A closure packages executable code with values captured from an enclosing scope. In C++, lambda expressions construct closures, making a small operation easy to keep beside the algorithm that uses it. section 2.3.4 section 2.4
Capture syntax controls how enclosing variables enter the closure: [=] captures used variables by value, while [&x] captures x by reference. Specific combinations such as [=, &x] and [y, &x] make the policy explicit. A closure can also capture nothing when its operation depends only on its parameter. section 2.3.4
Checking a row with a closure
std::all_of(row.begin(), row.end(), [](auto x) { return x >= 0; }) checks every integer without defining a separate named function. The same closure works with the ranges form: std::ranges::all_of(row, [](auto x) { return x >= 0; }). section 2.3.4
Listing 2.14 Making sure there are no negative numbers by using a view
auto negative = [](int x) { return x < 0; }; #1
auto negatives = row | std::views::filter(negative); #2
assert(negatives.empty()); #3
A named closure can be reused as a filter predicate, then applied through a view to verify that a row contains no negative values. section 2.3.4
