Dynamic Allocation in C++ Constexpr Functions

Modern C++ introduces the ability to perform dynamic memory allocation within constexpr functions through a rule known as transient allocation. Introduced in C++20, this feature allows developers to use new and delete expressions during compile-time evaluation, enabling standard library containers like std::vector and std::string to function in constant expressions, provided that all allocated memory is freed before the compile-time evaluation completes.

Transient Allocation Rule

The primary rule that makes dynamic allocation legal in a constant expression is that the allocation must be "transient." In constant evaluation, memory cannot leak into the runtime execution space, nor can a compile-time pointer point to a runtime heap address.

Every allocation initiated by new (or standard allocators) during constant evaluation must be deallocated by a corresponding delete before the constant expression finishes evaluating. If an allocation outlives the constant expression evaluation, the compiler issues a compile-time error indicating a memory leak.

Compiler-Level Virtual Memory Management

To support dynamic allocation at compile time, modern C++ compilers (such as GCC, Clang, and MSVC) incorporate an internal memory manager into their constant evaluators. When the compiler evaluates a constexpr context:

  1. It intercepts the call to the global operator new or standard allocator.
  2. It allocates memory within its own compile-time heap simulation rather than requesting memory from the operating system for a running binary.
  3. It tracks the lifetime, type, and bounds of the allocated storage to prevent undefined behavior, buffer overflows, and type-punning.
  4. When delete is called, the compiler verifies that the pointer refers to valid compile-time allocated memory and reclaims the simulated memory.

Enabling Language Features

Allowing new inside constexpr required several related enhancements to the core C++ language specification in C++20:

  • Constexpr Destructors: Deallocation requires invoking destructors. C++20 allowed destructors to be declared constexpr, ensuring non-trivial types clean up their resources during constant evaluation.
  • Constexpr Dynamic Polymorphism: Virtual function calls and virtual destructors were made legal in constexpr evaluation, allowing standard polymorphic designs and clean polymorphic deletion.
  • Constexpr std::allocator: The standard allocator (std::allocator<T>) was updated to have constexpr member functions (allocate and deallocate), integrating directly with the compiler's compile-time memory subsystem.
  • Type Safety Enforcement: Placement new and uninitialized storage handling were defined for compile-time execution via std::construct_at and std::destroy_at, providing a standard, type-safe path for containers to manage element lifetimes.

Practical Impact

Before this change, compile-time metaprogramming required static arrays or custom fixed-capacity data structures to manipulate collections of data. By allowing dynamic allocation via transient allocation rules, algorithms can dynamically scale their temporary working memory during compilation, run complex preprocessing routines, and produce finalized, immutable values stored in static memory formats.