The sad history of the C++ throw(…) exception specifier

Raymond Chen

When exceptions were introduced into the C++ language, a corresponding throw(...) dynamic exception specifier was introduced which annotated which exceptions could be throw by a function.

// this function might throw an integer or a pointer to char,
// but nothing else.
void foo() throw(int, char*);

This has made a lot of people very angry and has been widely regarded as a bad move.

According to the C++98 standard, if a function throws an exception not listed among the types specified in its dynamic exception specifier, the system called the std::unexpected() function, and the default behavior of std::unexpected() is to terminate the program by calling std::terminate(). As a special case, throw() means that the function shouldn’t throw any exceptions at all.

By C++11, the throw(...) dynamic exception specifier was deprecated, and in C++17, all support for dynamic exception specifiers was removed save for the special case of throw(). At the same time, they changed the penalty for throwing an exception when you said you wouldn’t: the runtime calls std::terminate() directly, rather than passing through std::unexpected().

But of course the Microsoft C++ compiler has to do things a little bit differently.

C++14 and earlier
Disallowed exception thrown
Standard behavior Microsoft behavior
Nonthrowing noexcept
std::unexpected std::unexpected
throw() std::unexpected undefined behavior 
Throwing noexcept(false) exception propagates exception propagates
throw(something) std::unexpected exception propagates 

The Microsoft C++ compiler treats the throw(...) exception specifier as a promise on the part of the programmer, but there is no enforcement. It trusts you to adhere to your self-imposed contract. If an exception is thrown when the function promised that no exceptions would be thrown, the behavior is undefined. If the function said that some exceptions could be thrown, the compiler doesn’t validate that the actual thrown exception is allowed; it just propagates the exception.

In practice, what happened is that the compiler performed optimizations on the assumption that no disallowed exception would be thrown. The most common such optimization is that the compiler won’t bother registering unwind codes for things that it “knows” will never require unwinding because there are no points where an exception could be thrown prior to the object’s destruction.

void Example()
   ObjectWithDestructor obj;
   // destructor runs here

If stuff_that_does_not_throw is marked as non-throwing, then the compiler can avoid having to register obj for unwinding during exception propagation, since you promised that no exception could escape.

And then you throw an exception and invalidate all those optimizations. The most common visible effect of this is that an exception propagated out of a function that should never have let an exception escape, and some object destructors failed to run.

But wait, all is not lost.

If you enable /std:c++17, then the Microsoft C++ compiler will implement the standard behavior for throw(...).

Disallowed exception thrown
Standard behavior Microsoft behavior
with /std:c++17
Nonthrowing noexcept
std::terminate std::terminate
throw() std::terminate std::terminate
Throwing noexcept(false) exception propagates exception propagates
throw(something) not supported not supported

Yes, it took a long time to get there, but better late than never.


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