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 to two versions of the same code.

1) With capture

int x = 4;  cout << "With capture  : Factorial of " << x << " = " << [x]() // <= Capture {     int r = 1;     for (int i = x; i > 1; i--) r = r * i;     return r; }() << endl; 

2) With parameter

int x = 4;  cout << "With parameter: Factorial of " << x << " = " << [](int x) // <= Parameter {     int r = 1;     for (int i = x; i > 1; i--) r = r * i;     return r; }(x) << endl; 

The output is:

With capture  : Factorial of 4 = 24 With parameter: Factorial of 4 = 24 

Since we can pass parameters to lambdas in the parameter list (just as with any C++ function), why do we need the capture list?

Can someone show me cases where parameter list doesn't work and only capture list does?

Answer 1

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.