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59 lines
No EOL
3.1 KiB
C++
59 lines
No EOL
3.1 KiB
C++
topic "String and WString";
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[2 $$0,0#00000000000000000000000000000000:Default]
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[i448;a25;kKO9;*@(64)2 $$1,0#37138531426314131252341829483380:class]
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[l288;2 $$2,2#27521748481378242620020725143825:desc]
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[a83;*R6 $$3,0#31310162474203024125188417583966:caption]
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[l288;i1121;b17;O9;~~~.1408;2 $$4,0#10431211400427159095818037425705:param]
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[i448;a25;kKO9;*@(64)2 $$5,0#37138531426314131252341829483370:item]
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[*+117 $$6,6#14700283458701402223321329925657:header]
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[l416;2 $$7,7#55548704457842300043401641954952:nested`-desc]
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[l288;i448;a25;kO9;*2 $$8,8#64691275497409617375831514634295:nested`-class]
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[{_}%EN-US
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[s3; String and WString&]
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[s0; Core implementation of String and WString is totally independent.&]
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[s0; &]
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[s0; Core implementation is represented by String0 and WString0 classes.
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Common methods shared by both are then added using AString template
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(which inherits either String0 or WString0). Implementation is
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then finalized in String and WString final classes.&]
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[s0; &]
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[s6; String&]
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[s0; String uses three step implementation to maximize performance.&]
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[s0; &]
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[s0; Strings up to 14 characters (bytes) can be stored using small`-string
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optimization inside String value. sizeof(String) is 16; first
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14 bytes (0`- 13) in this case are character data, byte 14 is
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reserved for zero terminator (and is always zero for this kind),
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byte 15 is the string len. Also, for this first kind of string,
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all characters len ... 14 are strictly kept zero `- this allows
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very fast implementation of equality comparison.&]
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[s0; &]
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[s0; For strings up to 31 characters, bytes 0`-7 are used for pointer
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to character data. Character data are allocated as 32 bytes block
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(one byte for zero terminator); String implementation is directly
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connected with heap allocator to bypass size processing here
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and achieve maximum performance. Length of String is stored in
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the bytes 8`-11. Byte 14 is in this case 31 (`"MEDIUM`") `- it
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is the maximum number allowed to be stored in the buffer. Note
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that non`-zero value in byte 14 signals that string is not `"small`".
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When copying String of this size, new memory block is always
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allocated and data is copied. With direct link to heap allocator,
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this is about as fast as using reference counting in single threaded
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mode and faster in multithreaded.&]
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[s0; &]
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[s0; Finally, if size exceeds 31 characters, String uses reference
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counting. Layout is similar to `"MEDIUM`", but memory block is
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of variable length and preceded with reference count and the
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size of block (`"alloc`") `- maximum number of characters the
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block can store. Note that as an optimization, for blocks smaller
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than 255 characters, this number is also stored in byte 14 for
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faster retrieval. If block is larger, 255 is in byte 14 and the
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`"alloc`" size has to be retrieved from the memory block.&]
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[s0; &]
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[s6; WString&]
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[s0; WString implementation does not use small string optimization,
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first step is 23 wchars stored in 48 bytes memory block without
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reference counting, for larger string once again reference counting
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is applied. Sizes are also directly stored in `"int`" member
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variables.&]
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[s0; ]] |