std::basic_filebuf::seekoff
protected: virtual pos_type seekoff( off_type off, std::ios_base::seekdir dir, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out ); | ? | ? |
|---|
如果可能的话，将文件指针重新定位到与之完全对应的位置。off文件%28的起始、结束或当前位置中的字符，取决于dir...
如果关联文件未打开%28is_open()==false立即失败。
如果多字节字符编码是状态相关的%28codecvt::encoding()退回来-1%29或可变长度%28codecvt::encoding()退回来?0?%29及抵销off不是?0?，立即失败：此函数无法确定对应于off人物。
如果dir不是std::basic_ios::cur或者抵消off不是?0?，而对这个filebuf对象执行的最不满的操作是输出%28，即PUT缓冲区不是空的，或者最近调用的函数是overflow()%29，然后呼叫std::codecvt::unshift以确定所需的未移位序列，并通过调用overflow()...
然后转换参数dir到一个值whence类型int详情如下：
value of dir | value of whence |
|---|---|
std::basic_ios::beg | SEEK_SET |
std::basic_ios::end | SEEK_END |
std::basic_ios::cur | SEEK_CUR |
然后，如果字符编码是固定宽度%28codecvt::encoding()返回一些正数width，将文件指针移动到std::fseek(file, width*off, whence)...
否则，将文件指针移动为std::fseek(file, 0, whence)...
大openmode参数，基类函数签名所需的参数通常被忽略，因为std::basic_filebuf只维护一个文件位置。
参数
off | - | relative position to set the position indicator 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 |
Constant | Explanation | |||||||||
beg | the beginning of a stream | |||||||||
end | the ending of a stream | |||||||||
cur | the current position of stream position indicator | |||||||||
which | - | defines which of the input and/or output sequences to affect. 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 | ||
Constant | Explanation | |||||||||
in | affect the input sequence | |||||||||
out | affect the output sequence |
返回值
新构造的类型对象pos_type存储结果文件位置的pos_type(off_type(-1))在失败的时候。
注记
seekoff()被std::basic_streambuf::pubseekoff，这是由std::basic_istream::seekg，，，std::basic_ostream::seekp，，，std::basic_istream::tellg，和std::basic_ostream::tellp...
例
二次
#include <iostream>
#include <fstream>
#include <locale>
int main()
{
// prepare a 10-byte file holding 4 characters in UTF8
std::ofstream("text.txt") << u8"z\u00df\u6c34\U0001d10b"; // or u8"z?水?"
// or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9d\x84\x8b";
// open using a non-converting encoding
std::ifstream f1("text.txt");
std::cout << "f1's locale's encoding() returns "
<< std::use_facet<std::codecvt<char, char, std::mbstate_t>>(f1.getloc()).encoding() << '\n'
<< "pubseekoff(3, beg) returns " << f1.rdbuf()->pubseekoff(3, std::ios_base::beg) << '\n'
<< "pubseekoff(0, end) returns " << f1.rdbuf()->pubseekoff(0, std::ios_base::end) << '\n';;
// open using UTF-8
std::wifstream f2("text.txt");
f2.imbue(std::locale("en_US.UTF-8"));
std::cout << "f2's locale's encoding() returns "
<< std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>(f2.getloc()).encoding() << '\n'
<< "pubseekoff(3, beg) returns " << f2.rdbuf()->pubseekoff(3, std::ios_base::beg) << '\n'
<< "pubseekoff(0, end) returns " << f2.rdbuf()->pubseekoff(0, std::ios_base::end) << '\n';
}二次
产出：
二次
f1's locale's encoding() returns 1
pubseekoff(3, beg) returns 3
pubseekoff(0, end) returns 10
f2's locale's encoding() returns 0
pubseekoff(3, beg) returns -1
pubseekoff(0, end) returns 10二次
另见
pubseekoff | invokes seekoff() (public member function of std::basic_streambuf) |
|---|---|
seekpos virtual | repositions the file position, using absolute addressing (virtual protected member function) |
fseek | moves the file position indicator to a specific location in a file (function) |
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