C++ remove_if on a vector of objects

Question

I have a vector (order is important) of objects (lets call them myobj class) where I'm trying to delete multiple objects at a time.

class vectorList {      vector<*myobj> myList;  };  class myobj {      char* myName;     int index;     bool m_bMarkedDelete; } 

I was thinking that the best way to do this would be to mark specific myobj objects for deletion and then call myList.remove_if() on the vector. However, I'm not exactly sure how to use predicates and such for this. Should I create a member variable in the object which allows me to say that I want to delete the myobj and then create a predicate which checks to see if the member variable was set?

How do I implement the predicate as a part of the vectorList class?

Answer 1

Should I create a member variable in the object which allows me to say that I want to delete the myobj and then create a predicate which checks to see if the member variable was set?

Haven't you already done that? Isn't that what m_bMarkedDelete is for? You would write the predicate like this:

bool IsMarkedToDelete(const myobj & o) {     return o.m_bMarkedDelete; } 

Then:

myList.erase(     std::remove_if(myList.begin(), myList.end(), IsMarkedToDelete),     myList.end()); 

Or, using lambdas:

myList.erase(     std::remove_if(myList.begin(), myList.end(),         [](const myobj & o) { return o.m_bMarkedDelete; }),     myList.end()); 

If your class doesn't actually have that member, and you're asking us if it should, then I would say no. What criteria did you use to decide to mark it for deletion? Use that same criteria in your predicate, for example:

bool IndexGreaterThanTen(const myobj & o) {     return o.index > 10; } 

note -- The functions I've written are of course invalid since all your members are private. So you'll need some way to access them.

Answer 2

A predicate is basically a conditional comparison. It can be a function or object. Here's an example using new C++ lambdas. This code will go through the vector and remove the values equal to 3.

int arg[6] = {1, 2, 3, 3, 3, 5}; std::vector<int> vec(arg, arg+6); vec.erase(    std::remove_if(       vec.begin(), vec.end(),       [](int i){ return i == 3;}),    vec.end()); 

Edit: For pointers let's say you had a vector or interfaces you could set them to nullptr then remove them in a batch with pretty much the same code. In VS2008 you won't have lambdas so make a comparison predicate function or struct instead.

bool ShouldDelete(IAbstractBase* i) {     return i == nullptr;     // you can put whatever you want here like:     // return i->m_bMarkedDelete; }  std::vector<IAbstractBase*> vec; vec.erase(    std::remove_if(       vec.begin(), vec.end(),       ShouldDelete),    vec.end());