Copy std::stack into an std::vector
Is the following code guaranteed by the standard to work(assuming st is not empty)?
#include <vector>
#include <stack>
int 开发者_开发问答main()
{
extern std::stack<int, std::vector<int> > st;
int* end = &st.top() + 1;
int* begin = end - st.size();
std::vector<int> stack_contents(begin, end);
}
Yes.
std::stack
is just a container adapter.
You can see that .top()
is actually (§23.3.5.3.1)
reference top() { return c.back(); }
Where c
is the container, which in this case is a std::vector
Which means that your code is basically translated into:
extern std::vector<int> st;
int* end = &st.back() + 1;
int* begin = end - st.size();
std::vector<int> stack_contents(begin, end);
And as std::vector
is guaranteed to be continuous there should be no problem.
However, that does not mean that this is a good idea. If you need to use "hacks" like this it is generally an indicator of bad design. You probably want to use std::vector
from the beginning.
Only std::vector
is guaranteed by C++03 to have contiguous elements (23.4.1). In C++1x this will be extended to std::string
as well (defect #530).
Yes, it's guaranteed. Vectors are guaranteed to use contiguous storage, so your code will work. It's a bit cludgy though - and if someone changes the underlying container type of the stack, your code will continue to compile without errors, yet the runtime behaviour will be broken.
I don't have a reference to the standard to back this up unfortunately, but there aren't many ways in which it could go wrong I guess:
- Specifying
std::vector<int>
as the container type means that the elements must be stored in astd::vector<int>
. st.top()
must return a reference to an element in the underlying container (i.e. an element in thestd::vector<int>
. Since the requirements on the container are that it supportsback()
,push_back()
andpop_back()
, we can reasonably assume thattop()
returns a reference to the last element in the vector.end
therefore points to one past the last element.start
therefore points to the beginning.
Conclusion: Unless the assumption was wrong, it must work.
EDIT: And given the other answer's reference to the standard, the assumption is correct, so it works.
According to this page, std::stack
uses a container class to store elements.
I guess what you suggest works only if the containter store its elements in a linear way (std::vector
).
As a default, std::stack
uses a std::deque
which, as far as I know, doesn't meet this requirement. But If you specify a std::vector
as a container class, I can't see a reason why it shoudln't work.
Edit: initial statement redacted, the standard actually does provide a full definition for the stack adaptor, nothing left to implentors. see top answer.
You want a container that has a push and pop method and allows you to inspect elements anywhere in the container and uses a std::vector
for storage. There is such a container in the standard template library
it is called std::vector
.
Use std::stack
only for bondage purposes
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