incorrect lambda usage in remove_if

Question

so I have a vector holding vectors like so:

vector<vector<int>> vec;

And I am trying to use a comparator (pred) to iterate through the vectors within vec and determine which vectors get deleted by remove_if and erase. My code so far is as follows:

vector<vector<int>> deleted_vecs;    

vec.erase(remove_if(vec.begin(), vec.end(),
                               ([vec, pred](vector<int> x) {
                                 if (pred(x)) {
                                     return true;
                                 }
                                 else {
                                     deleted_vecs.push_back(x);
                                     return false;
                                 }
                               },
                               vec.end())));

Is there something wrong I am doing syntax-wise here?

My compiler errors are on the lines "if (pred(x))" and "deleted_vecs.push_back(x);" and say there is no matching function call to pred and push_back, although my predicate is specifically intended to take in a vector and I've declared deleted_vecs as a vector>. This leads me to think it is a problem with how I've declared x, but I'm not sure.

Any help would be appreciated!

@JoachimPileborg oops, that was a typo in my code. However, when I fix it the problem still persists. — user313, Mar 25 '16 at 18:29
You *don't* need to capture `vec`, but you do need to capture `deleted_vecs`. By reference, or else you will add items to a copy of `deleted_vecs` and the rest of your program will never see it. — Ben Voigt, Mar 25 '16 at 18:39

score 0 · Accepted Answer · answered Mar 25 '16 at 18:35

You have an error with the parentheses as well as with the capture list.

You are capturing the used variables by value, which ends up in copying the objects. Try to capture the variables by reference, rather than by value. see http://en.cppreference.com/w/cpp/language/lambda

For example:

 vec.erase(
    std::remove_if(vec.begin(), vec.end(),
    [&](std::vector<int> x) {
      if (pred(x)) {
        return true;
      }
      else {
        deleted_vecs.push_back(x);
        return false;
      }
    }),
    vec.end());

incorrect lambda usage in remove_if

1 Answers1