Reuse format w/o "reparsing" it?

[ddevienne]

With Boost.Format, I can compile the format once, and use it several times.

boost::format fmt("some complex and long format");
for (const auto& item : items) {
  str(fmt % item.arg1 % item.arg2 % ...);
}

Despite CppFormat 2.0 being faster, isn't there an equivalent to the above? Thanks, --DD

[Spartan322]

There is not a cppformat format object (as far as I'm aware). The original idea was based on the python methods of formatting and thus takes the majority of the original design from it. I don't think there was anything in here inspired by Boost. (at least, I haven't seen a single Boost inspired part, but I'm not 100% sure)

[vitaut]

There is no equivalent in C++ Format for this. Moreover, I don't think it's worth doing for the following reasons:

1. Parsing time takes a relatively small fraction of formatting time. Comparing fmt::Writer+std::string to fmt::format performance numbers in http://zverovich.net/2013/09/07/integer-to-string-conversion-in-cplusplus.html suggests that it's no more than 7% (actually much less) on fast integer formatting. If you format something more complex like a floating-point number, parsing time will be negligible.
2. Compiling string in advance adds an overhead because one needs to create a copy of the format string and additional data structures. This will only pay off if you use it many times. I think this is one of the reason Boost.Format is so slow.

C++ Format is faster than Boost.Format, so you can simply do

for (const auto& item : items) {
  fmt::format("some complex and long format", item.arg1, item.arg2, ...);
}

and this will already be faster than the Boost version.

If you want to speed it up, you can avoid std::string construction which, unlike parsing, does take significant portion of time:

fmt::MemoryWriter writer;
for (const auto& item : items) {
  writer.write("some complex and long format", item.arg1, item.arg2, ...);
  writer.c_str(); // Access the formatted string as a C string to avoid std::string allocation
}

To speed this even further you can use the write API:

fmt::MemoryWriter writer;
for (const auto& item : items) {
  writer << "literal text " << item.arg1 << " more literal text " << item.arg2 << ...;
  writer.c_str(); // Access the formatted string as a C string to avoid std::string allocation
}

[ddevienne]

thank you for the thorough answer.