exception in string

Hello everyone,


I am reviewing the exception safety of some code and met with some issues about exception safety in string, which I can not find the answer at hand. I think the 3 code segments may all throw exceptions. Because the storage of string internal character data is on heap (using allocator for char?), so when there is low memory, there will be bad_alloc exception?

1. Initialization

Code:

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string str1 = "Hello"; // or string str ("Hello");

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2. Assignment

Code:

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string str2 = str1; // str1 is another string object or reference.

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3. Empty construction

Code:

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string str;

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I am not quite sure about (3).


thanks in advance,
George

I think it depends on implementation, Some can decide to allocate storage even in case of default constructor for internal use. Most I thinkwill not bother...

All of string's constructors may throw.
The base rule is that all functions may throw unless the standard explicitly says otherwise. From 17.3.4.8/3:

Quote:

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Any of the functions defined in the C++ Standard library that do not have an exception-specification may throw implementation-defined exceptions.

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Thanks vart,


So your answer to my question is all the scenarios may throw?

Quote:

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Originally Posted by vart

I think it depends on implementation, Some can decide to allocate storage even in case of default constructor for internal use. Most I thinkwill not bother...

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Thanks CornedBee,


So, you answer to my 3 scenarios is, all of them may throw?

Quote:

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Originally Posted by CornedBee

All of string's constructors may throw.
The base rule is that all functions may throw unless the standard explicitly says otherwise. From 17.3.4.8/3:

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regards,
George

There is no guarantee that #3 example DOESN'T allocate memory (take for example an implementation of string that maintains a valid C string at all times - it would require an empty string to have a single byte of the value zero to indicate an empty string).

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Mats

Provided it doesn't use small buffer optimization.

Thanks Mats and CornedBee,


Let me conclude, I think it is safe to assume all of the 3 code segments will (may) throw exception. Please comment if you do not agree. :-)

Quote:

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Originally Posted by CornedBee

Provided it doesn't use small buffer optimization.

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regards,
George

Quote:

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Originally Posted by CornedBee

Provided it doesn't use small buffer optimization.

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Of course - my point was more that UNLESS YOU KNOW the implementation, you can't know if the string is allocated or not. Small buffer optimization is an implementation that doesn't allocate storage for an empty string. Another option for the empty string is to not have a string stored at all - simply by the fact that the length is zero.

My example, by the way, is not necessarily how it is implemented in ANY version of std::string - but since there is no clear statement of how it should be implemented, we can't determine what it does in the empty constructor example, or in any other constructor for that matter, and whether that will lead to an exception or not.

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Mats

Thanks Mats,


So, I think you agree with my post #7. :-)

Quote:

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Originally Posted by matsp

Of course - my point was more that UNLESS YOU KNOW the implementation, you can't know if the string is allocated or not. Small buffer optimization is an implementation that doesn't allocate storage for an empty string. Another option for the empty string is to not have a string stored at all - simply by the fact that the length is zero.

My example, by the way, is not necessarily how it is implemented in ANY version of std::string - but since there is no clear statement of how it should be implemented, we can't determine what it does in the empty constructor example, or in any other constructor for that matter, and whether that will lead to an exception or not.

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Mats

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regards,
George

Yes, I do - there is no implicit guarantee that ANY string constructor are "exception safe".

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Mats

Thanks Mats,


My question is answered.

Quote:

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Originally Posted by matsp

Yes, I do - there is no implicit guarantee that ANY string constructor are "exception safe".

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Mats

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regards,
George