std::codecvt<InternT,ExternT,State>::max_length, do_max_length

< cpp‎ | locale‎ | codecvt
 
C++
 
本地化库
 
std::codecvt
 
定义于头文件 <locale>
(1)
public:
int max_length() const throw();
 (C++11 前)
public:
int max_length() const noexcept;
 (C++11 起)
(2)
protected:
virtual int do_max_length() const throw();
 (C++11 前)
protected:
virtual int do_max_length() const noexcept;
 (C++11 起)
1) 公开成员函数，调用最终派生类的成员函数 do_max_length 。
2) 返回对于任何合法范围 [from, from_end) 和任何 state 合法 ， do_length(state, from, from_end, 1) 所能返回的最大值。

返回值

若以 in() 转换生成一个 InternT 字符，能消耗的最大 ExternT 字符数。

不转换特化 std::codecvt<char, char, std::mbstate_t> 返回 1 。

注意

若编码依赖状态（ encoding() == -1 ），则可能消耗多于 max_length() 个外部字符生成一个内部字符。

示例

运行此代码
#include <locale>
#include <iostream>
#include <codecvt>
 
int main()
{
    std::cout << "In codecvt_utf8, the longest multibyte character is "
              << std::codecvt_utf8<wchar_t>().max_length() << " bytes long\n";
 
    std::cout << "In header-consuming codecvt_utf8, the longest multibyte character is "
              << std::codecvt_utf8<wchar_t,
                                   0x10ffff,
                                   std::consume_header>().max_length() << " bytes long\n";
 
    std::cout << "In this system's en_US.utf8, the longest multibyte character is "
              << std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>(
                    std::locale("en_US.utf8")
              ).max_length() << " bytes long\n";
 
    std::cout << "In this system's zh_CN.gb18030, the longest multibyte character is "
              << std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>(
                    std::locale("zh_CN.gb18030")
              ).max_length() << " bytes long\n";
}

输出： 

In codecvt_utf8, the longest multibyte character is 4 bytes long
In header-consuming codecvt_utf8, the longest multibyte character is 7 bytes long
In this system's en_US.utf8, the longest multibyte character is 6 bytes long
In this system's zh_CN.gb18030, the longest multibyte character is 4 bytes long

参阅

MB_CUR_MAX
 当前 C 本地环境中多字节字符的最大字节数
(宏变量)
[虚]
 返回产生一个 internT 字符所需的 externT 字符数，若此值为常数
(虚受保护成员函数)