do_narrow
STD：：Ctype：：窄，做[医]窄
| Defined in header | | |
|---|---|---|
| public: char narrow( CharT c, char dflt ) const; | (1) | |
| public: const CharT* narrow( const CharT* beg, const CharT* end, char dflt, char* dst ) const; | (2) | |
| protected: virtual char do_narrow( CharT c, char dflt ) const; | (3) | |
| protected: virtual const CharT* do_narrow( const CharT* beg, const CharT* end, char dflt, char* dst ) const; | (4) | |
1，2%29公共成员函数，调用受保护的虚拟成员函数do_narrow最派生的类。
3%29转换%28可能宽的%29字符c例如，如果字符可以用单个字节%28表示，那么UTF-8编码中的ASCII字符是单字节%29。回报dflt如果不存在这种转换。
4%29对字符数组中的每个字符[beg, end)，写入缩小字符%28或dflt每当将%29缩小到指向的字符数组中的连续位置时，dst...
缩窄总是成功的，并且通过调用widen()%29用于编写C++程序%29所需的基本源字符集%28 latin字母、数字和标点符号的所有字符。
如果收缩成功，则保留已知的所有字符分类类别。is()...
缩小任何数字字符可以保证如果从字符文字中减去结果'0'，此差异等于原始字符的数字值。
参数
| 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 |
返回值
1，3%29缩小字符或dflt如果收缩失败
2，4%29end
例
二次
#include <locale>
#include <iostream>
void try_narrow(const std::ctype<wchar_t>& f, wchar_t c)
{
char n = f.narrow(c, 0
if (n) {
std::wcout << '\'' << c << "' narrowed to " << +(unsigned char)n << '\n';
} else {
std::wcout << '\'' << c << "' could not be narrowed\n";
}
}
int main()
{
std::locale::global(std::locale("en_US.utf8")
std::wcout.imbue(std::locale()
std::wcout << std::hex << std::showbase << "In US English UTF-8 locale:\n";
auto& f = std::use_facet<std::ctype<wchar_t>>(std::locale()
try_narrow(f, L'A'
try_narrow(f, L'Ａ'
try_narrow(f, L'ě'
std::locale::global(std::locale("cs_CZ.iso88592")
auto& f2 = std::use_facet<std::ctype<wchar_t>>(std::locale()
std::wcout << "In Czech ISO-8859-2 locale:\n";
try_narrow(f2, L'A'
try_narrow(f2, L'Ａ'
try_narrow(f2, L'ě'
}
二次
产出：
二次
In US English UTF-8 locale:
'A' narrowed to 0x41
'Ａ' could not be narrowed
'ě' could not be narrowed
In Czech ISO-8859-2 locale:
'A' narrowed to 0x41
'Ａ' could not be narrowed
'ě' narrowed to 0xec
二次
另见
| widen | invokes do_widen (public member function) |
|---|---|
| narrow | narrows characters (public member function of std::basic_ios) |
| wctob | narrows a wide character to a single-byte narrow character, if possible (function) |
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