Evaluating Non-Trivial Destructors in C++ Constexpr

Modern C++ allows objects with non-trivial destructors to be created, manipulated, and destroyed during compile-time constant expression evaluation. Introduced primarily in C++20, this capability enables standard containers like std::vector and std::string to function inside constexpr and consteval contexts. The compiler achieves this by executing a compile-time interpreter that tracks object lifetimes, recursively evaluates destructor bodies, and strictly enforces memory and undefined-behavior checks before constant evaluation concludes.

The Evolution of Destructors in Constant Expressions

Prior to C++20, any type used in a constant expression had to be a literal type, which strictly required a trivial destructor. An object could not execute custom cleanup logic at compile time.

C++20 relaxed this restriction by allowing destructors to be explicitly declared constexpr. Under these updated rules, a class with a non-trivial destructor can be evaluated at compile time provided that:

  • The destructor is declared with the constexpr specifier.
  • The operations within the destructor body satisfy all constraints of constant expressions.
  • All subobjects, bases, and non-static data members have destructors that are also constexpr-evaluable.

How the Compile-Time Interpreter Evaluates Destructors

Compilers evaluate constant expressions using an internal abstract machine, often implemented as an AST (Abstract Syntax Tree) interpreter or bytecode virtual machine. When evaluating non-trivial destructors, the compiler follows specific operational steps:

1. Lifetime Tracking

The compiler maintains an internal environment mapping active objects and allocated storage. When a local variable is declared within a constexpr function, its lifetime begins. When the variable exits its lexical scope—or when an expression containing a temporary object finishes evaluation—the compiler automatically triggers the object's destructor.

2. Recursive Execution of Member and Base Cleanups

When invoking a constexpr non-trivial destructor, the compiler evaluates the statements inside the destructor body step-by-step:

  1. It executes the statements in the custom destructor body.
  2. It evaluates non-trivial destructors for direct non-static data members in reverse order of declaration.
  3. It calls non-trivial destructors for direct base classes in reverse order of initialization.

If the class uses virtual destructors, modern compilers perform compile-time dynamic dispatch. The compiler inspects the dynamic type assigned to the object during constant evaluation and resolves the correct destructor override.

3. Destruction of Dynamic Allocations

C++20 introduced compile-time dynamic memory allocation via std::allocator or expressions using new and delete. A primary role of non-trivial destructors in constant expressions is freeing this dynamically allocated memory (for example, clearing elements and releasing internal buffers in a std::vector).

When delete or std::destroy_at is encountered:

  • The compiler executes the targeted object's destructor.
  • The memory associated with the object is marked deallocated within the compiler's tracking map.

Strict Enforcement and Error Detection

The compiler treats any invalid operation during destruction as a failure of the constant expression, resulting in a compile-time error rather than runtime undefined behavior. During destructor evaluation, the compiler enforces the following constraints:

  • Transient Allocation Requirement: All memory allocated dynamically during constant evaluation must be deallocated before the evaluation completes. If a non-trivial destructor fails to release memory, the compiler detects a memory leak and aborts compilation.
  • No Access to Destroyed Objects: If a destructor attempts to read from a member that has already ended its lifetime, evaluation fails.
  • Double Free and Invalid Pointers: Passing an invalid address or freeing an already deallocated pointer inside a destructor halts evaluation immediately with a diagnostic message.

Through lifetime tracking, recursive subobject teardown, and rigorous undefined-behavior detection, the compiler ensures that non-trivial destructors run deterministically and safely during compilation.