How to join or concat ranges, C++26

Modern C++ continuously improves its range library to provide more expressive, flexible, and efficient ways to manipulate collections. Traditionally, achieving tasks like concatenation and flattening required manual loops, copying, or custom algorithms. With C++’s range adaptors, we now have an elegant and efficient way to process collections lazily without unnecessary allocations.

In this post, we will explore three powerful range adaptors introduced in different C++ standards:

• std::ranges::concat_view (C++26)
• std::ranges::join_view (C++20)
• std::ranges::join_with_view (C++23)

Let’s break down their differences, use cases, and examples.

std::ranges::concat_view (C++26)  

The concat_view allows you to concatenate multiple independent ranges into a single sequence. Unlike join_view, it does not require a range of ranges—it simply merges the given ranges sequentially while preserving their structure.

In short:

• Takes multiple independent ranges as arguments.
• Supports random access if all underlying ranges support it.
• Allows modification if underlying ranges are writable.
• Lazy evaluation: No additional memory allocations.

See the example:

#include <print>
#include <ranges>
#include <vector>
#include <array>

int main() {
    std::vector<std::string> v1{"world", "hi"};
    std::vector<std::string> v2 { "abc", "xyz" };
    std::string arr[]{"one", "two", "three"};
    
    auto v1_rev = v1 | std::views::reverse;
    auto concat = std::views::concat(v1_rev, v2, arr);

    concat[0] = "hello"; // access and write

    for (auto& elem : concat)
        std::print("{} ", elem);
}

See at @Compiler Explorer

hello world abc xyz one two three 

The example below shows how to concatenate three ranges, v1 is reversed and then combined with v2 and arr. Notice that we can also access the value at position 0 and update it.

And a bit more complex example:

#include <print>
#include <ranges>
#include <vector>
#include <list>
#include <string>

struct Transaction {
    std::string type;
    double amount;
};

int main() {
    std::vector<Transaction> bank_transactions{
       {"Deposit", 100.0}, 
       {"Withdraw", 50.0}, 
       {"Deposit", 200.0}
    };
    std::list<Transaction> credit_card_transactions{
        {"Charge", 75.0}, {"Payment", 50.0}
    };
    
    auto filtered_bank = bank_transactions 
        | std::views::filter([](const Transaction& t) {
        return t.amount >= 100.0;
    });
    
    auto filtered_credit = credit_card_transactions 
        | std::views::filter([](const Transaction& t) {
        return t.amount > 60.0;
    });
    
    auto all_transactions = std::views::concat(filtered_bank, filtered_credit);
    
    for (const auto& t : all_transactions)
        std::println("{} - {}$", t.type, t.amount);
}

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std::ranges::join_view (C++20)  

The join_view is designed for flattening a single range of ranges into a single sequence. It removes the structural boundaries between nested ranges.

• Works on a single range of ranges (e.g., std::vector<std::vector<int>>).
• Does not support operator[] (no random access).
• Eliminates boundaries between sub-ranges.
• Lazy evaluation, avoiding memory copies.

A simple example:

#include <iostream>
#include <ranges>
#include <vector>

int main() {
    std::vector<std::vector<int>> nested{{1, 2}, {3, 4, 5}, {6, 7}};
    auto joined = std::views::join(nested);
    
    for (int i : joined) 
        std::println(i);
}

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1 2 3 4 5 6 7

Of course, we can have different nested ranges… and this can be handy for string processing:

#include <print>
#include <ranges>
#include <vector>
#include <string>
#include <map>

int main() {
    std::vector<std::string> words { "Hello", "World", "Coding" };

    // regular:
    std::map<char, int> freq;
    for (auto &w : words)
        for (auto &c : w)
            freq[::tolower(c)]++;

    // join:
    std::map<char, int> freq2;
    for (auto &c : words | std::views::join)
        freq2[::tolower(c)]++;

    for (auto& [key, val] : freq2)
        std::println("{} -> {}", key, val);
}

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As you can see, thanks to views_join we can save one nested loop and iterate through a single range of characters.

std::ranges::join_with_view (C++23)  

The join_with_view works like join_view, but it allows inserting a delimiter between flattened sub-ranges.

• Works on a single range of ranges.
• Allows specifying a delimiter (single element or a range).
• Does not support random access.
• Useful for formatting strings or separating collections.

See the example below:

#include <iostream>
#include <ranges>
#include <vector>
#include <string>
#include <string_view>

std::string to_uppercase(std::string_view word) {
    std::string result(word);
    for (char& c : result) 
        c = std::toupper(static_cast<unsigned char>(c));
    return result;
}

int main() {
    std::vector<std::string_view> words{
        "The", "C++", "ranges", "library"
    };

    auto words_up = words | std::views::transform(to_uppercase);
    auto joined = std::views::join_with(words_up, std::string_view(" "));

    for (auto c : joined) 
        std::cout << c;
}

See at Compiler Explorer

THE C++ RANGES LIBRARY

Comparing concat_view, join_view, and join_with_view  

Summary  

C++’s range adaptors provide efficient ways to manipulate collections without unnecessary copying. Here’s a quick summary of when to use each view:

• Use concat_view when merging multiple independent ranges.
• Use join_view when flattening a range of ranges.
• Use join_with_view when flattening a range of ranges but needing a separator between elements.

Back to you

• Do you use ranges?
• What are your most useful views ald algorithms on ranges?

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