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40 lines
1.8 KiB
Text
40 lines
1.8 KiB
Text
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As we've seen with placement tt(new) objects can be constructed in blocks
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of memory of tt(sizeof(Class)) bytes large. And so, two objects of the same
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class each occupy tt(sizeof(Class)) bytes.
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If objects of our class can be swapped, and if our class's data members do
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not refer to data actually involved in the swapping operation then a very fast
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swapping method that is based on the fact that we know how large our objects
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are can be implemented.
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In this fast-swap method we merely swap the contents of the tt(sizeof(Class))
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bytes. This procedure may be applied to classes whose objects may be swapped
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using a member-by-member swapping operation and can (in practice, although
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this probabaly overstretches the allowed operations as described by the
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bf(C++) ANSI/ISO standard) also be used in classes having reference data
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members. It simply defines a buffer of tt(sizeof(Class)) bytes and performs a
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circular ti(memcpy) operation. Here is its implementation for a hypothetical
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class tt(Class). It results in very fast swapping:
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verb(
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#include <cstring>
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void Class::swap(Class &other)
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{
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char buffer[sizeof(Class)];
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memcpy(buffer, &other, sizeof(Class));
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memcpy(&other, this, sizeof(Class));
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memcpy(this, buffer, sizeof(Class));
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}
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)
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Here is a simple example of a class defining a reference data member and
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offering a tt(swap) member implemented like the one above. The reference data
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members are initialized to external streams. After running the program tt(one)
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contains two em(hello to 1) lines, tt(two) contains two em(hello to 2) lines
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(for brevity all members of tt(Reference) are defined inline):
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verbinclude(examples/fastswap.cc)
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Fast swapping should only be used for self-defined classes for which it can be
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proven that fast-swapping does not corrupt its objects, when swapped.
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