As expressed in the national body comment above, exception specifications have not proven useful in practice. There are numerous discussions of the problems with exception specifications in C++ (see, e.g., [Sutter02], [Boost03]), but the main issues are:
- Run-time checking: C++ exception specifications are checked at runtime rather than at compile time, so they offer no programmer guarantees that all exceptions have been handled. The run-time failure mode (calling std::unexpected()) does not lend itself to recovery.
- Run-time overhead: Run-time checking requires the compiler to produce additional code that also hampers optimizations.
- Unusable in generic code: Within generic code, it is not generally possible to know what types of exceptions may be thrown from operations on template arguments, so a precise exception specification cannot be written.
In practice, only two forms of exception-throwing guarantees are useful: an operation might throw an exception (any exception) or an operation will never throw any exception. The former is expressed by omitting the exception-specification entirely, while the latter can be expressed as throw() but rarely is, due to performance considerations.
[N3050] introduces a new kind of exception specification, noexcept, the specifies that the function will not throw any exceptions. Unlike throw(), noexcept does not require the compiler to introduce code to check whether an exception is thrown. Rather, if a function specified as noexcept is exited via an exception, the result is a call to std::terminate().
With the introduction of noexcept, programmers can now express the two kinds of exception guarantees that are useful in practice, without additional overhead. This paper therefore proposes to deprecate "dynamic" exception specifications, i.e., those that are written as throw(type-id-listopt).
