std::from_chars_result
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C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Text processing library
Regular expressions library (C++11)
Formatting library (C++20)
Defined in header
<charconv>
struct from_chars_result;
(since C++17)
std::from_chars_result is the return type of std::from_chars. It has no base classes, and only has the following members.
Contents
[edit] Data members
Member name
Definition
ptr
(public member object)
[edit] Member and friend functions
operator==(std::from_chars_result)
friend bool operator==( const from_chars_result&,
const from_chars_result& ) = default;
(since C++20)
const from_chars_result& ) = default;
Compares the two arguments using default comparisons (which uses operator== to compare ptr and ec respectively).
This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::from_chars_result is an associated class of the arguments.
The != operator is synthesized from operator==.
operator bool
constexpr explicit operator bool() const noexcept;
(since C++26)
Checks whether the conversion is successful. Returns ec == std::errc {}.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_to_chars |
201611L |
(C++17) | Elementary string conversions (std::to_chars, std::from_chars) |
202306L |
(C++26) | Testing for success or failure of <charconv> functions |
[edit] Example
Run this code
#include <cassert> #include <charconv> #include <iomanip> #include <iostream> #include <optional> #include <string_view> #include <system_error> int main() { for (std::string_view const str : {"1234", "15 foo", "bar", " 42", "5000000000"}) { std::cout << "String: " << std::quoted (str) << ". "; int result{}; auto [ptr, ec] = std::from_chars (str.data(), str.data() + str.size(), result); if (ec == std::errc ()) std::cout << "Result: " << result << ", ptr -> " << std::quoted (ptr) << '\n'; else if (ec == std::errc::invalid_argument ) std::cout << "This is not a number.\n"; else if (ec == std::errc::result_out_of_range ) std::cout << "This number is larger than an int.\n"; } // C++23's constexpr from_char demo / C++26's operator bool() demo: auto to_int = [](std::string_view s) -> std::optional <int> { int value{}; #if __cpp_lib_to_chars >= 202306L if (std::from_chars (s.data(), s.data() + s.size(), value)) #else if (std::from_chars (s.data(), s.data() + s.size(), value).ec == std::errc {}) #endif return value; else return std::nullopt ; }; assert (to_int("42") == 42); assert (to_int("foo") == std::nullopt ); #if __cpp_lib_constexpr_charconv and __cpp_lib_optional >= 202106 static_assert(to_int("42") == 42); static_assert(to_int("foo") == std::nullopt ); #endif }
Output:
String: "1234". Result: 1234, ptr -> "" String: "15 foo". Result: 15, ptr -> " foo" String: "bar". This is not a number. String: " 42". This is not a number. String: "5000000000". This number is larger than an int.