在初始化全局变量时使用assert宏

Question

In the following code, the first assert does not compile because, in MSVC2015RC at least, it is defined essentially as assert(expression) (void)(!!(expression)) .

int x = 1;
assert( x == 1 );
void foo()
{
    assert( x == 1 );
}

The error message is

C2062 type 'void' unexpected

The second assert compiles without problems but it is not executed unless foo() is called. I would like to use assert while the global variables are being initialized. Is there are any workaround?

Answer 1

Two issues here.

First, only declarations (eg variable, class, function) can go in global scope. assert() is not a declaration, that's why you can't do there.

That might lead to a workaround to just shove it into a declaration, like:

int x = 1;
namespace assert_x_is_1 {
    bool _ = (assert(x == 1), true);
}

On gcc, the above compiles fine and will assert if x is not 1. However, on clang, this leads to the second problem: assert() really wants to be used in a function - it uses a macro which is only defined in a function.

So for that, you could hack together a class declaration:

#define ASSERT_CLASS2(expr, ctr) namespace assert##ctr { struct Asserter { Asserter() { assert(expr); } } s; }
#define ASSERT_CLASS(expr) ASSERT_CLASS2(expr, __COUNTER__)

int x = 1;
ASSERT_CLASS(x == 1);

That will create a global variable in a semi-anonymous namespace that in its constructor will assert the expression. This works on both clang and gcc.

Answer 2

in c/c++, statement such as

x==1

can't be wrote beyond function

assert( x == 1 );

will be replaced during precompiling

(void)(!!(x==1));

it is a statement, can't be wrote beyond function definition