C++ remove_if on a vector of objects

问题:

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?

回答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.

回答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());