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Qt 4.8
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Qt 4.8
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Implements XSL-T's function fn:current().
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#include <qcurrentfn_p.h>
Properties | |
| ItemType::Ptr | m_itemType |
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compress() is the last stage performs in compiling an expression, done after the initial AST build and calling typeCheck(). compress() performs crucial simplifications, either by having drastic performance implications or that some expressions depend on it for proper behavior.
The default implementation performs a sparse conditional constant propagation. In short, a recursive process is performed in the AST which examines if the Expression's operands are constant values, and if so, performs a const fold(AST rewrite) into the result of evaluating the expression in question. This default behavior can be disabled by letting properties() return DisableElimination.
This compress() stage can be relative effective due to the design of XPath, in part because intrinsic functions are heavily used. Many Expressions override compress() and do optimizations specific to what they do. Also, many Expressions performs optimizations in their typeCheck().
| context | the static context. Supplies compile time information, and is the channel for communicating error messages. |
Reimplemented from QPatternist::Expression.
Definition at line 54 of file qcurrentfn.cpp.
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Reimplemented from QPatternist::Expression.
Definition at line 49 of file qcurrentfn.cpp.
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Reimplemented from QPatternist::FunctionCall.
Definition at line 67 of file qcurrentfn.cpp.
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This implementation guarantees to never rewrite away this Expression, but at most rewrite it as a child of another expression(that presumably have a type checking role). It is therefore always safe to override this function and call this implementation and not worry about that this Expression becomes deleted.
Many Expressions override typeCheck() and performs optimizations, as opposed to doing it in the compress() stage. This is due to that the design of those Expressions often are tied to that certain simplifications are done at the typeCheck() stage of the compilation process or that it in some other way is related to what the typeCheck() do. Also, the earlier the AST can be simplified, the better the chances are for subsequent optimizations.
It is important that the super class's typeCheck() is called before doing any custom type checking, since the call can change the children(notably, the childrens' static types). For example, if the Expression, MyExpression in the example, does not match the required type, typeCheck returns the Expression wrapped in for example ItemVerifier, CardinalityVerifier, or both.
typeCheck() may be called many times. typeCheck() must either raise an error if this Expression is an invalid expression. Thus, it is guaranteed that an Expression is valid after typeCheck() is called.
| context | supplies information, such as namespace bindings and available function signatures, that can be needed at compilation time. context is guaranteed by the caller to never null. |
| reqType | the static type that this Expression must match when evaluated. reqType is guaranteed by the caller to never null. |
Public Functions inherited from reqType Reimplemented from QPatternist::FunctionCall.
Definition at line 60 of file qcurrentfn.cpp.
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Definition at line 81 of file qcurrentfn_p.h.
Referenced by compress(), staticType(), and typeCheck().