FAQ-0529 How do I use _LIT() constants with the ?: operator?


FAQ-0529 How do I use _LIT() constants with the ?: operator?

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Classification:	C++	Category:	Development
Created:	10/19/2000	Modified:	09/11/2002
Number:	FAQ-0529
Platform:	Not Applicable

Question:
How do I use _LIT() constants with the ?: operator?

Answer:
Here's the code snippet in question:

_LIT(KDictName_zy, "zy.dct");
_LIT(KDictName_pinyin, "pinyin.dct");
...
err = iFile.Open(aRfs, iUseZY?KDictName_zy:KDictName_pinyin, EFileStream);
When this is compiled, the C++ compiler will object with an error message:

ambiguous overload for `bool ? const TLitC<7> & : const TLitC<11> &'
This message reflects the C++ rule that the type of the "bool ? X:Y" expression is the type of the X expression: either X and Y must be the same type, or there must be a conversion from Y to type X. The _LIT() macro uses a C++ template, so it produces a different type for each possible string length.

How To Solve the Problem:

Happily, the TLitC<> class has an operator() function which presents the object as a const TDesC&, so the fix is to change the code to

err = iFile.Open(aRfs, iUseZY?KDictName_zy():KDictName_pinyin(), EFileStream);
The operator() function is an inline function which just reassures the C++ compiler that everything is alright: it doesn't actually generate any extra code.

Footnote:

The old _L() macro produces a TPtrC object, so the two expressions had the same type. Unlike the _LIT() macro, which arranges to do all of the work at compile time, the _L() macro calls a TPtrC constructor at runtime, so it makes your code bigger and slower.