std::strstreambuf::underflow
来自cppreference.com
< cpp | io | strstreambuf
| protected: virtual int_type underflow(); |
||
从缓冲区的获取区读取下个字符。
若输入序列拥有可用读位置（ gptr() < egptr() ），则返回 (unsigned char)(*gptr()) 。
否则，若 pptr() 非空且 pptr() > egptr() （存在放置区，且它位于获取区之后），则以增加 egptr() 到 gptr() 和 pptr() 间的某个值扩展获取区的结尾，以包含最近写入到放置区中的字符，然后返回 (unsigned char)(*gptr()) 。
否则，返回 EOF 以指示失败。
参数
（无）
返回值
成功时为获取区中下个字符 (unsigned char)(*gptr()) ，失败时为 EOF 。
示例
运行此代码
#include <strstream> #include <iostream> struct mybuf : std::strstreambuf { int_type overflow(int_type c) { std::cout << "Before overflow(): size of the get area is " << egptr()-eback() << " size of the put area is " << epptr()-pbase() << '\n'; int_type rc = std::strstreambuf::overflow(c); std::cout << "After overflow(): size of the get area is " << egptr()-eback() << " size of the put area is " << epptr()-pbase() << '\n'; return rc; } int_type underflow() { std::cout << "Before underflow(): size of the get area is " << egptr()-eback() << " size of the put area is " << epptr()-pbase() << '\n'; int_type ch = std::strstreambuf::underflow(); std::cout << "After underflow(): size of the get area is " << egptr()-eback() << " size of the put area 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; // 读写动态 strstreambuf std::iostream stream(&sbuf); int n; stream >> n; stream.clear(); stream << "123"; stream >> n; std::cout << n << '\n'; }
可能的输出：
Before underflow(): size of the get area is 0 size of the put area is 0 After underflow(): size of the get area is 0 size of the put area is 0 underflow() returns EOF Before overflow(): size of the get area is 0 size of the put area is 0 After overflow(): size of the get area is 0 size of the put area is 32 Before underflow(): size of the get area is 0 size of the put area is 32 After underflow(): size of the get area is 3 size of the put area is 32 underflow() returns '1' Before underflow(): size of the get area is 3 size of the put area is 32 After underflow(): size of the get area is 3 size of the put area is 32 underflow() returns EOF 123
参阅
| [虚] |
从关联输入序列读取字符到获取区 ( std::basic_streambuf<CharT,Traits> 的虚受保护成员函数) |
| [虚] |
返回输入序列中可用的下一字符 ( std::basic_stringbuf<CharT,Traits,Allocator> 的虚受保护成员函数) |
| [虚] |
从关联文件读取 ( std::basic_filebuf<CharT,Traits> 的虚受保护成员函数) |
| 从输入序列读取一个字符，而不令序列前进 ( std::basic_streambuf<CharT,Traits> 的公开成员函数) | |
| 从流中读并取走（移除类似指针向下一个元素移动）一个字符 ( std::basic_istream<CharT,Traits> 的公开成员函数) |