An Overview of C++26: Core Language

C++26 has a lot to offer. Let me directly jump in and give you an overview.

This image gives you the first idea of what’s inside or outside C++26. Writing a blog post about the future is always challenging because the design freeze of C++26 is in the first quarter of 2025.

Fishing in Murky Waters

This image should only give you the first idea of what’s inside or outside C++26. I will adapt the image while writing the next months about C++26 if necessary. For example, the three powerful features reflection, contracts, and std::execution made a big step towards their standardization. On the contrary, I intentionally ignore pattern matching on the image.

Very impactful features such as reflection or contracts implement the agile idea of the minimum viable product. “A minimum viable product (MVP) is a version of a product with just enough features to be usable by early customers who can then provide feedback for future product development.” (https://en.wikipedia.org/wiki/Minimum_viable_product). This means C++26 is only the starting point for features such as reflection or contracts.

If possible, I show you the feature in action. Many features are already implemented in the brand-new C++ compilers. For the rest, I hope for prototype implementations.

Let me start with the core language.

Core Language

Reflection

Reflection is the ability of a program to examine, introspect, and modify its structure and behavior. This makes compile-time programming in C++ much more powerful.

I don’t want to annoy you with too much theory in this overview post. Therefore, I will show you my favorite example from the reflection proposal P2996R5.

A question I often have to answer in my classes is: How could I convert an enumerator to a string?

// enumString.cpp

#include <iostream>
#include <experimental/meta>
#include <string>
#include <type_traits>

// start 'expand' definition
namespace __impl {
  template<auto... vals>
  struct replicator_type {
    template<typename F>
      constexpr void operator>>(F body) const {
        (body.template operator()<vals>(), ...);
      }
  };

  template<auto... vals>
  replicator_type<vals...> replicator = {};
}

template<typename R>
consteval auto expand(R range) {
  std::vector<std::meta::info> args;
  for (auto r : range) {
    args.push_back(std::meta::reflect_value(r));
  }
  return substitute(^__impl::replicator, args);
}
// end 'expand' definition

template<typename E>
  requires std::is_enum_v<E>                      // (1)
constexpr std::string enum_to_string(E value) {
  std::string result = "<unnamed>";
  [:expand(std::meta::enumerators_of(^E)):] >>    // (2)
  [&]<auto e>{
    if (value == [:e:]) {
      result = std::meta::identifier_of(e);       // (3)
    }
  };
  return result;
}


int main() {

    std::cout << '\n';

    enum Color { red, green, blue };
    std::cout << "enum_to_string(Color::red): " << enum_to_string(Color::red) << '\n';
    // std::cout << "enum_to_string(42): " << enum_to_string(42) << '\n'; 

    std::cout << '\n';

}

This example uses the experimental features (std::meta) of the standard.

Modernes C++ Mentoring

"Fundamentals for C++ Professionals" (open)

"Design Patterns and Architectural Patterns with C++" (open)

"C++20: Get the Details" (open)

"Concurrency with Modern C++" (open)

"Generic Programming (Templates) with C++": October 2024

"Embedded Programming with Modern C++": October 2024

"Clean Code: Best Practices for Modern C++": March 2025

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Here’s the output of the program:

Let me say a few words about the function, template enum_to_string. The function expand is a workaround. The function call enum_to_string(Color(42)) breaks because the function requires an enum.: requires std::is_enum_v<E> (line 1).

Line (1) applies a reflection operator (^E) and calls the meta function enum_to_std::meta::enumerators_of(^E)). Finally, the so called splicer ([: refl :]) produces in line (2) grammatical elements for reflection. The second meta function in line (3) creates the string:std:meta::identifier_of(e)). The consteval meta functions are executed at compile time, and so is the reflection.

Contracts

A contract specifies interfaces for software components precisely and checkably. These software components are functions that fulfill preconditions, postconditions, and invariants.

Here’s a straightforward example from the proposal P2900.

int f(const int x)
  pre (x != 1) // a precondition assertion
  post(r : r != 2) // a postcondition assertion; r refers to the return value of f
{
  contract_assert (x != 3); // an assertion statement
  return x;
}

The function f has a precondition, postcondition, and invariant. The precondition is checked before the function Invocation, the postcondition after the function invocation, and the invariant precisely at the point of its invocation.

Invoking the function with arguments 1, 2, or 3 causes a contract violation. There are different ways to react to a contract violation.

void g()
{
  f(0); // no contract violation
  f(1); // violates precondition assertion of f
  f(2); // violates postcondition assertion of f
  f(3); // violates assertion statement within f
  f(4); // no contract violation
}

The core language of C++26 has more to offer besides reflection and contracts. Let me name them and present a code snippet from the corresponding proposals.

Placeholder and extended character set

auto [x, y, _] = f();

The _ stands for I don’t care and it can be used more than once.

static_assert extension

static_assert(sizeof(S) == 1,
std::format("Unexpected sizeof: expected 1, got {}", sizeof(S))

Template improvements

There are many template improvements in C++26. My favorite one is pack indexing:

template <typename... T>
constexpr auto first_plus_last(T... values) -> T...[0] {
  return T...[0](values...[0] + values...[sizeof...(values)-1]);
}

int main() {
  //first_plus_last(); // ill formed
  static_assert(first_plus_last(1, 2, 10) == 11);
}

delete with reason

delete("Should have a reason");

What’s Next?

In my next post, I will overview the C++26 library.

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