Explains how to create and use TLex objects with code fragments.
A TLex object may be constructed as an empty object, from another TLex, or from an existing string as below. The code fragments are taken from an example that implements a simple Reverse Polish Notation calculation engine.
TInt RPNCalc(const TDesC& aCommand, TReal& aReturnValue) { ... TLex input (aCommand) ; ...
Code can then proceed to move through the TLex data to:
mark positions for rolling back to later
delimit the start of lexical tokens
delete parts of the string held by the TLex object.
... input.Mark() ; // Remember where we are. input.SkipCharacters() ; // Move to end of character token. ... _LIT(KTextMemset,"MEMSET"); if ( input.TokenLength() != 0 ) // if valid potential token { TPtrC token = input.MarkedToken() ; // then extract token if ( token.CompareF(KTextMemset == 0) // and test. { ... } ... } ...
Analysis can also be done by character using functions that move through the TLex data, extracting, returning and jumping specified character lengths. For example:
... // ensure we are looking at a digit or sign if (!(input.Peek().IsDigit() || (input.Peek() == '.') ) ) { return KErrNotFound ; } ... // deal with sign if (input.Peek() == '+') { input.Inc(); }
Additionally, numeric conversion functions permit a variety of numeric formats to be extracted from the TLex data, with provision for conversion using the most common number systems (radixes).
... if (input.Val(extractUint) == KErrNone) { stack.Push(TReal(extractUint)); } else if (input.Val(extractReal) == KErrNone) { stack.Push(extractReal); } ...
where stack, is an instance of a class implementing a stack.
This converts an real number into a string, which is then assigned to a TLex.
TBuf<0x100> convertRealToString; // want a TLex from a value if (convertRealToString.Num(value,format) < KErrNone ) { ... } else { convertRealToString.ZeroTerminate(); TLex string(convertRealToString) ; }
This takes a descriptor as a function parameter, and copies it to a TLex.
TInt RPNCalc(const TDesC& iCommand, TReal& returnValue) { TLex input (iCommand) ; }
This shows a code flow decision made according to next character to be read from the TLex:
if (!(input.Peek()).IsDigit()) // found non-digit after decimal point if
This shows the use of the Inc() function to move past a character that has been peeked:
if (input.Peek() == '-') { input.Inc() ; // move past minus sign & flag negative = ETrue ; }
This shows the use of UnGet() to restore the previously "got" character.
.... if (input.Offset() > 0) // if not at start of line { input.UnGet() ; // restore 'got' character }
If the previous character is before the start of the string, then the function raises a USER 59 panic for the TLex8 variant and a USER 64 panic for the TLex16 variant.
This shows how to allow part of a TLex can be parsed again:
if (!(input.Peek()).IsDigit()) { // found non-digit after decimal point. Error, so rewind input.UnGetToMark(startMark); }
This parses a TLex for the next token:
input.Mark() ; // remember where we are input.SkipCharacters() ; // move to end of character token if ( input.TokenLength() != 0 ) // if valid potential token ...
This shows how TokenLength() is used to return the difference between the position of the next character and the extraction mark. This gives a check as to whether the token length is valid. An invalid token length implies an invalid token.
if ( input.TokenLength() != 0 ) // if valid token length ...
This extracts a marked token.
TPtrC token = input.MarkedToken() ; // extract token
This shows how to return the offset of the next character position from the start of the string.
if (input.Offset() > 0) // if not at start of line { input.UnGet() ; // restore 'got' character ... }
This example shows how to return the offset of the next character position from the start of the string.
if (input.Val(extractUint) == KErrNone) { stack.Push(TReal(extractUint)) ; } else { if (input.Val(extractReal) == KErrNone) { stack.Push(extractReal) ; } }
This extracts an unknown number type. Tries an integer first and then, if this fails, tries a real:
if (input.Val(extractUint) == KErrNone) stack.Push(TReal(extractUint)) ; else if (input.Val(extractReal) == KErrNone) stack.Push(extractReal) ;