- Generated by
1.9.8
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BDE 4.39.x Production Release
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Provide a value-semantic attribute type for position and length.
Provide a value-semantic attribute type for position and length.
This component provides a value-semantic, constrained attribute type IndexSpan that stores a position and a length. The constraint is that the sum of the two attributes must be representable by the type bsl::size_t.
position: this span starts at this position in a sequence.length: this span contains this many elements.For example, an IndexSpan describing the middle name in the string "James Tiberius Kirk" will have the position 6 (the T of "Tiberius" is the 7th character of the string) and the length 8 ("Tiberius" is 8 characters long).
Note that the sum of the two attributes must be representable by the bsl::size_t type.
This section illustrates intended use of this component.
Suppose that we work with file system paths and we frequently need the path, the base name, the extension, and the filename, as well as the full path itself. Therefore we want to create a class that stores the full path and a way to quickly access the individual parts. (To keep things simple we will make the example code work on POSIX-style paths so we do not need to deal with the difference in path separators.)
If we try the naive solution – storing a bsl::string of the full path and bslstl::StringRef for path, basename and extension – the class bslstl::StringRef members will reference back into memory owned by the object, and the result is the compiler-generated (and naively written) move and copy operations will be broken. In addition, explicitly implementing move and copy operations is quite difficult and error prone.
The simplest (most readable) solution to the problem is to store a position and a length for the path, basename, and extension (rather than a bslstl::StringRef) because that representation is independent of the location of the memory for the string. IndexSpan provides such a representation, and allow us to implement our class using a simple copy and move operations (or in other contexts, compiler supplied operations).
First, we define the members and interface of the class.
Next, to make the parsing code short and readable, we implement a helper function to create IndexSpan objects from two positions. (In practice we would use the higher level utility function IndexSpanStringUtil::createFromPosition.)
Then, we implement the parsing constructor using the create function.
Next, we implement the (now) simple copy constructor:
Then, to make the accessors simple (and readable), we implement a helper function that creates a StringRef from a StringRef and an IndexSpan. (In practice we would use the higher level utility function IndexSpanStringUtil::bind.)
Next, we implement the accessors:
Finally, we verify that the resulting class is copied properly. We do that by determining that the original and the copy object has equal but distinct strings and that the (other) accessors return references into those strings.
1.9.8