Functor vs. lambda

Here is a good page on MSDN about lambda expressions in the C++0x flavor.

A first consideration is about comparing the usage of a functor and a lambda expression when is required to perform some operation on a sequence. In our case we would like to check how many elements in a vector of int are even. Here is a comparison between the two solutions:

#include <algorithm>
#include <iostream>
#include <vector>

using std::vector;
using std::cout;
using std::endl;
using std::for_each;

namespace
{
class EvenCount
{
int count_;
public:
explicit EvenCount() : count_(0) {}

void operator()(int n)
{
cout << n;

if(n % 2 == 0)
{
cout << " is even " << endl;
++count_;
}
else
cout << " is odd " << endl;

}

const int getCount() const { return count_; }
};

}

// Count the number of even numbers in a vector
void l03()
{
vector<int> vec;
for (int i = 0; i < 10; ++i)
vec.push_back(i);

// count using a functor
cout << "The even numbers in the vector are " <<
for_each(vec.begin(), vec.end(), EvenCount()).getCount() << endl;

// count using a lambda expression
int even = 0;
for_each(vec.begin(), vec.end(), [&even] (int n) {
cout << n;

if (n % 2 == 0)
{
cout << " is even " << endl;
even++;
}
else
cout << " is odd " << endl;
});

cout << "There are " << even << " even numbers in the vector" << endl;
}

In this case, chosing one solution or the other is basically a matter of taste. Most probably I would have taken the functor way here, since it looks to me a bit clearer.

But let's say I just had to count the even element, and not printing their single status to console. This would be the code resulting for the lambda expression:

// just counting with lambda
even = 0;
for_each(vec.begin(), vec.end(), [&even] (int n) { if(n%2 == 0) ++even; });
cout << "There are " << even << " even numbers in the vector" << endl;

I reckon it would not make much sense creating a functor, when we could write such a compact and clear piece of code instead.

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