1 Introduction

std::from_chars accepts only a pair of raw char pointers in its overloads to provide a range of characters for conversion. The author proposes to add overloads taking std::string_view which is the main C++ vocabulary type to denote a range of characters.

2 Motivation and Scope

2.1 A naive approach

Unfortunately, it is common for an average C++ developer to provide the following implementation for converting a std::string_view to int via a std::from_chars.

void foo(std::string_view txt)
{
  int result;
  auto [ptr, ec] = std::from_chars(txt.begin(), txt.end(), result);
  if (ec == std::errc())
    // ...
}

The above will work for some implementations and build modes but will fail for others. This is caused by the fact that even though some vendors implement std::string_view::const_iterator in terms of const char*, such code is not portable. The C++ standard specifies std::string_view::const_iterator as implementation defined and only requires that the iterator’s value_type is char.

2.2 A hacker’s approach

After realizing the portability issue programmers often fix it in the following way which is far from what we want to teach:

void foo(std::string_view txt)
{
  int result;
  auto [ptr, ec] = std::from_chars(&*txt.begin(), &*txt.end(), result);
  if (ec == std::errc())
    // ...
}

2.3 A correct approach

A correct approach works on the underlying data rather than on iterators which is counter intuitive as we always taught to use iterators to denote ranges in C++:

void foo(std::string_view txt)
{
  int result;
  auto [ptr, ec] = std::from_chars(txt.data(), txt.data() + txt.size(), result);
  if (ec == std::errc())
    // ...
}

3 Before/After Comparison

Before	After
`void foo(std::string_view txt) { int result; auto [ptr, ec] = std::from_chars(txt.data(), txt.data() + txt.size(), result); if (ec == std::errc()) // ... }`	`void foo(std::string_view txt) { int result; auto [ptr, ec] = std::from_chars(txt, result); if (ec == std::errc()) // ... }`

Before

After

void foo(std::string_view txt)
{
  int result;
  auto [ptr, ec] = std::from_chars(txt.data(), txt.data() + txt.size(), result);
  if (ec == std::errc())
    // ...
}

void foo(std::string_view txt)
{
  int result;
  auto [ptr, ec] = std::from_chars(txt, result);
  if (ec == std::errc())
    // ...
}

4 Proposed Wording

The proposed changes are relative to the working draft of the standard as of [N4830].

Add the following to the end of synopsis in 20.19.1 [charconv.syn]:

from_chars_result from_chars(string_view txt, see below& value, int base = 10);
from_chars_result from_chars(string_view txt, float& value, chars_format fmt = chars_format::general);
from_chars_result from_chars(string_view txt, double& value, chars_format fmt = chars_format::general);
from_chars_result from_chars(string_view txt, long double& value, chars_format fmt = chars_format::general);

Update the following paragraphs in 20.19.3 [charconv.from.chars]:

1 All functions named from_chars analyze the provided string ~~[first, last)~~ for a pattern, where string is either a txt or [first, last) denoting~~[first, last) is required to be~~ a valid range.

  from_chars_result from_chars(const char* first, const char* last,
                               see below& value, int base = 10);
+ from_chars_result from_chars(string_view txt,
+                              see below& value, int base = 10);

  from_chars_result from_chars(const char* first, const char* last, float& value,
                               chars_format fmt = chars_format::general);
+ from_chars_result from_chars(string_view txt, float& value,
+                              chars_format fmt = chars_format::general);
  from_chars_result from_chars(const char* first, const char* last, double& value,
                               chars_format fmt = chars_format::general);
+ from_chars_result from_chars(string_view txt, double& value,
+                              chars_format fmt = chars_format::general);
  from_chars_result from_chars(const char* first, const char* last, long double& value,
                               chars_format fmt = chars_format::general);
+ from_chars_result from_chars(string_view txt, long double& value,
+                              chars_format fmt = chars_format::general);

5 Acknowledgements

Special thanks and recognition goes to Epam Systems for supporting my membership in the ISO C++ Committee and the production of this proposal.

6 References

[N4830] Richard Smith. 2019. Committee Draft, Standard for Programming Language C++.
https://wg21.link/n4830

`std::from_chars` should work with `std::string_view`

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