Question

Let’s have simplified class:

class A
{
   bool val_;
   public:
     A() : val_(true) {}
     bool isNew() const { return val_; }
     void setDirty() { val_ = false; }
};

and the vector of objects of such class:

vector<A> coll;
coll.push_back(A());
coll.push_back(A());
coll.push_back(A());
coll.push_back(A());
coll[1].setDirty();
coll[3].setDirty();

I need some elegant solution to rearrange(sort) elements in the vector, so that not modified objects will be grouped at the beginning of the sequence.

Answer 1

You can use Partition algorithm from standard library for that:

bool MyPredicate(A& a) { return a.isNew();}
...
// bound is iterator pointing to the first element for which predicate returns false
vector<A>::iterator bound = partition(coll.begin(), coll.end(), MyPredicate);

Or, as Christian Rau suggested solution without separate function:

std::partition(coll.begin(), coll.end(), std::mem_fun_ref(&A::isNew))