std::strstreambuf::underflow

 

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受保护的成员函数

strstreambuf::underflow

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protected: virtual int_type underflow();		(C++98 中已弃用) (C++26 中已移除)

从缓冲区的读取区域读取下一个字符。

如果输入序列有可用的读取位置 (gptr() < egptr(), 则返回 (unsigned char)(*gptr())。

否则，如果 pptr() 不为空且 pptr() > egptr() (存在写入区域且位于读取区域之后)，则通过将 egptr() 递增到 gptr() 和 pptr() 之间的某个值来扩展读取区域的末尾，以包含最近写入写入区域的字符，然后返回 (unsigned char)(*gptr())。

否则，返回 EOF 以指示失败。

参数

(无)

返回值

读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

运行此代码

#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; // read-write dynamic 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

参见

underflow [虚函数]	从关联的输入序列读取字符到读取区域 (std::basic_streambuf<CharT,Traits> 的虚保护成员函数)
underflow [虚函数]	返回输入序列中下一个可用的字符 (std::basic_stringbuf<CharT,Traits,Allocator> 的虚保护成员函数)
underflow [虚函数]	从关联的文件读取 (std::basic_filebuf<CharT,Traits> 的虚保护成员函数)
sgetc	从输入序列读取一个字符，但不推进序列 (std::basic_streambuf<CharT,Traits> 的公共成员函数)
get	提取字符 (std::basic_istream<CharT,Traits> 的公共成员函数)