how do I use STL algorithms with a vector of pointers

Question

I have a vector of pointers that are not owned by the container. How do I use algorithms on the targets of the pointers. I tried to use boost's ptr_vector, but it tries to delete the pointers when it goes out of scope.

Here is some code that needs to work:

vector<int*> myValues;
// ... myValues is populated
bool consistent = count(myValues.begin(), myValues.end(), myValues.front()) == myValues.size();
auto v = consistent ? myValues.front() : accumulate(myValues.begin(), myValues.end(), 0) / myValues.size();
fill(myValues.begin(), myValues.end(), v);
// etc.

I realize that for loops would work, but this happens in a bunch of places, so some kind of unary adapter? I wasn't able to find one. Thanks in advance!

Answer 1

You can look at boost::shared_ptr<> - a smart pointer with reference counting. It will not delete the pointer after it goes out of scope.

Answer 2

You could use Boost Indirect Iterator. When dereferenced (with operator*() ), it applies an extra dereference, so you end up with the value pointed by the pointer referenced by the iterator. For more information, you can also see this question about a dereference iterator.

Here's a simple example:

std::vector<int*> vec;

vec.push_back(new int(1));
vec.push_back(new int(2));

std::copy(boost::make_indirect_iterator(vec.begin()),
          boost::make_indirect_iterator(vec.end()),
          std::ostream_iterator<int>(std::cout, " "));     // Prints 1 2

Answer 3

bool consistent = count_if(myValues.begin(), myValues.end(), 
   bind2nd(ptr_fun(compare_ptr), *myValues.front())) == myValues.size();

int v = consistent ? *myValues.front() : accumulate(
   myValues.begin(), myValues.end(), 0, sum_int_ptr) / myValues.size();

for_each(myValues.begin(), myValues.end(), bind1st(ptr_fun(assign_ptr),v));

Fill can't take assign function (so that it would dereference pointers). Therefore for_each() was used. For optimization it would be wise to add if(!consistent) before running for_each(). Functions used in above STL one liners:

int sum_int_ptr(int total, int * a) { return total + *a; }    
void assign_ptr(int v, int *ptr) { *ptr = v; }    
bool compare_ptr(int* a, int pattern) { return *a == pattern; }