I borrowed N. Josuttis's code for a queue ("The C++ Standard Library",
1999, p. 450), with which he modifies the interface to read and
discard the front element on pop() and to throw an exception if
front() or pop() is called when the queue is empty.
I want to further modify the new queue class to accept an int as the
number of elements the underlying deque will allocate upon
construction. std::deque's got a constructor for this but I can't
figure out how to access it. I just don't understand templates
sufficiently clearly. Would you please suggest a syntax I can use and
explain how it works? Thanks.
Here's the relevant part of the code:
#include <deque>
#include <exception>
template <class T> class QUEUE2 {
protected:
std::deque<T> c; // the actual container
public:
/* *********
This sad business makes the code fail to compile. I've tried a
number of variations which result in redefinitions of c (the actual
container) or c not found.
// constructor - allow a pre-allocation for the deque's elements
QUEUE2( int prealloc) {
std::deque<T> c( prealloc ); // the actual container
}
********** */
// exception class
class read_empty : public std::exception {
public:
virtual const char * what() const throw() {
return "Read or popped an empty QUEUE2";
}
};
// read front element, return its value then discard it
T pop()
{
if ( c.empty())
throw read_empty();
// otherwise, we're OK
T elem( c.front() );
c.pop_front();
return elem;
}
// the rest of the usual queue funcs are implemented, e.g.,
// size()
// empty()
// push()
// front() like pop(), w/ the throw
};
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