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Overloading example
We can now modify earlier examples to use
function overloading. As stated before, an immediately useful place for
overloading is in constructors. You can see this in the following version of the
Stash class:
//: C07:Stash3.h
// Function overloading
#ifndef STASH3_H
#define STASH3_H
class Stash {
int size; // Size of each space
int quantity; // Number of storage spaces
int next; // Next empty space
// Dynamically allocated array of bytes:
unsigned char* storage;
void inflate(int increase);
public:
Stash(int size); // Zero quantity
Stash(int size, int initQuantity);
~Stash();
int add(void* element);
void* fetch(int index);
int count();
};
#endif // STASH3_H ///:~
The first Stash( )
constructor is the same as before, but the second one has a Quantity
argument to indicate the initial number of storage places to be allocated. In
the definition, you can see that the internal value of quantity is set to
zero, along with the storage pointer. In the second constructor, the call
to inflate(initQuantity) increases quantity to the allocated
size:
//: C07:Stash3.cpp {O}
// Function overloading
#include "Stash3.h"
#include "../require.h"
#include <iostream>
#include <cassert>
using namespace std;
const int increment = 100;
Stash::Stash(int sz) {
size = sz;
quantity = 0;
next = 0;
storage = 0;
}
Stash::Stash(int sz, int initQuantity) {
size = sz;
quantity = 0;
next = 0;
storage = 0;
inflate(initQuantity);
}
Stash::~Stash() {
if(storage != 0) {
cout << "freeing storage" << endl;
delete []storage;
}
}
int Stash::add(void* element) {
if(next >= quantity) // Enough space left?
inflate(increment);
// Copy element into storage,
// starting at next empty space:
int startBytes = next * size;
unsigned char* e = (unsigned char*)element;
for(int i = 0; i < size; i++)
storage[startBytes + i] = e[i];
next++;
return(next - 1); // Index number
}
void* Stash::fetch(int index) {
require(0 <= index, "Stash::fetch (-)index");
if(index >= next)
return 0; // To indicate the end
// Produce pointer to desired element:
return &(storage[index * size]);
}
int Stash::count() {
return next; // Number of elements in CStash
}
void Stash::inflate(int increase) {
assert(increase >= 0);
if(increase == 0) return;
int newQuantity = quantity + increase;
int newBytes = newQuantity * size;
int oldBytes = quantity * size;
unsigned char* b = new unsigned char[newBytes];
for(int i = 0; i < oldBytes; i++)
b[i] = storage[i]; // Copy old to new
delete [](storage); // Release old storage
storage = b; // Point to new memory
quantity = newQuantity; // Adjust the size
} ///:~
When you use the first constructor no
memory is allocated for storage. The allocation happens the first time
you try to add( ) an object and any time the current block of memory
is exceeded inside add( ).
Both constructors are exercised in the
test program:
//: C07:Stash3Test.cpp
//{L} Stash3
// Function overloading
#include "Stash3.h"
#include "../require.h"
#include <fstream>
#include <iostream>
#include <string>
using namespace std;
int main() {
Stash intStash(sizeof(int));
for(int i = 0; i < 100; i++)
intStash.add(&i);
for(int j = 0; j < intStash.count(); j++)
cout << "intStash.fetch(" << j << ") = "
<< *(int*)intStash.fetch(j)
<< endl;
const int bufsize = 80;
Stash stringStash(sizeof(char) * bufsize, 100);
ifstream in("Stash3Test.cpp");
assure(in, "Stash3Test.cpp");
string line;
while(getline(in, line))
stringStash.add((char*)line.c_str());
int k = 0;
char* cp;
while((cp = (char*)stringStash.fetch(k++))!=0)
cout << "stringStash.fetch(" << k << ") = "
<< cp << endl;
} ///:~
The constructor call for stringStash
uses a second argument; presumably you know something special about the
specific problem you’re solving that allows you to choose an initial size
for the Stash.
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