C++ High Performance

The capture block

Let's make this a little more advanced. In the previous example, we hard coded the value we wanted to count numbers above. What if we want to use an external variable inside the lambda instead? What we do is capture the external variables by putting them in the capture block, that is, the [] part of the lambda:

auto count_value_above(const std::vector<int>& vals, int th) { 
  auto is_above = [th](int v) { return v > th; }; 
  return std::count_if(vals.begin(), vals.end(), is_above); 
} 

In this example, we captured the th variable by copying it into the lambda function, if we want to declare the th as a reference, we put a & at the beginning like this:

auto is_above = [&th](int v) { return v > th; };

The variable is now merely ...