Check some compile-time definitions at compile time with older C++ implementations

Question

When working on a large legacy code base, I today suspected a duplicate definition, but the dependency was not obvious to me human since it depended on a lots of compile-time calculations.

enum { MAX_ITEMS = 4 }; // defined somewhere in my code universe
enum { ITEMS_MAX = COMPLICATED_CALCULATIONS }; // somewhere else

I remembered some cases with analogous sizeof questions, when I let the compiler speak.

I usually put some ad-hoc formulations like this in the code (in the IDE), then I press [Alt]+[F9]:

void check() {
    char bla[MAX_ITEMS == ITEMS_MAX]; // compiler-error shows difference
    // ...but it causes a compiler warning about bla being never used
}

...and that's only because my compiler (Borland C++ 5.6.4) lazy evaluates the typedef for arrays with non-literal dimension:

typedef char bla[0];       // immediate compiler error
typedef char bla[0 != 0];  // obvious, but no compiler error HERE

Is there a really easy-to-memorize way for checks like this? _{And, please don't blame an old brave compiler ;-)}

I would suggest you to use a free software C++11 compiler (recent [Clang](http://clang.llvm.org/) or recent [GCC](http://gcc.gnu.org/) ...) — Basile Starynkevitch, Sep 26 '14 at 12:14
@BasileStarynkevitch Thanks for the tip :) but the code base suffers from *compiler lock-in* :( — Wolf, Sep 26 '14 at 12:34
In the following link there are more detailed discussions about implementing one's own static assert: https://stackoverflow.com/questions/3385515/static-assert-in-c — Hari, May 31 '23 at 18:46

score 3 · Accepted Answer · answered Sep 26 '14 at 13:13

This works:

#define STATIC_ASSERT(x) typedef char foo[(x) ? 1 : -1];

I actually use the following setup borrowed from Boost, the purpose of this is to give each foo its own line number (otherwise a multiple definition error can happen):

#define BOOST_JOIN( X, Y ) BOOST_DO_JOIN( X, Y )
#define BOOST_DO_JOIN( X, Y ) BOOST_DO_JOIN2(X,Y)
#define BOOST_DO_JOIN2( X, Y ) X##Y
#define STATIC_ASSERT(x) \
    typedef char BOOST_JOIN(violation_on_line_,__LINE__) [(x) ? 1 : -1];

score 2 · Answer 2 · answered Sep 26 '14 at 11:36

2

You should try if BOOST_STATIC_ASSERT works on your compiler.

answered Sep 26 '14 at 11:36

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Check some compile-time definitions at compile time with older C++ implementations

2 Answers2