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# std::ctype::widen, do_widen

From cppreference.com
< cpp‎ | locale‎ | ctype

Defined in header `<locale>` |  |
public: CharT widen( char c ) const; | (1) |
public: const char* widen( const char* beg, const char* end, CharT* dst ) const; | (2) |
protected: virtual CharT do_widen( char c ) const; | (3) |
protected: virtual const char* do_widen( const char* beg, const char* end, CharT* dst ) const; | (4) |

1,2) public member function, calls the protected virtual member function `do_widen` of the most derived class.
3) Converts the single-byte character `c` to the corresponding wide character representation using the simplest reasonable transformation. Typically, this applies only to the characters whose multibyte encoding is a single byte (e.g. U+0000-U+007F in UTF-8).
4) For every character in the character array `[beg, end)`, writes the corresponding widened character to the successive locations in the character array pointed to by `dst`.

Widening always returns a wide character, but only the characters from the basic source character set (latin letters, digits, and punctuations required to write a C++ program) are guaranteed to have a unique, well-defined, widening transformation, which is also guaranteed to be reversible (by narrow()). In practice, all characters whose multibyte representation is a single byte are usually widened to their wide character counterparts, and the rest of the possible single-byte values are usually mapped into the same placeholder value, typically CharT(-1).

Widening, if successful, preserves all character classification categories known to is().

### [edit] Parameters

c | - | character to convert
dflt | - | default value to produce if the conversion fails
beg | - | pointer to the first character in an array of characters to convert
end | - | one past the end pointer for the array of characters to convert
dst | - | pointer to the first element of the array of char to fill

### [edit] Return value

1,3) widened character
2,4) `end`

### [edit] Example

Run this code

    #include <locale>
    #include <iostream>

    void try_widen(const std::ctype<wchar_t>& f, char c)
    {
        wchar_t w = f.widen(c);
        std::cout << "The single-byte character " << +(unsigned char)c
                  << " widens to " << +w << '\n';
    }

    int main()
    {
        std::locale::global(std::locale("cs_CZ.iso88592"));
        auto& f = std::use_facet<std::ctype<wchar_t>>(std::locale());
        std::cout << std::hex << std::showbase << "In Czech ISO-8859-2 locale:\n";
        try_widen(f, 'a');
        try_widen(f, '\xdf'); // German letter ß (U+00df) in ISO-8859-2
        try_widen(f, '\xec'); // Czech letter ě (U+011b) in ISO-8859-2

        std::locale::global(std::locale("cs_CZ.utf8"));
        auto& f2 = std::use_facet<std::ctype<wchar_t>>(std::locale());
        std::cout << "In Czech UTF-8 locale:\n";
        try_widen(f2, 'a');
        try_widen(f2, '\xdf');
        try_widen(f2, '\xec');
    }

Output:

    In Czech ISO-8859-2 locale:
    The single-byte character 0x61 widens to 0x61
    The single-byte character 0xdf widens to 0xdf
    The single-byte character 0xec widens to 0x11b
    In Czech UTF-8 locale:
    The single-byte character 0x61 widens to 0x61
    The single-byte character 0xdf widens to 0xffffffff
    The single-byte character 0xec widens to 0xffffffff

### [edit] See also

narrow | invokes `do_narrow` (public member function) [edit]
widen | widens characters (public member function of `std::basic_ios`) [edit]
btowc | widens a single-byte narrow character to wide character, if possible (function) [edit]

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