MemView¶

Synopsis¶

#include <ts/MemView.h>

class MemView¶

class StringView¶

These classes act as views in to already allocated memory. Internally in Traffic Server work must be done with string or memory entities that are embedded in larger pre-existing memory structures. These classes are designed to make that easier, more efficient, and less error prone.

Description¶

The term “view” will be used to mean an instance of MemView or StringView. Fundamentally both classes do the same thing, maintain a read only view of a contiguous region of memory. They differ in the methods and return types due to the conflicting requirements of raw memory operations and string based operations.

A view is constructed by providing a contiguous region of memory, either based on a start pointer and a length or a pair of pointers in the usual STL half open range style where the view starts at the first pointer and ends one short of the second pointer. A view can be empty and refer to no memory (which what default construction yields). A view attempts to act like a normal pointer in most situations. A view is only somewhat more expensive than a raw pointer but in most cases a count is needed as well making a view not any more costly than existing code. Any code that already keeps a pointer and a count is a good candidate for using MemView or StringView.

MemView and StringView inter-convert because the difference between them is simply the API to access the underingly memory in the view, the actual class internal data is identical.

StringView provides a variety of methods for manipulating the view as a string. These are provided as families of overloads differentiated by how characters are compared. There are four flavors.

Direct, a pointer to the target character.
Comparison, an explicit character value to compare.
Set, a set of characters (described by a StringView) which are compared, any one of which matches.
Predicate, a function that takes a single character argument and returns a bool to indicate a match.

If the latter three are inadequate the first, the direct pointer, can be used after finding the appropriate character through some other mechanism.

The increment operator for StringView shrinks the view by one character from the front which allows stepping through the view in normal way, although the string view itself should be the loop condition, not a dereference of it.

StringView v;
size_t hash = 0;
for ( ; v ; ++v) hash = hash * 13 + *v;

Or, because the view acts as a container of characters, this can be done non-destructively.

StringView v;
size_t hash = 0;
for (char c : v) hash = hash * 13 + c;

Views are cheap to construct therefore making a copy to use destructively is very inexpensive.

MemView provides a find method that searches for a matching value. The type of this value can be anything that is fixed sized and supports the equality operator. The view is treated as an array of the type and searched sequentially for a matching value. The value type is treated as having no identity and cheap to copy, in the manner of a integral type.

Parsing with StringView¶

A primary use of StringView is to do field oriented parsing. It is easy and fast to split strings in to fields without modifying the original data. For example, assume that value contains a null terminated string which is possibly several tokens separated by commas.

#include <ctype.h>
parse_token(const char* value) {
  StringView v(value); // construct assuming null terminated string.
  while (v) {
    StringView token(v.extractPrefix(','));
    token.trim(&isspace);
    if (token) {
      // process token
    }
  }
}

If value was bob ,dave, sam then token would be successively bob, dave, sam. After sam was extracted value would be empty and the loop would exit. token can be empty in the case of adjacent delimiters or a trailing delimiter. Note that no memory allocation at all is done because each view is a pointer in to value and there is no need to put nul characters in the source string meaning no need to duplicate it to prevent permanent changes.

What if the tokens were key / value pairs, of the form key=value? This is can be done as in the following example.

#include <ctype.h>
parse_token(const char* source) {
  StringView in(source); // construct assuming null terminated string.
  while (in) {
    StringView value(in.extractPrefix(','));
    StringView key(value.trim(&isspace).splitPrefix('=').rtrim(&isspace));
    if (key) {
      // it's a key=value token with key and value set appropriately.
      value.ltrim(&isspace); // clip potential space after '='.
    } else {
      // it's just a single token which is in value.
    }
  }
}

Nested delimiters are handled by further splitting in a recursive way which, because the original string is never modified, is straight forward.

History¶

The first attempt at this functionality was in the TSConfig library in the ts::Buffer and ts::ConstBuffer classes. Originally intended just as raw memory views, ts::ConstBuffer in particular was repeated enhanced to provide better support for strings. The header was eventually moved from lib/tsconfig to lib/ts and was used in in various part of the Traffic Server core.

There was then a proposal to make these classes available to plugin writers as they proved handy in the core. A suggested alternative was Boost.StringRef which provides a similar functionality using std::string as the base of the pre-allocated memory. A version of the header was ported to Traffic Server (by stripping all the Boost support and cross includes) but in use proved to provide little of the functionality available in ts::ConstBuffer. If extensive reworking was required in any case, it seemed better to start from scratch and build just what was useful in the Traffic Server context.

The next step was the StringView class which turned out reasonably well. It was then suggested that more support for raw memory (as opposed to memory presumed to contain printable ASCII data) would be useful. An attempt was made to do this but the differences in arguments, subtle method differences, and return types made that infeasible. Instead MemView was split off to provide a void* oriented view. String specific methods were stripped out and a few non-character based methods added.