In C++ exception handling, what is meant by an exception specification on a function declaration or definition?

Introduction / Context:
Older versions of C++ supported dynamic exception specifications, where a function declaration listed the types of exceptions it could throw. Although modern C++ discourages this form in favour of noexcept, interview questions still refer to exception specifications as a way to test understanding of exception safety and function contracts.

Given Data / Assumptions:

• We are discussing C++ functions that may throw exceptions during execution.
• An exception specification appears after the parameter list, for example void f() throw(std::runtime_error); in older C++ syntax.
• The specification lists exception types that the function promises to throw.

Concept / Approach:
An exception specification formally declares which exception types a function may emit. In the classic throw(Type1, Type2) form, if the function allows an exception to escape that is not in the list, unexpected() is called, often leading to program termination. This creates a contract between the function and its callers. While this feature has been deprecated and removed from modern standards, the conceptual idea remains that an exception specification limits what the function can throw.

Step-by-Step Solution:
Step 1: Consider a function declaration: void g() throw(std::bad_alloc);. Step 2: The exception specification indicates that g() may allow std::bad_alloc to propagate but not arbitrary other exception types. Step 3: If g() throws a different exception that is not caught inside g(), the C++ runtime calls unexpected(). Step 4: This behaviour enforces that the function adheres to the specified list of exception types. Step 5: Therefore, the correct description is that the function is limited to throwing only the listed exception types.

Verification / Alternative check:
In older compilers that implement dynamic exception specifications, you can test by writing a function with throw(std::runtime_error) and then deliberately throwing a different exception like std::logic_error without catching it. At run time, you will see unexpected behaviour such as a call to unexpected() or program termination, confirming that the specification constrains the allowed exceptions rather than allowing any type.

Why Other Options Are Wrong:
Option B confuses exception specifications with catch all handlers; a catch(...) handler is used inside try blocks to catch any exception, but this is unrelated to exception specifications on function declarations. Option C incorrectly states the opposite of the actual rule by claiming that any exception type is allowed. Option D is incorrect because option A accurately captures the meaning in the traditional C++ sense.

Common Pitfalls:
Many programmers misunderstand exception specifications and assume they behave like checked exceptions in some other languages. Another pitfall is relying on them for robust error handling, when in fact they can introduce unexpected program termination if violated. Modern C++ prefers the noexcept keyword to indicate functions that are not expected to throw at all, which is simpler and more predictable. Nevertheless, understanding legacy exception specifications is useful when reading older code or interview materials.

Final Answer:
An exception specification means that a function is limited to throwing only a specified list of exception types that appear in its exception specification.