std::strstreambuf:: underflow

从缓冲区的获取区域读取下一个字符。

如果输入序列存在可读取位置（ gptr ( ) < egptr ( ) ），则返回 ( unsigned char ) ( * gptr ( ) ) 。

否则，若 pptr() 非空且 pptr ( ) > egptr ( ) （存在输出区域且其位置位于输入区域之后），则通过将 egptr() 递增至介于 gptr ( ) 与 pptr() 之间的某个值，扩展输入区域的末端以包含最近写入输出区域的字符，随后返回 ( unsigned char ) ( * gptr ( ) ) 。

否则，返回 EOF 表示失败。

参数

(无)

返回值

获取区域中的下一个字符，成功时返回 ( unsigned char ) ( * gptr ( ) ) ，失败时返回 EOF 。

示例

#include <iostream>
#include <strstream>
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';
}

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

参见