c++ - Print macro values without knowing the amount of macros

Question

I have code that includes a generated file (I don't know in advance its content), there is just a convention upon which me and my users agreed on how to create this file so I can use it. This file looks like

#define MACRO0 "A"
#define MACRO1 "B"
#define MACRO2 "C"
...

I want to print all macros values. My current code looks like

#ifdef MACRO0
std::cout << "MACRO0 " << MACRO0 << std::endl;
#endif
#ifdef MACRO1
std::cout << "MACRO1 " << MACRO1 << std::endl;
#endif
#ifdef MACRO2
std::cout << "MACRO2 " << MACRO2 << std::endl;
#endif

My question is, how to iterate over the macros in the generated file so I don't need to duplicate my code so much

Answer 1

First of all, we know we can count on Boost.Preprocessor for our looping needs. However, the generated code must work on its own. Unfortunately, #ifdef cannot work as a result of macro expansion, so there's no way to generate the code in your question. Are we toasted?

Not yet! We can take advantage of the fact that your macros are all either nonexistent or a string literal. Consider the following:

using StrPtr = char const *;
StrPtr probe(StrPtr(MACRO1));

We're taking advantage of our old friend the most vexing parse here. The second line can be interpreted in two ways depending on whether MACRO1 is defined. Without it, it is equivalent to:

char const *probe(char const *MACRO1);

... which is a function declaration where MACRO1 is the name of the parameter. But, when MACRO1 is defined to be "B", it becomes equivalent to:

char const *probe = (char const *) "B";

... which is a variable initialized to point at "B". We can then switch on the type of what we just produced to see if a substitution occured:

if(!std::is_function<decltype(probe)>::value)
    std::cout << "MACRO1 " << probe << '
';

We could make use of if constexpr here, but std::cout can output a function pointer (it converts it to bool) so the dead branch is valid, and the compiler is clever enough to completely optimize it out.

Finally, we come back to Boost.Preprocessor to generate all that stuff for us:

#define PRINT_IF_DEFINED(z, n, data) 
    { 
        StrPtr probe(StrPtr(BOOST_PP_CAT(MACRO, n))); 
        if(!std::is_function<decltype(probe)>::value) 
            std::cout << "MACRO" BOOST_PP_STRINGIZE(n) " " << probe << '
'; 
    }

#define PRINT_MACROS(num) 
    do { 
    using StrPtr = char const *; 
    BOOST_PP_REPEAT(num, PRINT_IF_DEFINED, ~) 
    } while(false)

... voilà!

See it live on Coliru

Note: the Coliru snippet includes warning disablers for GCC and Clang, which warn against our poor pal the most vexing parse :(