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()gptr()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

#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

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)
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)
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)
提取字符
(std::basic_istream<CharT,Traits> 的公开成员函数)