Variadic templates and variadic functions combination

Question

I'm creating a logging function that on the one hand needs to be able to parse any number of parameters, but on the other hand will always be passed with __func__ and this (of the calling object).

If I use only variadic templates like this

template<typename... Args>
void
log_device_message_template(const class *object, 
               char const *func,
               char const *const format,
               Args const &... args)noexcept
{
 log(format, to_str(object).c_str(),format, to_str(args).c_str()...);
}

I have to call log_device_message_template like this : log_device_message_template(this,__func__,some format,some parameters)

so i've added the macro:

#define log_api_call(format, ...)                                                                                                                                                         \
    log_device_message_template(                                                                                                   \
        this, __func__, "%s::%s(" format ")", ##__VA_ARGS__)

The thing is that i'm getting a seg fault, probably due to a bad formatting somewhere. adding __attribiute__(format) doesn't work due to the use of variadic template...

here is the error from the python test testing the logger:

lookup in file=***** [0]
     28371: symbol=_ZN5***6logger27log_device_message_templateIINS_14*****E13****EEEvNS_21logger_component_eENS_17logger_level_eEPKcPKNS_9objectES7_DpRT_;  lookup in file=**** [0]
     28371: ****: error: symbol lookup error: undefined symbol: _ZN5***6logger27log_device_message_templateIINS_14****E13****EEEvNS_21logger_component_eENS_17logger_level_eEPKcPKNS_9objectES7_DpRT_ (fatal)

Answer 1

Here's another way to approach it which neatly avoids macros.

Note in this case I am emitting log records to stdout, but that can easily be altered. I am also using std::ostringstream to build the log message. It's not renowned for performance, so is a good candidate for a customisation point.

However, this should get you started if you like the approach:

#include <iostream>
#include <sstream>
#include <tuple>
#include <utility>

// define a constant index type
template<std::size_t N> using index_c = std::integral_constant<std::size_t, N>;

// emitting an item at index 0 has no prefix    
template<class T>
void emit_item(std::ostream &os, index_c<0>, T const &t)
{
    os << t;
}


// emitting an item at index N (!= 0) has a comma prefix
template<std::size_t N, class T>
void emit_item(std::ostream &os, index_c<N>, T const &t)
{
    os << ", " << t;
}

// emit args 0 .. N-1
template<class Tuple, std::size_t...Is>
void emit_arglist(std::ostream &sink, Tuple &&tuple, std::index_sequence<Is...>)
{
    using expand = int[];
    void(expand{0,
                (emit_item(sink, index_c<Is>(), std::get<Is>(tuple)), 0)...
    });

};

// emitting a 'more' at index 0 has a prefix
template<class T>
void emit_more(std::ostream &os, index_c<0>, T const &t)
{
    os << " : " << t;
}

// emitting a 'more' at index N (!= 0) has a space prefix
template<std::size_t N, class T>
void emit_more(std::ostream &os, index_c<N>, T const &t)
{
    os << " " << t;
}

template<class Tuple, std::size_t...Is>
void emit_more(std::ostream &sink, Tuple &&tuple, std::index_sequence<Is...>)
{
    using expand = int[];
    void(expand{0,
                (emit_more(sink, index_c<Is>(), std::get<Is>(tuple)), 0)...
    });

};

template<typename... Args, typename ...MoreStuff>
std::string
make_log_string(const char *object,
                char const *func,
                std::tuple<Args const &...> const& args,
               MoreStuff&&...morestuff) noexcept
{
    std::ostringstream ss;
    ss << object << "::" << func << '(';
    emit_arglist(ss, args, std::make_index_sequence<sizeof...(Args)>());
    ss << ')';
    emit_more(ss, std::tie(morestuff...), std::make_index_sequence<sizeof...(MoreStuff)>());
    return ss.str();
}

// syntactic sugar for indicating arguments    
template<class...Arg>
decltype(auto) args(Arg const&...args)
{
    return std::tie(args...);
}


int main()
{
    int a = 0, b = 1, c = 2;
    std::string sa = "xxx", sb = "yyy", sc = "zzz";

    const char* Class = "foo";
    const char* Func = "var";

    std::cout << make_log_string(Class, Func, args(a, b, c)) << std::endl;
    std::cout << make_log_string(Class, Func, args(sa, b, sc)) << std::endl;
    std::cout << make_log_string(Class, Func, args(sa, b, sc), "more stuff") << std::endl;
    std::cout << make_log_string(Class, Func, args(), "empty", "argument", "list") << std::endl;

}

expected output:

foo::var(0, 1, 2)
foo::var(xxx, 1, zzz)
foo::var(xxx, 1, zzz) : more stuff
foo::var() : empty argument list

And with a bit more boilerplate we can write this:

std::cout << make_log_string(method(Class, Func)(a, b, c)) << std::endl;
std::cout << make_log_string(method(Class, Func)(sa, b, sc)) << std::endl;
std::cout << make_log_string(method(Class, Func)(sa, b, sc), "more stuff") << std::endl;
std::cout << make_log_string(method(Class, Func)(), "empty", "argument", "list") << std::endl;

Here it is:

#include <iostream>
#include <sstream>
#include <tuple>
#include <utility>

template<std::size_t N> using index_c = std::integral_constant<std::size_t, N>;

template<class T>
void emit_item(std::ostream &os, index_c<0>, T const &t)
{
    os << t;
}

template<std::size_t N, class T>
void emit_item(std::ostream &os, index_c<N>, T const &t)
{
    os << ", " << t;
}

template<class Tuple, std::size_t...Is>
void emit_arglist(std::ostream &sink, Tuple &&tuple, std::index_sequence<Is...>)
{
    using expand = int[];
    void(expand{0,
                (emit_item(sink, index_c<Is>(), std::get<Is>(tuple)), 0)...
    });

};

template<class T>
void emit_more(std::ostream &os, index_c<0>, T const &t)
{
    os << " : " << t;
}

template<std::size_t N, class T>
void emit_more(std::ostream &os, index_c<N>, T const &t)
{
    os << " " << t;
}

template<class Tuple, std::size_t...Is>
void emit_more(std::ostream &sink, Tuple &&tuple, std::index_sequence<Is...>)
{
    using expand = int[];
    void(expand{0,
                (emit_more(sink, index_c<Is>(), std::get<Is>(tuple)), 0)...
    });

};

template<class...Args>
struct method_with_args;

struct method
{
    constexpr method(const char* c, const char* f) : klass(c), func(f) {}
    const char* klass;
    const char* func;

    template<class...Args>
    auto operator()(Args const&...args) -> method_with_args<Args...>;

    friend std::ostream& operator<<(std::ostream& os, const method& m)
    {
        return os << m.klass << "::" << m.func;
    }
};

template<class...Args>
struct method_with_args
{
    friend std::ostream& operator<<(std::ostream& os, method_with_args const& ma)
    {
        os << ma.m << '(';
        emit_arglist(os, ma.args, std::make_index_sequence<sizeof...(Args)>());
        return os << ')';
    }

    method m;
    std::tuple<Args const&...> args;
};

template<class...Args>
auto method::operator()(Args const&...args) -> method_with_args<Args...>
{
    return method_with_args<Args...>{*this, std::tie(args...)};

}

struct function
{
    const char* name;
};

template<typename Method, typename ...MoreStuff>
std::string
make_log_string(Method m,
                MoreStuff &&...morestuff) noexcept
{
    std::ostringstream ss;
    ss << m;
    emit_more(ss, std::tie(morestuff...), std::make_index_sequence<sizeof...(MoreStuff)>());
    return ss.str();
}


int main()
{
    int a = 0, b = 1, c = 2;
    std::string sa = "xxx", sb = "yyy", sc = "zzz";

    const char *Class = "foo";
    const char *Func = "var";

    std::cout << make_log_string(method(Class, Func)(a, b, c)) << std::endl;
    std::cout << make_log_string(method(Class, Func)(sa, b, sc)) << std::endl;
    std::cout << make_log_string(method(Class, Func)(sa, b, sc), "more stuff") << std::endl;
    std::cout << make_log_string(method(Class, Func)(), "empty", "argument", "list") << std::endl;
}