Re: exceptions and container modifiers
subramanian100in@yahoo.com, India <subramanian100in@yahoo.com>, on
23/07/2010 18:51:50, wrote:
* Howard Hinnant<howard.hinn...@gmail.com> wrote:
If a copy or assignment throws an exception, the container will be
left in a valid but unspecified state. A client who catches the
exception before the container is destructed may inspect the state of
the container, and may perform any operation on the container for
which the unspecified state meets all of the requirements for said
operation (e.g. container.size(), container.clear(), etc.).
-Howard
Thanks for your reply. Kindly clarify the following portion in your
reply.
How do I inspect the state of the container(before it gets
destructed) ? Please give sample program.
What operation can be performed on the container, what are the
requirements of the unspecified state and how to check if it meets the
requirements ? Kindly explain with code sample.
"Valid but unspecified" means that it is just valid as before, only that
there is no guarantee about the results of the operation that threw the
exception. The container could have been cleared, could have been left
completely unchanged, could have received only part of the data and so
forth.
Since it's a completely valid object, you can perform all the
(reasonable) operations as you would with any other object. Being able
to inspect it you can check whether any work has been done, and if so,
how much of it.
This is a silly example which very unlikely would return anything
different from zero:
#include <vector>
#include <exception>
int main() {
std::vector<int> v, w;
try {
w.push_back(42);
v = w;
} catch(std::exception) {
if(v.size()) {
// v.size() elements have been copied
// it is safe to access the elements:
return v[0];
}
if(w.size()) {
// w.size() elements have been inserted
// it is safe to access the elements:
return w[0];
}
}
return 0;
}
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