Print macro values without knowing the amount of m

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 << '\n';

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 << '\n'; \
    }

#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 :(

Answer 2

I ran into the same kind of need a long time ago.

My solution was to use the preprocessor, but not to get the answer "within the code".

For example, clang++ -dM -E test.cpp will output all the macros. (At the time, I used gcc, but the same technique works for GCC, CLang, and Visual Studio's CL.EXE ... the compiler switches may vary.)

Ahh, drat, that also includes all the predefined macros.

So I would produce a "blacklist" file of the predefined macros that I did not care about, and then use that to filter out those results (using grep -v).

The other problem I ran into was that sometimes someone would #undef IMPORTANT_MACRO which would then get missed in the dump. For those infrequent situations... and then the murders began.

Answer 3

This answer is written taking a follow-up question into account.

C++ has support for generic programming that often eliminates need for preprocessor. In this case it would be better to make a set of type traits declaring properties of parameters that need to be handled reducing role of preprocessor to conditional compilation (or eliminating it completely if this code is supposed to be generated every time):

enum class
t_Param
{
    begin, a = begin, b, c, d, e, z, end
};

template<t_Param param, typename TEnabled = void> class
t_ParamIsEnabled final: public ::std::true_type
{};

template<t_Param param> class
t_ParamIsEnabled
<
    param
,   typename ::std::enable_if
    <
        (t_Param::end == param)
        #ifndef A1
        || (t_Param::a == param)
        #endif
        #ifndef B2
        || (t_Param::b == param)
        #endif
        #ifndef C3
        || (t_Param::c == param)
        #endif
        #ifndef D4
        || (t_Param::d == param)
        #endif
        #ifndef E5
        || (t_Param::e == param)
        #endif
    >::type
> final: public ::std::false_type
{};

template<t_Param param> class
t_ParamTrait;

template<> class
t_ParamTrait<t_Param::a> final
{
    public: static constexpr auto const & num{"1"};
    public: static constexpr auto const & val{"A"};
};

template<> class
t_ParamTrait<t_Param::b> final
{
    public: static constexpr auto const & num{"2"};
    public: static constexpr auto const & val{"B"};
};

template<> class
t_ParamTrait<t_Param::c> final
{
    public: static constexpr auto const & num{"3"};
    public: static constexpr auto const & val{"C"};
};

template<> class
t_ParamTrait<t_Param::d> final
{
    public: static constexpr auto const & num{"4"};
    public: static constexpr auto const & val{"D"};
};

template<> class
t_ParamTrait<t_Param::e> final
{
    public: static constexpr auto const & num{"5"};
    public: static constexpr auto const & val{"E"};
};

template<> class
t_ParamTrait<t_Param::z> final
{
    public: static constexpr auto const & num{"26"};
    public: static constexpr auto const & val{"ZZ"};
};

This will allow you to iterate over parameters and query their properties using generic code:

template<t_Param param> typename ::std::enable_if<t_ParamIsEnabled<param>::value>::type
Echo(void)
{
    ::std::cout << t_ParamTrait<param>::val << ":" << t_ParamTrait<param>::num << ::std::endl;
}

template<t_Param param> typename ::std::enable_if<!t_ParamIsEnabled<param>::value>::type
Echo(void)
{
    //  Do nothing
}

template<int param_index = 0> void
Echo_All(void)
{
    Echo<static_cast<t_Param>(param_index)>();
    Echo_All<param_index + 1>();
}

template<> void
Echo_All<static_cast<int>(t_Param::end)>(void)
{
    //  Do nothing.
}

int main()
{
    Echo_All();
    return 0;
}

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