queue<char> and memcpy()

[Alex Vinokur]

Is using memcpy() with queue<char> safe?

------ C++ code ------
#include <cstring>
#include <iostream>
#include <queue>
using namespace std;

int main()
{
queue<char> qch;
char ach[100];

qch.push ('h');
qch.push ('e');
qch.push ('l');
qch.push ('l');
qch.push ('o');
qch.push (' ');
qch.push ('w');
qch.push ('o');
qch.push ('r');
qch.push ('l');
qch.push ('d');

memset (ach, 0, sizeof(ach));

memcpy (ach, &qch.front(), 5); // Is this safe?
cout << ach << endl;

qch.pop();
qch.pop();
qch.pop();
qch.pop();
qch.pop();
qch.pop();

memcpy (ach, &qch.front(), 5);
cout << ach << endl;

return 0;
}

----------------------

--- Output ---

hello
world

--------------


Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn

 

[Dietmar Kuehl]

Is using memcpy() with queue<char> safe?

No: the default container underlying a 'std::queue<T>' is 'std::deque<T>'
which is not contiguous at all. Well, it may be contiguous if all
elements fit into on block but even this is not guaranteed as objects
are not rearranged when removing the from the front.

 

[Alex Vinokur]

No: the default container underlying a 'std::queue<T>' is 'std::deque<T>'
which is not contiguous at all. Well, it may be contiguous if all
elements fit into on block but even this is not guaranteed as objects
are not rearranged when removing the from the front.

[snip]

So, how to copy data chunks from queue<char> into char[] ?
One-to-one?

queue<char> qch;
char ach[100];

for (int i = 0; i < 5; i++)
{
char = qch.front();
qch.pop();
}

Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn

 

[peter koch]

No: the default container underlying a 'std::queue<T>' is 'std::deque<T>'
which is not contiguous at all. Well, it may be contiguous if all
elements fit into on block but even this is not guaranteed as objects
are not rearranged when removing the from the front.

[snip]

So, how to copy data chunks from queue<char> into char[] ?
One-to-one?

Right. But you could wrap it in a function if you'd like to. You
know... memcpy is likely to copy one-to-one as well.

/Peter

queue<char> qch;
char ach[100];

for (int i = 0; i < 5; i++)
{
char = qch.front();
qch.pop();
}

Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn