Does GCC have a built-in compile time assert?

问题:

Our existing compile-time assert implementation is based on negative array index, and it provides poor diagnostic output on GCC. C++0x's static_assert is a very nice feature, and the diagnostic output it provides is much better. I know GCC has already implemented some C++0x features. Does anyone know if static_assert is among them and if it is then since what GCC version?

回答1:

According to this page, gcc has had static_assert since 4.3.

回答2:

If you need to use a gcc version which does not support it you can use

#include <boost/static_assert.hpp>  BOOST_STATIC_ASSERT( /* assertion */ ) 

Basically, what boost does is this:

Declare (but don't define!) a

template< bool Condition > struct STATIC_ASSERTION_FAILURE; 

Define a specialization for the case that the assertion holds:

template <> struct STATIC_ASSERTION_FAILURE< true > {}; 

Then you can define STATIC_ASSERT like this:

#define STATIC_ASSERT(Condition) \    enum { dummy = sizeof(STATIC_ASSERTION_FAILURE< (bool)(Condition) > } 

The trick is that if Condition is false the compiler needs to instantiate the struct

STATIC_ASSERTION_FAILURE< false > 

in order to compute its size, and this fails since it is not defined.

回答3:

The following code works as expected with g++ 4.4.0 when compiled with the -std=c++0x flag:

int main() {     static_assert( false, "that was false" ); } 

it displays:

x.cpp: In function 'int main()': x.cpp:2: error: static assertion failed: "that was false" 

回答4:

If you have an older gcc or use an older C++ standard, or use C, then you can emulate static_assert as described here: http://www.pixelbeat.org/programming/gcc/static_assert.html

回答5:

This doesn't really answer the question, but I like compile-time asserts based on switch-case better, e.g.

#define COMPILE_TIME_ASSERT(cond) do { switch(0) { case 0: case cond: ; } } while (0) 

Works also in C and not only in C++.

回答6:

you can always play around with templates and non-existant strutures via template-specialization. This is how boost does it as far as I know. This is what I use as a static_assert, it's pretty straight forward.

namespace Internal  { template<bool x> struct SASSERT_F; template<      > struct SASSERT_F <true> {}; template<int  x> struct SASSERT_P        {}; #define STATIC_ASSERT(B)            \      typedef Internal::SASSERT_P <( \      sizeof (Internal::SASSERT_F <( \          ((B)? true : false))    >) \                                  )> \          StaticAssert##__LINE__  () } 

Example Of Use

int main(int argc, char **argv) {     static_assert(sizeof(int) == 1)           // Error     static_assert(sizeof(int) == sizeof(int)) // OK } 

回答7:

NSPR does:

#define PR_STATIC_ASSERT(condition) \     extern void pr_static_assert(int arg[(condition) ? 1 : -1]) 

which fails if condition is false because it declares an array of negative length.

回答8:

Both

    BOOST_STATIC_ASSERT(x)     BOOST_STATIC_ASSERT_MSG(x, msg) 

will use the C++11 static_assert if your compiler supports it

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