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constexpr Cast From void*

Executive Abstract

constexpr Cast From void* is a core language enhancement introduced in C++26 (via proposal P2738R1) that permits casting a void* pointer back to a typed object pointer (static_cast<T*>(void_ptr)) during compile-time constant evaluation. In C++20 and C++23, although constexpr dynamic memory allocations (new/delete) and placement construction (std::construct_at) were permitted, type-erased pointer conversions through void* remained strictly illegal inside constant expressions. Standardized in C++26, this feature enables type-erased memory allocators, custom memory arenas (std::pmr), object pools, and custom data containers to execute entirely during compilation.

Motivation & Historical Context

Pre-C++26 Deficiencies: The void* Type-Erasure Barrier

C++20 introduced compile-time dynamic memory allocation (constexpr new/delete). However, low-level memory allocation architecture relies fundamentally on type-erased void* pointers:

// C++20/C++23 Compilation Failure
constexpr void* allocate_raw_bytes(std::size_t bytes) {
    return ::operator new(bytes); // Returns void*
}

constexpr int test_allocation() {
    void* raw_ptr = allocate_raw_bytes(sizeof(int));

    // C++20/C++23 FATAL COMPILE ERROR:
    // "cast from 'void*' is not allowed in a constant expression!"
    int* typed_ptr = static_cast<int*>(raw_ptr);

    ::operator delete(raw_ptr);
    return 42;
}

Deficiencies of C++20/C++23 Constant Evaluation Rules:

  1. Conservative Pointer Restriction: Early constexpr specification rules conservatively banned casting from void* to T* to prevent raw memory type-punning in the compiler's constant expression interpreter.
  2. Blockage of Custom Allocators and Arenas: Standard Library containers (std::vector) functioned inside constexpr because compilers hardcoded compiler-intrinsic hooks specifically for std::allocator<T>. However, custom user-defined allocators, std::pmr::memory_resource pools, and type-erased memory containers could not function in constexpr because their interfaces operated on void* or std::byte* buffers!

Solution: ISO C++26 proposal P2738R1 updated [expr.const] and [expr.static.cast], permitting static_cast<T*>(void_ptr) in constant expressions provided the target type $T$ matches the active underlying object type in compile-time memory.

Technical Mechanics & Execution Invariants

1. The P2738R1 Execution Rules ([expr.static.cast])

$$\text{constexpr } \mathbf{\text{static_cast< T* >( void_ptr )}}$$

During compile-time constant evaluation, a static_cast<T*>(void_ptr) expression is well-formed if and only if:

  1. Active Object Match: The pointer void_ptr points to a valid, active object $U$ in compile-time memory.
  2. Type Compatibility Invariant: The target type $T$ is identical to $U$ (ignoring cv-qualifiers) or is a pointer to a valid base or derived subobject of $U$.

2. Compile-Time Type Safety Protection

constexpr casting from void* does NOT permit unsafe type punning:

constexpr int bad_type_pun() {
    float f = 3.14f;
    void* void_ptr = &f;

    // C++26 COMPILE ERROR: The compiler tracks that void_ptr points to 'float'!
    // Attempting to cast to 'int*' violates constant evaluation type safety rules!
    int* int_ptr = static_cast<int*>(void_ptr);
    return *int_ptr;
}

If the underlying object type in memory ($U$) does not match the cast target type ($T$), the compiler halts compilation immediately with a diagnostic error.

Evolution Across Standards (C++20 to C++26/C++29)

timeline
    title Constexpr Cast From void* Evolution
    C++20 : Introduced constexpr new / delete and std::construct_at
          : Casting void* -> T* strictly PROHIBITED in constant expressions
          : Custom allocators and std::pmr blocked from constexpr usage
    C++26 : Introduced constexpr Cast From void* P2738R1
          : static_cast<T*>void_ptr allowed in constexpr evaluation
          : Enables custom constexpr memory allocators and arenas
Loading 

---

## Comprehensive Code Examples

### Example 1: Type-Erased Custom Memory Allocator in `constexpr`
Demonstrates writing a custom type-erased bump-pointer allocator that executes at compile time in C++26.

```cpp
#include <iostream>
#include <cstddef>
#include <memory>

// Custom Type-Erased Bump-Pointer Allocator
class ConstexprArenaAllocator {
private:
    alignas(alignof(std::max_align_t)) char m_buffer[1024]{};
    std::size_t m_offset = 0;

public:
    constexpr ConstexprArenaAllocator() = default;

    // Type-erased allocation returning void*
    constexpr void* allocate(std::size_t bytes) {
        if (m_offset + bytes > 1024) throw "Arena Out of Memory!";
        void* ptr = &m_buffer[m_offset];
        m_offset += bytes;
        return ptr;
    }
};

struct Vector3D {
    double x, y, z;
};

constexpr Vector3D create_arena_vector() {
    ConstexprArenaAllocator arena;

    // 1. Allocate raw type-erased memory buffer (returns void*)
    void* raw_mem = arena.allocate(sizeof(Vector3D));

    // 2. C++26: static_cast<Vector3D*> from void* is VALID in constexpr!
    Vector3D* typed_ptr = static_cast<Vector3D*>(raw_mem);

    // 3. Construct object via std::construct_at
    std::construct_at(typed_ptr, 1.0, 2.0, 3.0);

    return *typed_ptr;
}

int main() {
    // Evaluated entirely at COMPILE TIME!
    constexpr Vector3D vec = create_arena_vector();

    static_assert(vec.x == 1.0 && vec.y == 2.0 && vec.z == 3.0,
                  "C++26 constexpr void* cast failed");

    std::cout << "Compile-Time Arena Vector: (" << vec.x << ", " << vec.y << ", " << vec.z << ")\n";
    return 0;
}

Example 2: Placement Construction with std::construct_at and void* Slices

Demonstrates using C++26 void* casts to implement a type-erased optional wrapper in constant evaluation.

#include <iostream>
#include <memory>
#include <utility>

template <typename T>
class TypeErasedBox {
private:
    alignas(T) char m_storage[sizeof(T)]{};
    bool m_has_value = false;

public:
    constexpr TypeErasedBox() = default;

    template <typename... Args>
    constexpr void emplace(Args&&... args) {
        void* void_addr = m_storage;

        // C++26: Casting void* address to T* inside constexpr!
        T* typed_addr = static_cast<T*>(void_addr);

        std::construct_at(typed_addr, std::forward<Args>(args)...);
        m_has_value = true;
    }

    constexpr const T& value() const {
        const void* void_addr = m_storage;

        // C++26: Const void* cast
        const T* typed_addr = static_cast<const T*>(void_addr);
        return *typed_addr;
    }

    constexpr ~TypeErasedBox() {
        if (m_has_value) {
            T* typed_addr = static_cast<T*>(static_cast<void*>(m_storage));
            std::destroy_at(typed_addr);
        }
    }
};

int main() {
    constexpr auto res = []() constexpr {
        TypeErasedBox<int> box;
        box.emplace(42);
        return box.value();
    }();

    static_assert(res == 42, "Compile-time type-erased box failed");

    std::cout << "Type-erased constexpr box value: " << res << "\n";
    return 0;
}

Engineering Best Practices & Common Pitfalls

  1. Use static_cast<T*> for Type-Erased constexpr Memory Buffers:

    • In C++26 and later, write static_cast<T*>(void_ptr) directly inside custom memory managers, object pools, and serialization layers.

  2. Type Safety Is Strictly Enforced at Compile Time:

    • Do not attempt to use void* casts to perform bitwise type punning (float $\to$ int). The compiler's constexpr interpreter tracks original object types in memory and will reject invalid type casts. Use std::bit_cast for bit-level type reinterpretation.

  3. Pair with std::construct_at and std::destroy_at:

    • Always use std::construct_at(static_cast<T*>(void_ptr), args...) and std::destroy_at(typed_ptr) to manage object lifetimes correctly inside placement memory buffers.

Relationships

Back to C++26 section.