Static Metaprogramming in C++
Mateus Krepsky Ludwich
[email protected] http://www.lisha.ufsc.br September 27, 2010
September 27, 2010
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Introduction
Meta: Greek ● Means: ● E.g.:
● In
"after" or "beyond"
Metaphysics, Metapsychology
linguistics: just means "being about" something
● E.g.:
A metalanguage is a language to describe another language
Metaprograms ● Programs
that represent and manipulate other programs or themselves
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Metaprograms execution Post-runtime
Static metaprograms
Compile time Information from static analysis only
Linking
Loading
Runtime
Complete execution history
Dynamic metaprograms September 27, 2010
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Dynamic Metaprogramming
Reflection ● ...”The
ability of a program to manipulate as data something representing the state of a program during its own execution.” [Gabriel, B. W., 1993]
Reification ● Encoding
Introspection ● Observe
state as data
/ reason about its own state
Intercession ● Modify
its own execution state or its own interpretation or meaning
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Examples of levels of reflection
Smalltalk ● Meta-objects:
provide language concepts (methods, classes, execution stacks, the processor) in the form of libraries ● High level of reflection
Java ● Reflection
API: used to discover methods and attributes at runtime ● No direct modification of classes or methods
C++ ● RTTI: Runtime ● Dynamic cast
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type information
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Static Metaprogramming
“Run“ before load time of the code they manipulate – usually compile time Most common examples ● Compilers ● AST
→ Assembly
● Preprocessors ● Ling1
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→ Ling2
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Types of SMP
Open Compilers ● May
provide access to its parts (parser, code generator) ● E.g.:
OpenC++, MPC++, Magik, Xroma
● Transformation ● Provides
systems
an interface to write transformations on AST
Two-level languages ● Static code: “runs” at compile time ● Dynamic code: runs at runtimme ● E.g.: Templates C++
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C++ as Two-Level Language
Static code ● Templates ●*
e.g.: conditional operator: “?”
Dynamic code ● “ordinary”
features)
+ other C++ features*
C++ (all the others constructions and
Static code (subset of C++) is Turing-complete ● Conditional construction → Template specialization ● Loop construction → Template recursion
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Factorial example
Dynamic factorial
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Factorial example
Static factorial
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“Functional flavor of the static level” Class templates as functions
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“Functional flavor of the static level” Class templates as functions Integer and types as data
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“Functional flavor of the static level” Class templates as functions Integer and types as data Template recursion instead of loops
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“Functional flavor of the static level” Class templates as functions Integer and types as data Template recursion instead of loops Constant initialization instead of assignment
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Template Metaprogramming
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Template Metaprogramming
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Template Metaprogramming
factorial
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Template Metaprogramming
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Template Metaprogramming
IF
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Metaprogrammed IF
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Template Metaprogramming
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Template Metaprogramming
Recursive code expansion
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Recursive code expansion
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Recursive code expansion power(m,n) power(m)
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Template Metaprogramming
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Template Metaprogramming Lists and Trees as Nested templates
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Metaprogrammed list (cons 1 (cons 2 (cons 3 (cons 9 nil)))) Lisp
creates: [1, 2, 3, 9] Cons
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C++
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Metaprogrammed list
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Template Metaprogramming
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Expression templates
Allows for optimized code generation for adding a number of vectors ● V4
= v1 + v2 + v3
● Simply
overloading the + operator is inefficient (temporary vector for each +)
Can be used to implement compile-time domain-specific checks ● E.g.
“an expression cannot contain more than five + operators”
Useful to implement domain-specific languages
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Conclusions
Metaprogramming ● Dynamic: reflection ● Static: template metaprogramming
Static metaprogramming in C++ ● Turing-complete ● Metainformation ● Metafunction ● Expression templates
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References
Czarnecki, K. and Eisenecker, U. W. 2000 Generative Programming: Methods, Tools, and Applications. ACM Press/Addison-Wesley Publishing Co. ● Chapter
10: Static Metaprogramming in C++
Stroustrup, B. 2000 The C++ Programming Language. 3rd. Addison-Wesley Longman Publishing Co., Inc. ● Chapter
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13: Templates
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Exercises
Implementing fibonacci metafunction
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