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The All-in-One Guide to C++20

Function Composition

Explore how to use function composition with the C++20 ranges library to manipulate data structures like std::map. Understand how views and the pipe operator simplify accessing, reversing, and filtering keys for clearer and more efficient code.

We'll cover the following...

In the example below, I use a std::map, because the ordering of the keys is crucial.

C++
#include <iostream>
#include <ranges>
#include <string>
#include <map>
int main() {
    std::map<std::string, int> freqWord{ {"witch", 25}, {"wizard", 33},
                                         {"tale", 45}, {"dog", 4},
                                         {"cat", 34}, {"fish", 23} };
    std::cout << "All words: ";
    for (const auto& name : std::views::keys(freqWord)) { std::cout << name << " "; }
    std::cout << '\n';
    std::cout << "All words, reversed: ";
    for (const auto& name : std::views::keys(freqWord)
                          | std::views::reverse) { std::cout << name << " "; }
    std::cout << '\n';
    std::cout << "The first 4 words: ";
    for (const auto& name : std::views::keys(freqWord)
                          | std::views::take(4)) { std::cout << name << " "; }
    std::cout << '\n';
    std::cout << "All words starting with w: ";
    auto firstw = [](const std::string& name){ return name[0] == 'w'; };
    for (const auto& name : std::views::keys(freqWord)
                        | std::views::filter(firstw)) { std::cout << name << " "; }
    std::cout << '\n';
}

I’m only interested in the keys. I display all of them ( ...