Sentinels and Concepts with Ranges Algorithms

The following example uses sentinels for a C-string and a std::vector<int>.

// sentinel.cpp

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

struct Space {                        // (1)
bool operator== (auto pos) const {
        return *pos == ' '; 
    }
};

struct NegativeNumber {               // (2)
    bool operator== (auto num) const {
        return *num < 0;   
    }
};

struct Sum {                          // (7)
    void operator()(auto n) { sum += n; }
    int sum{0};
};

int main() {

    std::cout << '\n';

    const char* rainerGrimm = "Rainer Grimm";
   
    std::ranges::for_each(rainerGrimm, Space{}, [] (char c) { std::cout << c; });  // (3)
    std::cout << '\n';
    for (auto c: std::ranges::subrange{rainerGrimm, Space{}}) std::cout << c;      // (4)
    std::cout << '\n';

    std::ranges::subrange rainer{rainerGrimm, Space{}};                            // (5)
    std::ranges::for_each(rainer, [] (char c) { std::cout << c << ' '; });         // (6)
    std::cout << '\n';
    for (auto c: rainer) std::cout << c << ' ';
    std::cout << '\n';
  

    std::cout << "\n";


    std::vector<int> myVec{5, 10, 33, -5, 10};

    for (auto v: myVec) std::cout << v << " ";
    std::cout << '\n';

    auto [tmp1, sum] = std::ranges::for_each(myVec, Sum{});
    std::cout << "Sum: " << sum.sum << '\n';                                   // (8)

    auto [tmp2, sum2] = std::ranges::for_each(std::begin(myVec), NegativeNumber{}, 
                                              Sum{} );            
    std::cout << "Sum: " << sum2.sum << '\n';                                   // (9)

    std::ranges::transform(std::begin(myVec), NegativeNumber{},                 // (10)
                           std::begin(myVec), [](auto num) { return num * num; });
    std::ranges::for_each(std::begin(myVec), NegativeNumber{},                  // (11)
                          [](int num) { std::cout << num << " "; });
    std::cout << '\n';
    for (auto v: std::ranges::subrange{ std::begin(myVec), NegativeNumber{}}) { // (12)
        std::cout << v << " ";
    }

    std::cout << "\n\n";
    
}

The program defines two sentinels: Space (line 1) and NegativeNumber (line 2). Both define the equal operator. Thanks to the <compare> header, the compiler auto-generates the non-equal operator. The non-equal operator is required when using algorithms such as std::ranges_for_each or std::ranges::transform with a sentinel. Let me start with the sentinel Space.

Line (3) applies the sentinel Space{} directly onto the string “rainerGrimm“. Creating a std::ranges::subrange (line 4) allows it to use the sentinel in a range-based for-loop. You can also define a std::ranges::subrange and use it directly in the algorithm std::ranges::for_each (line 5) or in a range-based for-loop (line 6).

My second example uses a std::vector<int>, filled with the values {5, 10, 33, -5, 10}. The sentinel NegativeNumber checks if a number is negative. First, I sum up all values using the function object Sum (line 7). std::ranges::for_each returns a pair (it, func).  it is the successor of the sentinel and func the function object applied to the range. Thanks to the structured binding, I can directly define the variables sum and sum2 and display their values (lines 8 and 9). std::ranges::for_each uses the sentinel NegativeNumber. Consequently, sum2 has the sum up to the sentinel. The call std::ranges::transform (line 10) transforms each element to its square: [](auto num){ return num * num}. The transformation stops with the sentinel NegativeNumber. Line 11 and line 12 display the transformed values.

Finally, here is the output of the program.

Let’s start with a comparison of the classical std::sort and the new std::ranges::sort. std::sort and std::ranges::sort require a random-access iterator that can access each range element in constant time. Here are the two relevant overloads for std::sort and std::ranges::sort.

template< class RandomIt >
constexpr void sort( RandomIt first, RandomIt last );

template <std::random_access_iterator I, std::sentinel_for<I> S,
         class Comp = ranges::less, class Proj = std::identity>
requires std::sortable<I, Comp, Proj>
constexpr I sort(I first, S last, Comp comp = {}, Proj proj = {});