std::strstreambuf::seekoff
std：：strStreambuf：：Sekoff
| protected: virtual pos_type seekoff(off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out | | |
|---|
再定位std::basic_streambuf::gptr和/或std::basic_streambuf::pptr，如果可能的话，它的位置正好对应于off缓冲区的GET和/或PUT区域的起始、结束或当前位置中的字符。
- 如果
which包括ios_base::in这个缓冲区是打开的，然后重新定位读取指针。std::basic_streambuf::gptr在GET区域内，如下所述
- 如果
which包括ios_base::out这个缓冲区是打开的，然后重新定位写指针。std::basic_streambuf::pptr在PUT区域内，如下所述
- 如果
which包括两者ios_base::in和ios_base::out并且缓冲区是开放的，既可以读也可以写，而且when是ios_base::beg或ios_base::end，然后重新定位读和写指针，如下所述。
- 否则，此函数将失败。
如果指针%28gptr或pptr或者两种%29都被重新定位，其操作如下：
1%29如果要重新定位的指针是空指针和新偏移量newoff如果是非零，则此函数将失败。
2%29新指针偏移量newoff类型off_type已确定
A%29如果way == ios_base::beg，然后newoff是零
B%29way == ios_base::cur，然后newoff指针%28的当前位置。gptr()-eback()或pptr()-pbase()%29
C%29如果way == ios_base::end，然后newoff是缓冲区%28的整个初始化部分的长度(如果使用过分配，则高水印指针减去起始指针%29)。
3%29newoff + off为负值或超出缓冲区初始化部分的界限，则函数将失败。
4%29否则，指针将被分配为gptr() = eback() + newoff + off或pptr() = pbase() + newoff + off
参数
| off | - | relative position to set the next pointer(s) to |
|---|---|---|
| dir | - | defines base position to apply the relative offset to. It can be one of the following constants: Constant Explanation beg the beginning of a stream end the ending of a stream cur the current position of stream position indicator |
| Constant | Explanation | |
| beg | the beginning of a stream | |
| end | the ending of a stream | |
| cur | the current position of stream position indicator | |
| which | - | defines whether the input sequences, the output sequence, or both are affected. It can be one or a combination of the following constants: Constant Explanation in affect the input sequence out affect the output sequence |
| Constant | Explanation | |
| in | affect the input sequence | |
| out | affect the output sequence |
返回值
pos_type(newoff)在成功的时候，pos_type(off_type(-1))论失败与失败[医]类型不能表示结果流位置。
例
二次
#include <iostream>
#include <strstream>
int main()
{
char a[] = "123";
std::strstream ss(a, sizeof a // in/out
std::cout << "put pos = " << ss.tellp()
<< " get pos = " << ss.tellg() << '\n';
// absolute positioning both pointers
ss.rdbuf()->pubseekoff(1, std::ios_base::beg // move both forward
std::cout << "put pos = " << ss.tellp()
<< " get pos = " << ss.tellg() << '\n';
// try to move both pointers 1 forward from current position
if(-1 == ss.rdbuf()->pubseekoff(1, std::ios_base::cur))
std::cout << "moving both pointers from current position failed\n";
std::cout << "put pos = " << ss.tellp()
<< " get pos = " << ss.tellg() << '\n';
// move the write pointer 1 forward, but not the read pointer
// can also be called as ss.seekp(1, std::ios_base::cur
ss.rdbuf()->pubseekoff(1, std::ios_base::cur, std::ios_base::out
std::cout << "put pos = " << ss.tellp()
<< " get pos = " << ss.tellg() << '\n';
ss << 'a'; // write at put position
std::cout << "Wrote 'a' at put position, the buffer is now: '";
std::cout.write(a, sizeof a
std::cout << "'\n";
char ch;
ss >> ch;
std::cout << "reading at get position gives '" << ch << "'\n";
}
二次
产出：
二次
put pos = 0 get pos = 0
put pos = 1 get pos = 1
moving both pointers from current position failed
put pos = 1 get pos = 1
put pos = 2 get pos = 1
Wrote 'a' at put position, the buffer is now: '12a'
reading at get position gives '2'
二次
另见
| seekpos virtual | repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function of std::basic_streambuf) |
|---|---|
| seekoff virtual | repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of std::basic_streambuf) |
| seekoff virtual | repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of std::basic_stringbuf) |
| seekoff virtual | repositions the file position, using relative addressing (virtual protected member function of std::basic_filebuf) |
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