C++26: Reflection, Memory Safety, Contracts, and a New Async Model

Source: Hacker News

The C++26 standard draft is now complete, reports Herb Sutter, long‑time C++ expert and former chair of the ISO C++ standards committee. The finalized draft introduces reflection, enhances memory safety without requiring code rewrites, adds contracts with pre‑conditions and post‑conditions alongside a new assertion statement, and establishes a unified framework for concurrency and parallelism.

Reflection

Reflection is a mechanism that, as Sutter explains, gives developers the keys to C++ internal machinery, enabling the language to describe itself and generate code, and thus provide a solid foundation for metaprogramming. True to C++‘s spirit, reflection comes with no runtime overhead.

A basic example shows how reflection enables a specialized syntax for declaring C++ interfaces:

class(interface) IFoo {
    int f();
    void g(std::string);
};

//-- will be translated into the "classical":

class IFoo {
public:
    virtual int f() = 0;
    virtual void g(std::string) = 0;
    virtual ~IFoo() = default;
    IFoo() = default;
protected:
    IFoo(IFoo const&) = default;
    void operator=(IFoo const&) = default;
};

The interface abstraction is part of cppfront, a compiler that builds to pure ISO C++, which Sutter created to provide a faster way to experiment with proposals. cppfront also includes abstractions such as copyable (expressing a type with copy/move construction/assignment), ordered (defining a totally ordered type with operator), union (a tagged union with names), regex, and more.

Reflection can simplify C++’s future evolution by reducing the need for many bespoke new language features, since many can now be expressed as reusable compile‑time libraries—faster to design, easier to test, and portable from day one.

Memory Safety

C++26 delivers significant improvements in memory safety. The changes eliminate undefined behavior when reading uninitialized local variables and provide bounds safety for most standard‑library types, including vector, span, string, and string_view. These enhancements have already been deployed in production at Apple and Google, covering hundreds of millions of lines of C++ code.

• At Google alone, the new safety mechanisms have fixed over 1,000 bugs, are projected to prevent 1,000–2,000 bugs per year, and have reduced the segfault rate across the production fleet by 30 %.
• The benefits were achieved simply by recompiling existing code with the new compiler; only seven cases required developers to opt out of memory safety via a fine‑grained API.

Contracts

C++26 introduces contracts to bring defensive programming into the language, improving both functional and memory safety. Contracts let developers express pre‑conditions and post‑conditions that must not be violated across a method execution. They also allow assertions to be placed in function declarations, making them visible to callers and static‑analysis tools.

C++ offers four ways to handle contract violations:

1. ignore – do nothing.
2. observe – report the violation at runtime.
3. enforce – terminate the program.
4. quick enforce – fast termination without diagnostic information.

In addition, a native assertion mechanism replaces C’s assert macro, providing a more integrated way to enforce invariants.

Concurrency and Parallelism

C++26 introduces std::execution as a framework to express and control concurrency and parallelism. The three main abstractions are schedulers, senders, and receivers, which can be composed through a set of customizable asynchronous algorithms. std::execution is designed to work seamlessly with C++20 coroutines and makes it easier to write programs that use structured (rigorously lifetime‑nested) concurrency and parallelism, achieving data‑race‑free execution by construction.

Compiler Support

The two major C++ compilers, GCC and Clang, have already implemented most of the C++26 features during the standardization process, and these features will soon make their way into mainline releases.