of data-member in read-only structure, class in STL set

Question

The minimal example of the problem I\'m having is reproduced below:

#include 
using namespace std;

class foo {
public:
  int value, x;
  foo(const in         


        

Answer 1

Objects in a set are immutable; if you want to modify an object, you need to:

1. make a copy of the object from the set,
2. modify the copy,
3. remove the original object from the set, and
4. insert the copy into the set

It will look something like this:

std::set<int> s;
s.insert(1);

int x = *s.begin(); // (1)
x+= 1;              // (2)
s.erase(s.begin()); // (3)
s.insert(x);        // (4)

Answer 2

From the definition of the operator< (i.e. considering only the value return value < rhs.value and ignoring the x), I am wondering whether you want a map instead of a set. In map, the second value is mutable.

Answer 3

Given that the "x" variable is not involved in the less-than comparison, it would be safe in this case to make "x" mutable, allowing you to modify it from within the set. Your class definition would then become:

class foo {
public:
  int value;
  mutable int x;

  foo(const int & in_v) : value(in_v), x(0) { }
  bool operator<(const foo & rhs) const {
    return value < rhs.value; 
  }
};

And you can now use it in the std::set and modify x as you like. In this case it is pointless to keep two copies of the data structure as the previous poster has suggested.