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rockenstein
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 Posted: Mon Aug 18, 2003 5:21 pm Post subject: question on std::stack |
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Hi,
I have a question on using std::stack (StlPort):
-------------begin PseudoCode---------
std::stack<myData>;
//push some elements on stack
// get reference to top element
myData elem = stack.top();
// remove top element from stack
stack.pop();
// do something with elem. Is elem still valid after popping?
-------------end PseudoCode---------
According to the documentation top() returns a reference to the top element
of the stack and
pop() removes the first element from the stack. So elem should be invalid
after calling pop?
Or am i missing something here?
greets
Michael
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Kai Ruhnau
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| Posted: Tue Aug 19, 2003 12:18 am Post subject: Re: question on std::stack |
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rockenstein wrote:
| Quote: | I have a question on using std::stack (StlPort):
-------------begin PseudoCode---------
std::stack<myData>;
//push some elements on stack
// get reference to top element
myData elem = stack.top();
// remove top element from stack
stack.pop();
// do something with elem. Is elem still valid after popping?
-------------end PseudoCode---------
According to the documentation top() returns a reference to the top element
of the stack and
pop() removes the first element from the stack. So elem should be invalid
after calling pop?
Or am i missing something here?
|
Your 'elem' is not declared as reference, so stack::top() returns a
reference, which is then copie(constructe)d to elem. So elem remains valid
even if you pop() away the top-Element of your stack.
Greetings
Kai
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ulrich Eckhardt
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| Posted: Tue Aug 19, 2003 12:22 am Post subject: Re: question on std::stack |
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rockenstein wrote:
| Quote: | -------------begin PseudoCode---------
not really that pseudo ... *shrug* |
| Quote: | std::stack<myData>;
//push some elements on stack
// get reference to top element
myData elem = stack.top();
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This in fact performs a copy.
| Quote: | // remove top element from stack
stack.pop();
// do something with elem. Is elem still valid after popping?
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Yes. Note also that doing anything to elem will not alter the topmost
element of the stack.
If you want to keep the reference, use a syntax like this:
myData& elem_ref = stack.top();
For that reference, all your assumptions about invalidation are true though.
cheers
Uli
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Ron
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| Posted: Tue Aug 19, 2003 10:27 am Post subject: Re: question on std::stack |
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| Quote: | I have a question on using std::stack (StlPort):
-------------begin PseudoCode---------
std::stack<myData>;
//push some elements on stack
// get reference to top element
myData elem = stack.top();
// remove top element from stack
stack.pop();
// do something with elem. Is elem still valid after popping?
-------------end PseudoCode---------
According to the documentation top() returns a reference to the top element
of the stack and
pop() removes the first element from the stack. So elem should be invalid
after calling pop?
Or am i missing something here?
|
"elem" is a variable of type myData, not a reference. top () does,
indeed, return a reference, but the compiler copies the item to which
it refers into elem, so elem remains valid after calling pop (). Note
that elem would be *invalid* after pop () if it had been declared as a
reference to a myData:
myData &elem = stack.top();
-- Ron
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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