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Difference between revisions of "cpp/header/charconv"

From cppreference.com
< cpp‎ | header
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{{cpp/header/title|charconv}}
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{{cpp/header/title|charconv|notes={{mark c++17}}}}
 
{{cpp/header/navbar}}
 
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===Classes===
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This header is part of the [[cpp/string|strings]] library.
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{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/utility/dsc chars_format}}
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{{dsc h1|Classes}}
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{{dsc end}}
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{{dsc inc|cpp/utility/dsc chars_format}}
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{{dsc inc|cpp/utility/dsc from_chars_result}}
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{{dsc inc|cpp/utility/dsc to_chars_result}}
  
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===Functions===
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{{dsc h1|Functions}}
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{{dsc inc|cpp/utility/dsc from_chars}}
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{{dsc begin}}
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{{dsc inc|cpp/utility/dsc to_chars}}
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{{dsc inc | cpp/utility/dsc from_chars}}
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{{dsc inc | cpp/utility/dsc to_chars}}
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{{dsc end}}
 
{{dsc end}}
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===Warning===
 
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libstdc++ and libc++ do not support from_chars/to_chars for floating points. VC's implementation assumes long double is as long as double (64 bits). Ryu algorithm does not support 128 bits fixed/precision floating points or etc. In general, it is questionable why ISO C++ committee decides to add something which is theoretically impossible and useless.
 
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<charconv> is a huge mistake. There are no algorithms in the world to implement to_chars/from_chars effectively or even correctly. The API encourages buffer overflow since it manipulates through raw pointers and buffer size is not deterministic. It also cannot be used under constexpr context.
 
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The fundamental problems are that <cstdio> and <iostream> are notoriously dangerous for tons of security vulnerability (like buffer overflow, vtable pointer injection, type confusion) and also notoriously slow. Everyone can easily write their own integers to strings with at least a 10x performance boost. C++ desperately needs a new IO library to replace stdio and iostream to fix the C/C++ historical problems for at least 50 years.
 
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Though the library is locale-dependent, the result is still not deterministic on EBCDIC machines. The results are nowhere near portable.
 
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In general, charconv considered harmful and should be avoided.
 
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===Synopsis===
 
===Synopsis===
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{{source|1=
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{{cpp/synopsis/charconv}}
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namespace std {
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    // floating-point format for primitive numerical conversion
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    enum class chars_format {
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        scientific = /*unspecified*/ ,
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        fixed = /*unspecified*/ ,
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        hex = /*unspecified*/ ,
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        general = fixed {{!}} scientific
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    };
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    // primitive numerical output conversion
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    struct to_chars_result {
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        char* ptr;
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        errc ec;
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    };
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    to_chars_result to_chars(char* first, char* last,
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                            /*see description*/ value, int base = 10);
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    to_chars_result to_chars(char* first, char* last, float value);
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    to_chars_result to_chars(char* first, char* last, double value);
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    to_chars_result to_chars(char* first, char* last, long double value);
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    to_chars_result to_chars(char* first, char* last, float value,
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                            chars_format fmt);
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    to_chars_result to_chars(char* first, char* last, double value,
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                            chars_format fmt);
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    to_chars_result to_chars(char* first, char* last, long double value,
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                            chars_format fmt);
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    to_chars_result to_chars(char* first, char* last, float value,
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                            chars_format fmt, int precision);
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    to_chars_result to_chars(char* first, char* last, double value,
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                            chars_format fmt, int precision);
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    to_chars_result to_chars(char* first, char* last, long double value,
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                            chars_format fmt, int precision);
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    // primitive numerical input conversion
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    struct from_chars_result {
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        const char* ptr;
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        errc ec;
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    };
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    from_chars_result from_chars(const char* first, const char* last,
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                                /*see description*/ & value, int base = 10);
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    from_chars_result from_chars(const char* first, const char* last, float& value,
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                                chars_format fmt = chars_format::general);
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    from_chars_result from_chars(const char* first, const char* last, double& value,
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                                chars_format fmt = chars_format::general);
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    from_chars_result from_chars(const char* first, const char* last, long double& value,
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                                chars_format fmt = chars_format::general);
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}
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}}
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{{langlinks|ja|zh}}
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{{langlinks|es|ja|ru|zh}}

Latest revision as of 07:08, 27 November 2023

 
C++
 
Standard library headers
General utilities
<any> (C++17)
<bitset>
<bit> (C++20)
<charconv> (C++17)
<expected> (C++23)
<format> (C++20)
<functional>
<optional> (C++17)
<tuple> (C++11)
<typeindex> (C++11)
<utility>
<variant> (C++17)
Containers
<array> (C++11)
<deque>
<flat_map> (C++23)
<flat_set> (C++23)
<forward_list> (C++11)
<inplace_vector> (C++26)   
<list>
<map>
<mdspan> (C++23)
<queue>
<set>
<span> (C++20)
<stack>
<unordered_map> (C++11)
<unordered_set> (C++11)
<vector>
Iterators
<iterator>
Ranges
<generator> (C++23)
<ranges> (C++20)
 

This header is part of the strings library.

Classes

(C++17)
 specifies formatting for std::to_chars and std::from_chars
(enum) [edit]
(C++17)
 the return type of std::from_chars
(class) [edit]
(C++17)
 the return type of std::to_chars
(class) [edit]

Functions

(C++17)
 converts a character sequence to an integer or floating-point value
(function) [edit]
(C++17)
 converts an integer or floating-point value to a character sequence
(function) [edit]

[edit] Synopsis

namespace std {
  // floating-point format for primitive numerical conversion
  enum class chars_format {
    scientific = /* unspecified */,
    fixed = /* unspecified */,
    hex = /* unspecified */,
    general = fixed | scientific
  };
 
 
  // primitive numerical output conversion
  struct to_chars_result { // freestanding
    char* ptr;
    errc ec;
    friend bool operator==(const to_chars_result&, const to_chars_result&) = default;
    constexpr explicit operator bool() const noexcept { return ec == errc{}; }
  };
 
  constexpr
  to_chars_result to_chars(char* first, char* last, // freestanding
                           /* integer-type */ value, int base = 10);
  to_chars_result to_chars(char* first, char* last, // freestanding
                           bool value, int base = 10) = delete;
  to_chars_result to_chars(char* first, char* last, // freestanding-deleted
                           /* floating-point-type */ value);
  to_chars_result to_chars(char* first, char* last, // freestanding-deleted
                           /* floating-point-type */ value, chars_format fmt);
  to_chars_result to_chars(char* first, char* last, // freestanding-deleted
                           /* floating-point-type */ value,
                           chars_format fmt, int precision);
 
  // primitive numerical input conversion
  struct from_chars_result { // freestanding
    const char* ptr;
    errc ec;
    friend bool operator==(const from_chars_result&, const from_chars_result&) = default;
    constexpr explicit operator bool() const noexcept { return ec == errc{}; }
  };
 
  constexpr
  from_chars_result from_chars(const char* first, // freestanding
                               const char* last, /* integer-type */& value,
                               int base = 10);
  from_chars_result from_chars(const char* first, // freestanding-deleted
                               const char* last, /* floating-point-type */& value,
                               chars_format fmt = chars_format::general);
}
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