Minimal test-case for undefined compiler behaviour when using the C/C++ defined() operator

c-defined-operator-testcase.cpp

/**
 * Simple testcase for compiler-specific behaviour with the defined() C++ operator
 * The spec states that if defined() appears as part of macro expansion, the behaviour is undefined
 *
 * On GNU cpp treats it as a genuine define() operator. Visual C++ doensn't.
 * This generates many warnings when compiling e.g. the BSON library under Windows
 *
 * See https://gcc.gnu.org/onlinedocs/cpp/Defined.html for details
 * 
 * On Visual C++ this program generates the following;
 *
 * > Test case for define() operator
 * > (1) Fooey is not defined
 * > (2) Fooey is not defined
 * > (3) Fooey is defined
 * > (4) Fooey is not defined
 *
 * On GNU C apparently older versions would have compiled this O.K, but given
 * a -Wexpansion-to-defined warning. As of 5.4.0 I get a compiler error;
 * error: operator "defined" requires an identifier
 * 
 */

#include <iostream>

#define DEFINED_MACRO(test) (defined(test))

int main(int argc, char **argv)
{
    std::cout << "Test case for define() operator" << std::endl;
    
#if defined(FOOEY)
    std::cout << "(1) Fooey is defined" << std::endl;
#else
    std::cout << "(1) Fooey is not defined" << std::endl;
#endif

#if DEFINED_MACRO(FOOEY)
	std::cout << "(2) Fooey is defined" << std::endl;
#else
	std::cout << "(2) Fooey is not defined" << std::endl;
#endif

#define FOOEY 1
    
#if defined(FOOEY)
    std::cout << "(3) Fooey is defined" << std::endl;
#else
    std::cout << "(3) Fooey is not defined" << std::endl;
#endif
    
#if DEFINED_MACRO(FOOEY)
    std::cout << "(4) Fooey is defined" << std::endl;
#else
    std::cout << "(4) Fooey is not defined" << std::endl;
#endif
    
    return 0;
}