diff options
author | Thiago Macieira <thiago.macieira@intel.com> | 2013-08-28 18:22:22 -0700 |
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committer | The Qt Project <gerrit-noreply@qt-project.org> | 2013-08-31 23:02:59 +0200 |
commit | e13e90fb55149833aa96e46087f47e4dd9ca0a75 (patch) | |
tree | 791f54f16ad8fa4ddc09a1cf3d24d8cbbb98d32e /src/corelib/tools/qbitarray.cpp | |
parent | 873ae53d47b34e51c0e44303499fad23737f9e5b (diff) |
Add a construction note on why QBitArray has a +1 everywhere
It took me several minutes reading the code to figure out why it was
there. It wasn't immediately obvious.
Change-Id: Ic36b3fd24ce84a1b08c73986d3b7ab8a5bfff133
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Diffstat (limited to 'src/corelib/tools/qbitarray.cpp')
-rw-r--r-- | src/corelib/tools/qbitarray.cpp | 14 |
1 files changed, 14 insertions, 0 deletions
diff --git a/src/corelib/tools/qbitarray.cpp b/src/corelib/tools/qbitarray.cpp index e432322aeb..cae8267772 100644 --- a/src/corelib/tools/qbitarray.cpp +++ b/src/corelib/tools/qbitarray.cpp @@ -117,6 +117,20 @@ QT_BEGIN_NAMESPACE \sa isEmpty() */ +/* + * QBitArray construction note: + * + * We overallocate the byte array by 1 byte. The first user bit is at + * d.data()[1]. On the extra first byte, we store the difference between the + * number of bits in the byte array (including this byte) and the number of + * bits in the bit array. Therefore, it's always a number between 8 and 15. + * + * This allows for fast calculation of the bit array size: + * inline int size() const { return (d.size() << 3) - *d.constData(); } + * + * Note: for an array of zero size, *d.constData() is the QByteArray implicit NUL. + */ + /*! Constructs a bit array containing \a size bits. The bits are initialized with \a value, which defaults to false (0). |