std::basic_stringbuf<CharT,Traits,Allocator>::underflow
来自 cppreference.cn
< cpp | io | basic stringbuf
| protected: virtual int_type underflow() |
||
从缓冲区的获取区读取下一个字符。
具体地，
1) 如果输入序列有可用的读取位置（egptr() > gptr()），则返回 Traits::to_int_type(*gptr())
2) 否则，如果 pptr() > egptr()（自上次 overflow() 更改 egptr() 以来，流中插入了一些字符），则通过将 egptr() 更改为等于 pptr() 来扩展获取区的末尾以包含最近插入的字符，然后返回 Traits::to_int_type(*gptr())
3) 否则，返回 Traits::eof()。
缓冲区中任何已初始化的字符，无论其是源自构造函数中传递的字符串，还是通过 overflow() 添加的，都被视为输入序列的一部分。
目录 |
[编辑] 参数
(无)
[编辑] 返回值
成功时返回 Traits::to_int_type(*gptr())（获取区中下一个要读取的字符），失败时返回 Traits::eof()。
[编辑] 示例
运行此代码
#include <iostream> #include <sstream> struct mybuf : std::stringbuf { mybuf(const std::string& new_str, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out) : std::stringbuf(new_str, which) {} int_type overflow(int_type c) { std::cout << "Before overflow(): get area size is " << egptr() - eback() << ", the put area size is " << epptr() - pbase() << '\n'; int_type rc = std::stringbuf::overflow(c); std::cout << "After overflow(): get area size is " << egptr() - eback() << ", put area size is " << epptr() - pbase() << '\n'; return rc; } int_type underflow() { std::cout << "Before underflow(): get area size is " << egptr() - eback() << ", put area size is " << epptr() - pbase() << '\n'; int_type ch = std::stringbuf::underflow(); std::cout << "After underflow(): get area size is " << egptr() - eback() << ", put area size is " << epptr() - pbase() << '\n'; if (ch == EOF) std::cout << "underflow() returns EOF\n"; else std::cout << "underflow() returns '" << char(ch) << "'\n"; return ch; } }; int main() { mybuf sbuf("123"); // read-write stream std::iostream stream(&sbuf); int n; stream >> n; // calls sgetc() four times // three calls return the characters '1', '2', '3' // the fourth call, gptr() == egptr() and underflow() is called // underflow returns EOF std::cout << "n = " << n << '\n'; stream.clear(); // clear the eofbit stream << "123456"; // sputc() is called 6 times // first three calls store "123" in the existing buffer // 4th call finds that pptr() == epptr() and calls overflow() // overflow() grows the buffer and sets egptr() to 4 // 5th and 6th calls store '5' and '6', advancing pptr() stream >> n; // calls sgetc() 4 times // 1st call returns the '4' that was made available by overflow() // on the 2nd call, egptr() == egptr() and underflow() is called // underflow advances egptr() to equal pptr() (which is 6) // 3rd sgetc() returns '6' // 4th sgetc() finds gptr() == egptr(), calls underflow() // underflow() returns EOF std::cout << "n = " << n << '\n'; }
可能的输出
Before underflow(): get area size is 3, put area size is 15 After underflow(): get area size is 3, put area size is 15 underflow() returns EOF n = 123 Before underflow(): get area size is 3, put area size is 15 After underflow(): get area size is 6, put area size is 15 underflow() returns '4' Before underflow(): get area size is 6, put area size is 15 After underflow(): get area size is 6, put area size is 15 underflow() returns EOF n = 456
[编辑] 缺陷报告
下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。
| 缺陷报告 | 应用于 | 发布时的行为 | 正确的行为 |
|---|---|---|---|
| LWG 432 | C++98 | 不清楚通过 overflow() 添加的字符 是否被视为输入序列的一部分 |
已明确 |
[编辑] 参阅
| [virtual] |
从关联输入序列中读取字符到获取区 ( std::basic_streambuf<CharT,Traits> 的虚保护成员函数) |
| [virtual] |
从关联文件读取 ( std::basic_filebuf<CharT,Traits> 的虚保护成员函数) |
| [virtual] |
从输入序列中读取一个字符，但不推进下一个指针 ( std::strstreambuf 的虚保护成员函数) |
| 从输入序列中读取一个字符而不推进序列 ( std::basic_streambuf<CharT,Traits> 的公有成员函数) |
