C++26 New Features: Expected Features and Proposals

C++26 New Features: Expected Features and Proposals

C++26 is the upcoming C++ standard (expected 2026). This guide covers expected features based on current proposals and working drafts. Note: Features may change before finalization.

Core Language Features

1. Static Reflection

Reflection capabilities for compile-time introspection.

// Expected C++26: Static reflection
#include <experimental/reflect>

struct Point {
    int x;
    int y;
};

// Get type information
constexpr auto point_type = reflexpr(Point);
constexpr auto members = get_public_data_members(point_type);

// Iterate over members
for_each(member : members) {
    std::cout << get_name(member) << std::endl;
}

// Create from reflection
template<typename T>
void print_structure() {
    constexpr auto type = reflexpr(T);
    std::cout << "Struct: " << get_name(type) << std::endl;
    for_each(member : get_public_data_members(type)) {
        std::cout << "  " << get_name(member) << ": " 
                  << get_type(member) << std::endl;
    }
}

print_structure<Point>();

2. Pattern Matching

Expressive pattern matching syntax.

// Expected C++26: Pattern matching
#include <experimental/match>

// Match on value
int result = match(value) {
    case 0 => 1;
    case 1 => 2;
    case 2 => 3;
    default => 0;
};

// Match on type (variant)
std::variant<int, double, std::string> v = 42;
auto result = match(v) {
    case int i => i * 2;
    case double d => d / 2.0;
    case std::string s => s.length();
};

// Match on structure
struct Point { int x, y; };
Point p{10, 20};

auto result = match(p) {
    case Point{0, 0} => "origin";
    case Point{x, 0} => "on x-axis";
    case Point{0, y} => "on y-axis";
    case Point{x, y} if x == y => "on diagonal";
    case Point{x, y} => "general point";
};

// Match on optional
std::optional<int> opt = 42;
auto result = match(opt) {
    case std::some(int x) => x * 2;
    case std::none => 0;
};

3. Contracts

Preconditions, postconditions, and assertions.

// Expected C++26: Contracts
int divide(int a, int b)
    [[pre: b != 0]]           // Precondition
    [[post r: r == a / b]]    // Postcondition
{
    return a / b;
}

void process(int* ptr)
    [[pre: ptr != nullptr]]
    [[post: *ptr > 0]]
{
    *ptr = 42;
}

// Contract levels
void func(int x)
    [[pre audit: x > 0]]      // Checked in debug builds
    [[pre: x < 100]]           // Always checked
{
    // ...
}

4. Named Parameters

Named function arguments.

// Expected C++26: Named parameters
struct create_window_params {
    int width = 800;
    int height = 600;
    std::string title = "Window";
    bool resizable = true;
};

void create_window(create_window_params params);

// Usage
create_window({
    .width = 1024,
    .height = 768,
    .title = "My App"
});

// Or with designated initializers
create_window({
    width: 1024,
    height: 768,
    title: "My App"
});

5. Value Semantics Improvements

Better support for value types.

// Expected C++26: Value semantics
class Value {
    int data_;
public:
    Value(int d) : data_(d) {}
    
    // Automatic value semantics
    friend auto operator<=>(const Value&, const Value&) = default;
    
    // Value-based operations
    Value operator+(const Value& other) const {
        return Value(data_ + other.data_);
    }
};

// Improved move semantics
Value func() {
    Value v(42);
    return v;  // Guaranteed move or elision
}

6. operator[] with Multiple Arguments

// Expected C++26: Enhanced subscript
class Matrix {
    int data_[10][10];
public:
    // Already in C++23, but improvements expected
    int& operator[](size_t i, size_t j) {
        return data_[i][j];
    }
    
    // With default arguments
    int& operator[](size_t i, size_t j = 0) {
        return data_[i][j];
    }
};

7. comparable Concept

// Expected C++26: Comparable concept
template<std::comparable T>
void sort(T& container) {
    // T must be comparable
}

// Custom comparable types
struct MyType {
    int value;
    auto operator<=>(const MyType&) const = default;
};

static_assert(std::comparable<MyType>);

8. std::meta::info

Reflection metadata type.

// Expected C++26: Reflection metadata
#include <experimental/meta>

template<typename T>
constexpr std::meta::info get_type_info() {
    return reflexpr(T);
}

template<std::meta::info TypeInfo>
void process_type() {
    // Work with type information
}

---

Standard Library

9. Networking Library

Standard networking support.

// Expected C++26: Networking
#include <experimental/net>

namespace net = std::experimental::net;

// TCP server
net::io_context ctx;
net::ip::tcp::acceptor acceptor(ctx, net::ip::tcp::endpoint(
    net::ip::tcp::v4(), 8080));

void accept_connection() {
    auto socket = std::make_shared<net::ip::tcp::socket>(ctx);
    acceptor.async_accept(*socket, [socket](std::error_code ec) {
        if (!ec) {
            handle_client(socket);
        }
        accept_connection();
    });
}

// HTTP client
net::http::client client(ctx);
auto response = client.get("https://example.com");

10. std::hive (Planned)

High-performance container for frequent insertions/deletions.

// Expected C++26: std::hive
#include <hive>

std::hive<int> container;

// Optimized for frequent insertions/deletions
container.insert(1);
container.insert(2);
container.erase(container.begin());

// Maintains iterator stability
auto it = container.begin();
container.insert(3);  // it still valid

11. std::flat_map and std::flat_set

Flat associative containers.

// Expected C++26: Flat containers
#include <flat_map>
#include <flat_set>

std::flat_map<std::string, int> map;
map["one"] = 1;
map["two"] = 2;

// Better cache locality than std::map
std::flat_set<int> set = {1, 2, 3, 4, 5};

12. std::ranges::zip Improvements

// Expected C++26: Enhanced zip
#include <ranges>

std::vector<int> vec1 = {1, 2, 3};
std::vector<std::string> vec2 = {"a", "b", "c"};
std::vector<double> vec3 = {1.5, 2.5, 3.5};

// Zip multiple ranges
for (auto [a, b, c] : std::views::zip(vec1, vec2, vec3)) {
    std::cout << a << " " << b << " " << c << std::endl;
}

13. std::ranges::enumerate

Enumerate range elements.

// Expected C++26: Enumerate
#include <ranges>

std::vector<int> vec = {10, 20, 30};

for (auto [index, value] : std::views::enumerate(vec)) {
    std::cout << index << ": " << value << std::endl;
}

14. std::ranges::adjacent and std::ranges::pairwise

Adjacent element views.

// Expected C++26: Adjacent views
#include <ranges>

std::vector<int> vec = {1, 2, 3, 4, 5};

// Adjacent pairs
for (auto [a, b] : vec | std::views::adjacent<2>) {
    std::cout << a << ", " << b << std::endl;
}

// Pairwise
for (auto [a, b] : vec | std::views::pairwise) {
    std::cout << a << ", " << b << std::endl;
}

15. std::ranges::chunk_by Improvements

// Expected C++26: Enhanced chunk_by
#include <ranges>

std::vector<int> vec = {1, 2, 3, 10, 11, 12, 20, 21};

// Chunk by predicate with more control
auto chunks = vec | std::views::chunk_by([](int a, int b) {
    return b - a <= 5;
});

16. std::ranges::join_with Improvements

// Expected C++26: Enhanced join
#include <ranges>

std::vector<std::vector<int>> vecs = {{1, 2}, {3, 4}, {5, 6}};

// Join with custom separator
auto joined = vecs | std::views::join_with(std::vector{-1});

17. std::ranges::repeat_n

Repeat value N times.

// Expected C++26: Repeat N times
#include <ranges>

// Repeat value N times
for (auto val : std::views::repeat_n(42, 5)) {
    std::cout << val << " ";  // 42 42 42 42 42
}

18. std::ranges::stride

Stride view.

// Expected C++26: Stride
#include <ranges>

std::vector<int> vec = {1, 2, 3, 4, 5, 6, 7, 8};

// Every 2nd element
for (auto val : vec | std::views::stride(2)) {
    std::cout << val << " ";  // 1 3 5 7
}

19. std::ranges::cycle

Cycle through range.

// Expected C++26: Cycle
#include <ranges>

std::vector<int> vec = {1, 2, 3};

// Cycle infinitely
for (auto val : vec | std::views::cycle | std::views::take(10)) {
    std::cout << val << " ";  // 1 2 3 1 2 3 1 2 3 1
}

20. std::ranges::sample

Sample elements from range.

// Expected C++26: Sample
#include <ranges>
#include <random>

std::vector<int> vec = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

// Sample 3 random elements
std::mt19937 gen(std::random_device{}());
auto sampled = vec | std::views::sample(3, gen);

21. std::ranges::chunk Improvements

// Expected C++26: Enhanced chunk
#include <ranges>

std::vector<int> vec = {1, 2, 3, 4, 5, 6, 7, 8};

// Chunk with overlap
for (auto chunk : vec | std::views::chunk(3, 1)) {  // Overlap of 1
    // ...
}

22. std::ranges::group_by

Group by key function.

// Expected C++26: Group by
#include <ranges>

std::vector<int> vec = {1, 2, 2, 3, 3, 3, 4, 4};

// Group by value
for (auto group : vec | std::views::group_by(std::equal_to{})) {
    for (auto val : group) {
        std::cout << val << " ";
    }
    std::cout << std::endl;
}

23. std::ranges::adjacent_transform

Transform adjacent elements.

// Expected C++26: Adjacent transform
#include <ranges>

std::vector<int> vec = {1, 2, 3, 4, 5};

// Differences between adjacent elements
auto diffs = vec | std::views::adjacent_transform<2>([](int a, int b) {
    return b - a;
});

24. std::ranges::windows

Sliding window view.

// Expected C++26: Windows
#include <ranges>

std::vector<int> vec = {1, 2, 3, 4, 5};

// Sliding window of size 3
for (auto window : vec | std::views::windows(3)) {
    for (auto val : window) {
        std::cout << val << " ";
    }
    std::cout << std::endl;
}

25. std::ranges::concat

Concatenate ranges.

// Expected C++26: Concat
#include <ranges>

std::vector<int> vec1 = {1, 2, 3};
std::vector<int> vec2 = {4, 5, 6};
std::vector<int> vec3 = {7, 8, 9};

// Concatenate multiple ranges
for (auto val : std::views::concat(vec1, vec2, vec3)) {
    std::cout << val << " ";  // 1 2 3 4 5 6 7 8 9
}

26. std::ranges::interleave

Interleave ranges.

// Expected C++26: Interleave
#include <ranges>

std::vector<int> vec1 = {1, 3, 5};
std::vector<int> vec2 = {2, 4, 6};

// Interleave elements
for (auto val : std::views::interleave(vec1, vec2)) {
    std::cout << val << " ";  // 1 2 3 4 5 6
}

27. std::ranges::round_robin

Round-robin through ranges.

// Expected C++26: Round robin
#include <ranges>

std::vector<int> vec1 = {1, 4};
std::vector<int> vec2 = {2, 5};
std::vector<int> vec3 = {3, 6};

// Round-robin
for (auto val : std::views::round_robin(vec1, vec2, vec3)) {
    std::cout << val << " ";  // 1 2 3 4 5 6
}

28. std::ranges::take_while and std::ranges::drop_while Improvements

// Expected C++26: Enhanced take/drop while
#include <ranges>

std::vector<int> vec = {1, 2, 3, 4, 5, 6, 7, 8};

// Take while condition
auto taken = vec | std::views::take_while([](int x) { return x < 5; });

// Drop while condition
auto dropped = vec | std::views::drop_while([](int x) { return x < 3; });

29. std::ranges::split Improvements

// Expected C++26: Enhanced split
#include <ranges>

std::string str = "hello,world,test";

// Split by delimiter
for (auto part : str | std::views::split(',')) {
    std::cout << std::string_view(part.begin(), part.end()) << std::endl;
}

30. std::ranges::elements and std::ranges::keys/values

Extract elements from pairs/tuples.

// Expected C++26: Elements extraction
#include <ranges>

std::vector<std::pair<int, std::string>> pairs = {
    {1, "one"}, {2, "two"}, {3, "three"}
};

// Extract keys
auto keys = pairs | std::views::keys;  // 1, 2, 3

// Extract values
auto values = pairs | std::views::values;  // "one", "two", "three"

// Extract nth element
auto firsts = pairs | std::views::elements<0>;
auto seconds = pairs | std::views::elements<1>;

---

Other Expected Features

31. std::ranges::as_const and std::ranges::as_mutable

Const/mutable views.

// Expected C++26: Const/mutable views
#include <ranges>

std::vector<int> vec = {1, 2, 3, 4, 5};

// Const view
auto const_view = vec | std::views::as_const;

// Mutable view
auto mutable_view = const_view | std::views::as_mutable;

32. std::ranges::cache1

Cache first element.

// Expected C++26: Cache first
#include <ranges>

auto expensive_range = /* expensive to compute */;

// Cache first element
auto cached = expensive_range | std::views::cache1;

33. std::ranges::drop_last and std::ranges::take_last

// Expected C++26: Drop/take last
#include <ranges>

std::vector<int> vec = {1, 2, 3, 4, 5};

// Drop last N
auto dropped = vec | std::views::drop_last(2);  // {1, 2, 3}

// Take last N
auto taken = vec | std::views::take_last(2);  // {4, 5}

34. std::ranges::reverse Improvements

// Expected C++26: Enhanced reverse
#include <ranges>

std::vector<int> vec = {1, 2, 3, 4, 5};

// Reverse view
auto reversed = vec | std::views::reverse;

// Reverse with custom comparator
auto custom_reversed = vec | std::views::reverse_with([](int a, int b) {
    return a > b;
});

---

Summary Table

Feature	Category	Status
Static Reflection	Language	Proposed
Pattern Matching	Language	Proposed
Contracts	Language	Proposed
Named Parameters	Language	Proposed
Networking Library	Library	Proposed
std::hive	Library	Proposed
std::flat_map	Library	Proposed
Enhanced Ranges	Library	Proposed
std::ranges::enumerate	Library	Proposed
std::ranges::adjacent	Library	Proposed

---

Important Notes

⚠️ C++26 is still in development. Features listed here are based on:

• Current proposals (P-papers)
• Working drafts
• Committee discussions

Features may:

• Change before finalization
• Be deferred to later standards
• Be removed entirely

Check the latest C++ standards documents for authoritative information.

---

Expected Timeline

• 2024-2025: Feature proposals and refinement
• 2025-2026: Finalization and voting
• 2026: Expected publication

---

Compiler Support

As C++26 is not yet finalized, compiler support is experimental:

• GCC: Experimental support for some features
• Clang: Experimental support for some features
• MSVC: Experimental support for some features

Use experimental flags: -std=c++2c or -fconcepts-ts (for specific features)

---

C++26 promises to be another major update, with reflection, pattern matching, contracts, and extensive networking/ranges improvements. Stay tuned for the final standard!