Introduction

UK-136

Exception specifications have proven close to worthless in practice, while adding a measurable overhead to programs. The feature should be deprecated. The one exception to the rule is the empty throw specification which could serve a legitimate optimizing role if the requirement to call the runtime unexpected mechanism was relaxed in this case.

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-list_opt).

Approach

To aid in the transition from dynamic exception specifications to noexcept, the wording provides somewhat loose compatibility rules for redeclarations of functions that have exception specifications. Two rules stand out:

1) All "non-throwing" forms of exception specifications (throw(), noexcept, noexcept(true)) are considered compatible, but the exception specification on the definition is what affects code generation:

// header ultramodern.h
void f() noexcept;

// source plodding.cpp
#include "ultramodern.h"
struct X { };
void f() throw() { // okay, compatible with noexcept
  throw X(); // calls std::unexpected()
}

2) noexcept(false) is considered compatible with throw( type-id-list ):

// header ultramodern.h
void g() noexcept(false);

// source plodding.cpp
#include "ultramodern.h"
struct X { };
void g() throw(X) { // okay, compatible with noexcept(false)
  throw X(); // okay
}

These compatibility rules allow a gradual migration from dynamic exception specifications to noexcept, since a declaration of a function can choose to use the new or old syntax independently in the declaration and in the definition.

Proposed Changes to Standard Wording

The wording in this paper is based on the current working paper (N3035) as amended by N3050.