变异模板和变异功能组合

这是对其进行整齐避免宏的另一种方法。

请注意，在这种情况下，我将日志记录发射到STDOUT上，但可以很容易地更改。我还使用std::ostringstream来构建日志消息。它不是以性能而闻名的，因此是定制点的好候选人。

但是，如果您喜欢这种方法，这应该可以让您入门：

#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;
}

预期输出：

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

，使用更多样式板，我们可以写下：

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;

这里是：

#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;
}