boost::format with variadic template arguments

Question

Suppose I have a printf-like function (used for logging) utilizing perfect forwarding:

template<typename... Arguments>
void awesome_printf(std::string const& fmt, Arguments&&... args)
{
    boost::format f(fmt);
    f % /* How to specify `args` here? */;
    BlackBoxLogFunction(boost::str(f).c_str());
}

(I didn't compile this but my real function follows this guideline)

How can I "unroll" the variadic argument into the boost::format variable f?

Answer 1

I did some googling and found an interesting solution:

#include <iostream>
#include <boost/format.hpp>

template<typename... Arguments>
void format_vargs(std::string const& fmt, Arguments&&... args)
{
    boost::format f(fmt);
    int unroll[] {0, (f % std::forward<Arguments>(args), 0)...};
    static_cast<void>(unroll);

    std::cout << boost::str(f);
}

int main()
{
    format_vargs("%s %d %d", "Test", 1, 2);
}

I don't know if this is a recommended solution but it seems to work. I don't like the hacky static_cast usage, which seems necessary to silence the unused variable warnings on GCC.

Answer 2

As is usual with variadic templates, you can use recursion:

std::string awesome_printf_helper(boost::format& f){
    return boost::str(f);
}

template<class T, class... Args>
std::string awesome_printf_helper(boost::format& f, T&& t, Args&&... args){
    return awesome_printf_helper(f % std::forward<T>(t), std::forward<Args>(args)...);
}

template<typename... Arguments>
void awesome_printf(std::string const& fmt, Arguments&&... args)
{
    boost::format f(fmt);

    auto result = awesome_printf_helper(f, std::forward<Arguments>(args)...);

    // call BlackBoxLogFunction with result as appropriate, e.g.
    std::cout << result;
}

Demo.

---

In C++17, simply (f % ... % std::forward<Arguments>(args)); will do.

Answer 3

Just to summarize the void.pointer's solution and the hints proposed by Praetorian, T.C. and Jarod42, let me provide the final version (online demo)

#include <boost/format.hpp>
#include <iostream>

template<typename... Arguments>
std::string FormatArgs(const std::string& fmt, const Arguments&... args)
{
    boost::format f(fmt);
    std::initializer_list<char> {(static_cast<void>(
        f % args
    ), char{}) ...};

    return boost::str(f);
}

int main()
{
    std::cout << FormatArgs("no args\n"); // "no args"
    std::cout << FormatArgs("%s; %s; %s;\n", 123, 4.3, "foo"); // 123; 4.3; foo;
    std::cout << FormatArgs("%2% %1% %2%\n", 1, 12); // 12 1 12
}

Also, as it was noted by T.C., using the fold expression syntax, available since C++17, the FormatArgs function can be rewritten in the more succinct way

template<typename... Arguments>
std::string FormatArgs(const std::string& fmt, const Arguments&... args)
{
    return boost::str((boost::format(fmt) % ... % args));
}