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From Mark Brown <>
Subject Re: question on the examples for the source/forward/backward incompatible changes
Date Sat, 24 Nov 2007 18:03:41 GMT
Martin Sebor wrote:
> Farid Zaripov wrote:
>>   Whether these examples should be abstract, or based on C++ Standard
>> Library
>> declarations/definitions?
> Examples using the C++ standard library classes or functions would
> be great, but any other sort will, in my opinion, work just as well.
> The point is to highlight the general kind of changes people might
> easily think are compatible when in reality they're not.
>>   Since the source  incompatible changes involves changes in
>> function/class mebmers
>> prototype, or changes in global data/class data members declaration, or
>> changing the
>> place of the some declaration (i.e. moving the declaration from one
>> header file to the another),
>> but requirements of the all of this are stated in C++ Standard and,
>> accordingly, any of these
>> changes are impossible.
> Let me give one example where this isn't true. The standard leaves
> it up to each implementation to decide whether to define overloads
> of certain functions or whether to make use of default function
> arguments. For instance, vector::resize() can be declared in one
> of two ways:
>     resize(size_type, value_type = value_type());
> or
>     resize(size_type);
>     resize(size_type, value_type);
> Changing from one form to the other is detectable and might qualify
> as a breaking change. First, programs that explicitly instantiate
> vector on a user-defined type that's not default constructible will
> compile with the first declaration but not with the second (because
> the definition of the first overload must default-construct the
> element). Second, programs that take the address of such functions
> will not compile. The standard leaves the exact type of library
> member functions unspecified so it's up to us whether we want to
> go a step beyond and provide a stronger guarantee or not.

Or adding overloads with different behavior to functions that are
a better match for calls in existing programs. For instance, if we
added an overload for the ostream inserter for wchar_t* that did
something different than print the value of the pointer it would
break programs that insert wchar_t pointers into narrow streams.
So code like this: std::cout << L"insert a pointer"; inserts the
address of the wide string today, but if we added the overload it
would write the string itself.


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