text: With C++17 we get another facility to handle the conversion between text and numbers. Why should we care about the new routines? Are they better in any way?
  Intro
  C++, before C++17, offered several options when it comes to string conversion:
  sprintf/
  snprintf
  sscanf
  atol
  strtol
  strstream
  stringstream
  to_string
  stoiand similar functions
  And with C++17 you get another option:
  std::from_chars! Wasn’t the old stuff good enough? Why do we need new methods?
  In short: because
  from_chars is low-level, and offers the best possible performance.
  The new conversion routines are:
  - non-throwing
  - non-allocating
  - no locale support
  - memory safety
  - error reporting gives additional information about the conversion outcome
  The API might not be the most friendly to use, but it’s easy enough to wrap it into some facade.
  A simple example:
  const std::string str { "12345678901234" }; int value = 0; std::from_chars(str.data(),str.data() + str.size(), value); // error checking ommited...
  If you want to read more about existing conversion routines, new ones and also see some benchmarks you can see two great posts at @fluentcpp:
  How to Convert a String to an int in C++ and How to *Efficiently* Convert a String to an int in C++ written by JFT. (this post)
  - Working with
  std::filesystem
  Let’s have a look at the API now.
  Converting From Characters to Numbers:
  from_chars
  std::from_chars is a set of overloaded functions: for integral types and floating point types.
  For integral types we have the following functions:
  std::from_chars_result from_chars(const char* first, const char* last, TYPE &value, int base = 10);
  Where
  TYPE expands to all available signed and unsigned integer types and
  char.
  base can be a number ranging from 2 to 36.
  Then there’s the floating point version:
  std::from_chars_result from_chars(const char* first, const char* last, FLOAT_TYPE& value, std::chars_format fmt = std::chars_format::general);
  FLOAT_TYPE expands to
  float,
  double or
  long double.
  chars_format is an enum with the following values:
  scientific,
  fixed,
  hex and
  general (which is a composition of
  fixed and
  scientific).
  The return value in all of those functions (for integers and floats) is
  from_chars_result:
  struct from_chars_result { const char* ptr; std::errc ec; };
  from_chars_result holds valuable information about the conversion process.
  Here’s the summary:
  The new routines are very low level, so you might be wondering why is that. Titus Winters added a great summary in comments:
  The intent of those APIs is *not* for people to use them directly, but to build more interesting/useful things on top of them. These are primitives, and we (the committee) believe they have to be in the standard because there isn't an efficient way to do these operations without calling back out to nul-terminated C routines
  Examples
  Here are two examples of how to convert a string into a number using
  from_chars, to
  int and
  float.
  Integral types
  #include <charconv> // from_char, to_char #include <string> #include <iostream> int main() { const std::string str { "12345678901234" }; int value = 0; const auto res = std::from_chars(str.data(), str.data() + str.size(), value); if (res.ec == std::errc()) { std::cout << "value: " << value << ", distance: " << res.ptr - str.data() << '\n'; } else if (res.ec == std::errc::invalid_argument) { std::cout << "invalid argument!\n"; } else if (res.ec == std::errc::result_out_of_range) { std::cout << "out of range! res.ptr distance: " << res.ptr - str.data() << '\n'; } }
  The example is straightforward, it passes a string
  str into
  from_chars and then displays the result with additional information if possible.
  Run the code below and change the
  str value to see the output:
  Does "12345678901234" fit into the number? Or you see some errors from the conversion API?
  Floating Point
  To get the floating point test, we can replace the top lines of the previous example with:
  // works with MSVC only so far (Dec 2018) const std::string str { "16.78" }; double value = 0; const auto format = std::chars_format::general; const auto res = std::from_chars(str.data(), str.data() + str.size(), value, format);
  Here’s the example output that we can get:
  The
  general format is a combination of
  fixed and
  scientific so it handles regular floating point string with the additional support for
  e+num syntax.
  A bit about Performance
  I did some benchmark, and the new routines are blazing fast!
  Some numbers:
  - On GCC it’s ~4,5x faster than
  stoi, 2,2x faster than
  atoiand almost 50x faster than
  istringstream.
  - On Clang it’s ~3,5x faster than
  stoi, 2.7x faster than
  atoiand 60x faster than
  istringstream!
  - MSVC performs ~3x faster than
  stoi, ~2x faster than
  atoiand almost 50x faster than
  istringstream
  The whole benchmark might be however a story for another post, so I’m not sharing the code yet.
  Summary
  If you want to convert text into a number and you don’t need any extra stuff like locale support then
  std::from_chars might be the best choice. It offers great performance, and what’s more, you’ll get a lot of information about the conversion process (for example how much characters were scanned).
  The routines might be especially handy with parsing JSON files, 3d textual model representation (like OBJ file formats), etc.
  Your Turn
  Have you played with the new conversion routines?
  What do you usually use to convert text into numbers?