C++0x static assert Asger Bruun Generisk programmering og - - PowerPoint PPT Presentation

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

• nly 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.

1is 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

• bstruction 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

2the 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

4Boost is the only solution that works properly in namespace scope. 3MS 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