Old snapshot `BigIntegerLibrary-2005.01.06'; see the ChangeLog file.

[bigint/bigint.git] / NumberlikeArray.hh

diff --git a/NumberlikeArray.hh b/NumberlikeArray.hh
index 10b7262..79d46df 100644 (file)
--- a/NumberlikeArray.hh
+++ b/NumberlikeArray.hh
@@ -3,9 +3,22 @@
 * http://mysite.verizon.net/mccutchen/bigint/
 */
 
+/*
+* This mechanism prevents files from being included twice.
+* Each file gets its own `id' (here `NUMBERLIKEARRAY').
+* When `#include'd, this file checks whether its `id' has
+* already been flagged.  If not, it flags the `id' and
+* loads the declarations.
+*/
 #ifndef NUMBERLIKEARRAY
 #define NUMBERLIKEARRAY
 
+// An essential memory-management constant.
+// I wish this were built into C++ just as it is in Java.
+#ifndef NULL
+#define NULL 0
+#endif
+
 /*
 * A NumberlikeArray<Block> object holds a dynamically
 * allocated array of Blocks.  It provides certain basic
@@ -34,22 +47,53 @@ class NumberlikeArray {
        Index cap; // The current allocated capacity of this NumberlikeArray (in blocks)
        Index len; // The actual length of the value stored in this NumberlikeArray (in blocks)
        Blk *blk; // Dynamically allocated array of the blocks
+       /*
+       * Change made on 2005.01.06:
+       *
+       * If a zero-length NumberlikeArray is desired, no array is actually allocated.
+       * Instead, `blk' is set to `NULL', and `cap' and `len' are zero as usual.
+       *
+       * `blk' is never dereferenced if the array has zero length.  Furthermore,
+       * `delete NULL;' does nothing and causes no error. Therefore, we can use
+       * `NULL' as if it were a zero-length array from `new'.
+       *
+       * This is a great convenience because the only code that need be changed
+       * is the array allocation code.  All other code will still work file.
+       */
        
        // MANAGEMENT
-       NumberlikeArray(int, Index c) : cap(c), len(0) { // Creates a NumberlikeArray with a capacity
-               blk = new Blk[cap];
+       NumberlikeArray(Index c) : cap(c), len(0) { // Creates a NumberlikeArray with a capacity
+               blk = (cap > 0) ? (new Blk[cap]) : NULL;
        }
        void allocate(Index c); // Ensures the array has at least the indicated capacity, maybe discarding contents
        void allocateAndCopy(Index c); // Ensures the array has at least the indicated capacity, preserving its contents
        
-       NumberlikeArray() : cap(0), len(0) { // Default constructor (empty array)
-               blk = new Blk[0];
+       /*
+       * Default constructor.
+       *
+       * If a class derived from NumberlikeArray knows at initializer time what size array
+       * it wants, it can call the first constructor listed above in an initializer.
+       *
+       * Otherwise, this default constructor will be implicitly invoked, pointing `blk' to
+       * `NULL', a fake zero-length block array.  The derived class can allocate the desired
+       * array itself and overwrite `blk'; it need not `delete [] blk' first.
+       *
+       * This change fixes a memory leak reported by Milan Tomic on 2005.01.06.
+       * Integer-type-to-BigUnsigned (and BigInteger) conversion constructors have always
+       * allocated their own array of length 0 or 1 after seeing whether the input is zero.
+       * But when the NumberlikeArray transition occurred, these constructors contained an
+       * implicit initializer call to the old NumberlikeArray default constructor, which
+       * created a real `new'-allocated zero-length array.  This array would then be lost,
+       * causing a small but annoying memory leak.
+       */
+       NumberlikeArray() : cap(0), len(0) {
+               blk = NULL;
        }
        NumberlikeArray(const NumberlikeArray<Blk> &x); // Copy constructor
        void operator=(const NumberlikeArray<Blk> &x); // Assignment operator
        NumberlikeArray(const Blk *b, Index l); // Constructor from an array of blocks
        ~NumberlikeArray() { // Destructor
-               delete [] blk;
+               delete [] blk; // Does nothing and causes no error if `blk' is null.
        }
        
        // PICKING APART