std::from_chars_result

 

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Primitive numeric conversions

to_chars (C++17) to_chars_result (C++17) from_chars (C++17) from_chars_result (C++17) chars_format (C++17)

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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.

Data members

Member name	Definition
ptr	a pointer of type constchar* (public member object)
ec	an error code of type std::errc (public member object)

Member and friend functions

operator==(std::from_chars_result)

friendbool operator==(const from_chars_result&,                         const from_chars_result&)=default;		(since C++20)

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

constexprexplicit operator bool()constnoexcept;		(since C++26)

Checks whether the conversion is successful. Returns ec ==std::errc{}.

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

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_viewconst 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';elseif(ec ==std::errc::invalid_argument)std::cout<<"This is not a number.\n";elseif(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 >= 202306Lif(std::from_chars(s.data(), s.data()+ s.size(), value))#elseif(std::from_chars(s.data(), s.data()+ s.size(), value).ec==std::errc{})#endifreturn value;elsereturnstd::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.

See also

from_chars (C++17)

converts a character sequence to an integer or floating-point value (function)