C++0x static assert Asger Bruun Generisk programmering og biblioteksudvikling. Datalogisk Institut, Københavns Universitet. 9. maj 2008
The problem The standard ways in C++ to test for software assertions ◮ before compile: use the #error preprocessor directive ◮ compile time: missing! ◮ after compile: use the runtime assert A compile time check makes it possible for a template library to detect a wrong template instantiation and tell the user what is wrong. Without it, then the library can hope or make sure for an internal compilation failure, that will eventually prevent the user in going runtime.
Work arounds The problem can be solved with some less beautiful compiler hacks ◮ Boost: BOOST STATIC ASSERT(expr) ◮ Loki: LOKI STATIC CHECK(expr,err-mesg-type) ◮ CPH STL: static assert(expr,err-mesg-type) Inadequacies: a) name space polution because of new types is the only way to communicate error messages to the user, b) worse, Boost basically tell the user no more than something went wrong in the library and that the user should consult this library source 1 . 1 is the same as removing the explanation, not matter how internal it was
The proposal The proposal is a new keyword in namespace scope, block scope, and class member declaration lists, static assert-declaration: static assert ( constant-expression , string-literal ) ; The proposal eliminates the compiler twisting of present days.
Test Get most compiler libraries possibly into system in advance ◮ MS VS2008: a) install Windows SDK first (to avoid obstruction of TR1 another day from this known threat). b) install Visual C++ 2008 Feature Pack which includes all the implemented parts of TR1. ◮ or GCC: the newest update will have all standards included the implemented parts of C++ TR1.
Use case 1, of proposal ◮ BOOST STATIC ASSERT(sizeof(long) > = 8); // 64-bit code generation is not enabled/supported. ◮ LOKI STATIC CHECK(sizeof(long) > = 8, 64 bit code generation not enabled supported); ◮ CPHSTL: static assert(sizeof(long) > = 8, 64 bit code generation not enabled supported); ◮ C++0x: static assert(sizeof(long) > = 8, ′′ 64-bit code generation is not enabled/supported. ′′ ); 2 2 the size of long is 32 bit in MS VC x64 code generation.
Use case 1, compiler error messages 3 error C2027: use of undefined type ′ boost::STATIC ASSERTION FAILURE < x > ′ with [ x=false ] 4 — error C2079: ′ ERROR 64 bit code generation not enabled supported ′ uses undefined struct ′ Loki::CompileTimeError < formal > ′ with [ formal=0 ] — error C2440: ′ < function-style-cast > ′ : cannot convert from ′ case1::tst::ERROR 64 bit code generation not enabled supported ′ to ′ type ′ No constructor could take the source type, or constructor overload resolution was ambiguous 4 Boost is the only solution that works properly in namespace scope. 3 MS VC x86 code generation.
Result Consequences ◮ the new keyword, static assert, is probably useful ◮ a new keyword may create collisions with existing code Is it an ill-timed quick fix for the general limitation that the only valid template arguments are types and integral constants?
References The C++ Standards Committee, N1720: Proposal to Add Static Assertions to the Core Language (Revision 3) , 2004-10-20. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2521.pdf The C++ Standards Committee, N2521: Working Draft, Standard for Programming Language C++ , 2008-02-04. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2521.pdf CPH STL Library, static assert , 2001-. http://loki-lib.sourceforge.net/index.php?n=Idioms.CompileTimeCheck Loki Library, LOKI STATIC CHECK , 2001-. http://loki-lib.sourceforge.net/index.php?n=Idioms.CompileTimeCheck Boost Library, BOOST STATIC ASSERT , 2000-. http: //www.boost.org/doc/libs/1_35_0/doc/html/boost_staticassert.html
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