[Development] Proposal: adding Q_DECL_NOEXCEPT to many methods

[Thiago Macieira]

I'd like to propose we add Q_DECL_NOEXCEPT to many methods in our API.

We already took the decision to turn exception off in most modules, except 
QtCore and where exceptions were used, like QtXmlPatterns. This is the next 
step.

The Q_DECL_NOEXCEPT macro expands to noexcept, which is a new C++11 keyword. 
It is equivalent to noexcept(true), also new in C++11, and it means the method 
declares that it does not throw any exceptions.

The benefits are:
 - callers do not need to emit exception handlers around such functions
 - the compiler may assume that no exception unexpectedly happens inside that 
function body, foregoing exception handlers inside it as well.

The first behaviour is present with a C++03's empty exception specification 
(i.e., throw() in the function declaration), but the second behaviour is new 
in C++11. In the previous standard, the compiler was forced to emit exception 
code for the case when exceptions did happen even when they shouldn't. For 
that reason, the C++03 exception specification is deprecated.

The drawback is that it makes the code ugly.

The macro expands to nothing in C++98 mode. That means code using the API so 
marked and compiling in C++98 mode will simply not gain the benefits of the 
keyword, but should see no side effects.

The presence of the keyword does not affect binary compatibility. With the 
Itanium C++ ABI, it's not present in the mangling. MSVC2010 does not supoprt 
it yet, so I cannot verify what it does. However, since C++11 support cannot 
be turned on or off on it, the keyword will be enabled or it won't depending on 
the compiler version, which means that binary compatibility is irrelevant. But 
if it does encode it in the mangling, we will not be able to add the keyword 
to methods in 5.1 or later.


The question that remains is: what methods shall we add this to? We can add it 
to any method for which we can *prove* that it will never throw, which are:
 - leaf functions
 - functions calling only C functions or other noexcept functions

Outside of QtCore, I propose we add it only to methods that are called often 
and frequently. In QtCore, I propose we add it to most tool methods that are 
provably noexcept. For example, the qHash functions, QMutex, our memory 
allocation routines (the throwing is in inline code), etc.

PS: to be clear: new throws.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel Open Source Technology Center
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358
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