Rethrow Expression

A rethrow expression is a throw statement omitting its operand, utilized to propagate the currently active exception object to the next dynamically enclosing exception handler. It reactivates the exception handling mechanism while strictly preserving the original exception’s dynamic type and state.

Syntax

throw;

Technical Mechanics

• Dynamic Type Preservation: When an exception is caught by reference to a base class, throwing the named variable (e.g., throw e;) invokes the copy constructor of the base class, resulting in object slicing. A rethrow expression (throw;) bypasses this by propagating the exact original exception object, fully retaining its derived dynamic type.
• Object Identity: The expression does not allocate or initialize a new exception object. It extends the lifetime of the existing exception object in memory until the next handler in the call stack completes its execution.
• Execution Flow: Upon evaluation, control immediately exits the current catch block. The C++ runtime resumes stack unwinding to locate the next matching catch clause in the call chain.

Scope and Evaluation Rules

• Valid Context: A rethrow expression is valid only when an exception is currently active. While typically found lexically nested within a catch block, it is also valid if evaluated dynamically inside a function invoked directly or indirectly from within a catch block.
• Invalid Context: If a rethrow expression is evaluated when no exception is currently being handled (i.e., outside the dynamic scope of an active exception handler), the C++ runtime immediately invokes std::terminate(), which aborts the program.

Behavioral Comparison

try {
    // ...
} catch (BaseException& e) {
    throw e; // INCORRECT: Copies 'e' as a BaseException, slicing any derived data.
}

try {
    // ...
} catch (BaseException& e) {
    throw;   // CORRECT: Rethrows the original object, preserving its derived type.
}

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