std::strstreambuf::underflow

< cpp‎ | io‎ | strstreambuf
 
C++
 
输入/输出库
 
std::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)