C++ Lambdas: capture list vs. parameter list

Question

According to the C++11 standard, lambda expressions may use variables in the enclosing scope, by means of the capture list, the parameter list or both.

So, let\'s look

Answer 1

The point is that with capture, you can keep a state (just as a hand written class with operator() ) in a function-like object. @dau_sama gives a good answer. Here is another example:

#include <iostream>
using namespace std;

int main() {

  const auto addSome = [](double some){
    return [some](double val){ return some+val; } ;
  };

  const auto addFive = addSome(5);

  std::cout << addFive(2) << std::endl;
}

Answer 2

For example using stl algorithms:

std::vector<int> items;
int factor;
auto foundItem = std::find_if(items.begin(), items.end(), 
[&factor](int const& a) 
{ 
   return a * factor == 100; 
});

In this case you're called in the lambda for every item in the container and you return if the value multiplied by a captured factor is 100. The code doesn't make much sense, it's just to show you an example where capture and parameter lists matter.