How to Use The Newest C++ String Conversion Routines: std::from_chars

This article was originally posted at the author's blog: bfilipek.com

Intro

C++, before C++17, offered several options when it comes to string conversion:

• sprintf / snprintf
• sscanf
• atol
• strtol
• strstream
• stringstream
• to_string
• stoi and 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
• have no locale support
• memory safe
• error reporting, which 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...

The new methods are available in the following compilers:

• Visual Studio 2017 15.9 - full support (from_chars and also to_chars) (see notes about changes in 15.8 and in 15.9)
• GCC - 8.0 - still in progress, only integer support
• Clang 7.0 - still in progress, only integer support

The Series

This article is part of my series about C++17 Library Utilities. Here’s the list of the topics in the series:

• Refactoring with std::optional
• Using std::optional
• Error handling and std::optional
• Aboutstd::variant
• Aboutstd::any
• In place construction for std::optional, std::variant and std::any
• std::string_view Performance and follow-up and another post about string initialization
• C++17 string searchers and follow-up
• Conversion utilities (this post)
• Working with std::filesystem

Resources about C++17 STL:

• C++17 In Detail by Bartek!
• C++17 - The Complete Guide by Nicolai Josuttis
• C++ Fundamentals Including C++ 17 by Kate Gregory
• Practical C++14 and C++17 Features - by Giovanni Dicanio
• C++17 STL Cookbook by Jacek Galowicz

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:

Examples

Here are two examples of how to convert a string into a number using from_chars, to int andfloat.

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:

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 benchmarking and the new routines are blazing fast!

Some numbers:

• On GCC it’s ~4.5x faster than stoi, 2.2x faster than atoi and almost 50x faster than istringstream.
• On Clang it’s ~3.5x faster than stoi, 2.7x faster than atoi and 60x faster than istringstream!
• MSVC performs ~3x faster than stoi, ~2x faster than atoi and almost 50x faster than istringstream

The whole benchmark might, however, be 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 many characters were scanned).

The routines might be especially handy with parsing JSON files, 3D textual model representation (like OBJ file formats), etc.

More from the Author:

Bartek recently published a book - "C++17 In Detail"- learn the new C++ Standard in an efficient and practical way. The book contains more than 300 pages filled with C++17 content!

Your Turn

Have you played with the new conversion routines? What do you usually use to convert text into numbers?