template question: preallocation for the underlying deque in user-defined queue

[Heck]

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 Tclass QUEUE2 {
protected:
std::deque<Tc; // 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<Tc( 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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[Victor Bazarov]

Heck wrote:

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 Tclass QUEUE2 {
protected:
std::deque<Tc; // 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<Tc( prealloc ); // the actual container

If you want to give your 'c' some size to begin with (not sure why
you would want this), you need to _initialise_ it, not try to declare
a local variable 'c' in the constructor's body.

I am guessing you're not familiar with *initialiser lists*. Read up
on proper implementations of constructors. Read the FAQ as well.

If I were you, I would still revisit your intent to give 'c' some
initial size. What for? Are you going to do assignment instead of
'push_back' until the size grows up to your 'prealloc'? Why bother?
I say, let 'std::deque' worry about allocations, it usually does
a very good job.

}
********** */
// 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
};

V
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[red floyd]

Heck wrote:

I borrowed N. Josuttis's code for a queue ("The C++ Standard Library",

/* *********
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) : c(prealloc) {
std::deque<Tc;

QUEUE2( int prealloc) {
std::deque<Tc( prealloc ); // the actual container
}

Read your C++ book (not Josuttis -- it's a library ref) , especially
where it discusses initialization lists.

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