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# C++ Constexpr Destructor
A `constexpr` destructor is a destructor declared with the `constexpr` specifier, introduced in C++20, which permits an object to be destroyed during constant evaluation. It modifies the language rules for literal types by allowing objects with non-trivial destructors to be instantiated, manipulated, and safely destroyed entirely at compile-time.
## Syntax
The `constexpr` keyword is applied directly to the destructor declaration. It can be used on user-defined destructors or explicitly defaulted ones:
```cpp theme={"dark"}
class NonTrivialLiteral {
public:
constexpr NonTrivialLiteral() {}
// User-defined constexpr destructor
constexpr ~NonTrivialLiteral() {
// Cleanup logic valid in constant evaluation
}
};
class TrivialLiteral {
public:
constexpr TrivialLiteral() = default;
// Defaulted constexpr destructor
constexpr ~TrivialLiteral() = default;
};
```
## Technical Mechanics and Constraints
To be valid and executable during constant evaluation, a `constexpr` destructor must adhere to strict language rules:
1. **Constant Expression Compliance:** The body of the destructor cannot contain operations that are invalid in a constant expression context. It cannot call non-`constexpr` functions, execute inline assembly, or invoke undefined behavior.
2. **Transient Memory Deallocation:** If the object allocated dynamic memory during constant evaluation (via a `constexpr` constructor or member function), that exact memory must be deallocated before the constant evaluation completes. The `constexpr` destructor is the mechanism that executes the `delete` expression at compile-time. Memory cannot "escape" compile-time to run-time.
3. **Virtual Destructors:** As of C++20, virtual destructors can also be declared `constexpr`. This allows polymorphic destruction during constant evaluation, provided the dynamic type of the object is known at compile-time.
4. **Implicit `constexpr`:** A defaulted destructor (`= default`) is implicitly `constexpr` if the destructors of all its base classes and non-static data members are also `constexpr`.
## Execution Behavior
When an object with a `constexpr` destructor is created within a `constexpr` function or a `consteval` context, the compiler tracks its lifecycle. When the object goes out of scope, the compiler evaluates the destructor just as it would evaluate a standard `constexpr` function.
```cpp theme={"dark"}
class CompileTimeBuffer {
int* buffer;
public:
constexpr CompileTimeBuffer(int size) : buffer(new int[size]) {}
// Evaluated by the compiler to prevent compile-time memory leaks
constexpr ~CompileTimeBuffer() {
delete[] buffer;
}
};
constexpr bool evaluate_destruction() {
// Object created in constant evaluation context
CompileTimeBuffer temp_buffer(10);
// As temp_buffer goes out of scope, ~CompileTimeBuffer() is invoked.
// If the destructor were not constexpr, or if it failed to delete the memory,
// the compilation would fail.
return true;
}
// Forces constant evaluation
static_assert(evaluate_destruction());
```
## Triviality and Literal Types
Declaring a destructor `constexpr` does not inherently make it non-trivial. A `constexpr` destructor can still be trivial (for example, if it is implicitly generated or explicitly defaulted via `constexpr ~T() = default;`). The `constexpr` specifier simply allows a destructor to contain executable logic (making it non-trivial) while remaining valid for execution during the translation phase.
In C++20, having a `constexpr` destructor is a *necessary condition* for a class to be classified as a literal type. Prior to C++20, literal types strictly required trivial destructors. However, a `constexpr` destructor is not a *sufficient condition* on its own. To be a literal type, the class must still satisfy all other literal type requirements, including:
* Having at least one `constexpr` constructor (that is not a copy or move constructor).
* Ensuring all base classes are literal types.
* Ensuring all non-static data members are literal types.
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