I/O: std::ios_base::openmode

本文详细介绍了C++中std::ios_base不同模式下文件流的行为特点,包括读写模式、追加模式、定位模式等,并针对不同组合模式下文件是否创建及文件内容的变化进行了深入解析。

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 std::ios_base::openmode

std::ios_base::in:  打开文件进行读操作,即读取文件中的数据

如果指定路径中没有包含该文件,不会创建,而且设置std::ios_base::badbit.

 

std::ios_base::out: 打开文件进行写操作,即写入数据到文件。

如果指定路径中并没有包含该文件,会创建一个新的.

如果指定的路径中包含有该文件,那么打开后,会清空文件中内容,其实就是暗含了std::ios_base::truc.

 

std::ios_base::in | std::ios_base::out: 打开文件进行读写操作

如果指定的路径没有包含该文件,也不会创建,并且会设置std::ios_base::badbit.

 

std::ios_base::app: 每次进行写入操作的时候都会重新定位到文件的末尾.

如果指定路径不存在该文件那么创建一个.

 

std::ios_base::ate: 打开文件之后立即定位到文件末尾

如果指定的路径不存在该文件不会创建.

 

std::ios_base::trunc: 打开文件,若文件已存在那么,清空文件内容.

如果指定的路径不存在该文件不会创建.

如果指定的路径含有该文件那么清空文件中的内容.

 

std::ios_base::binary: 以二进制的方式对打开的文件进行读写.

如果指定的路径不存在该文件不会创建.

 

总结:

1,凡包含std::ios_base::out mode的操作方式,如果文件不存在都是会创建.

std::ios_base::out | std::ios_base::app/ate/trunc 这些组合的方式来操作文件,如果指定的路径该文件不存在就会创建一个空的.

2,如果std::ios_base::in 和 std::ios_base_out同时使用就会按照 std::ios_base_in,也就是即使文件不存在也不会创建.

3,std::ios_base::in | std::ios_base::out | std::ios_base::app/ate/trunc 这些组合的方式来操作文件, 如果指定路径的文件不存在也不会创建而且会把当前stream的state设置为std::ios_base::badbit.

4,由于std::ios_base::out和std::ios_base::trunc单独使用的时候在打开已存在文件的情况下都会清空文件内容,因此我们使用的时候要格外注意.例如:我们可以通过std::ios_base::out | std::ios_base::app组合的形式来消除打开已存在文件时候的清空动作.

转载于:https://my.oschina.net/SHIHUAMarryMe/blog/899167

49725@DESKTOP-CB4LRLP /cygdrive/f/S $ g++ -o screensaver_clock F:/S/screensaver_clock.cpp -lgdiplus -mwindows F:/S/screensaver_clock.cpp: In function 'void SetFontSize(HWND)': F:/S/screensaver_clock.cpp:21:54: error: no matching function for call to 'std::basic_ofstream<wchar_t>::open(std::__cxx11::wstring&, const openmode&)' logFile.open(g_runtimeFilePath, std::ios::app); ^ In file included from F:/S/screensaver_clock.cpp:2: D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:851:7: note: candidate: 'void std::basic_ofstream<_CharT, _Traits>::open(const char*, std::ios_base::openmode) [with _CharT = wchar_t; _Traits = std::char_traits<wchar_t>; std::ios_base::openmode = std::_Ios_Openmode]' open(const char* __s, ios_base::openmode __mode = ios_base::out) ^~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:851:7: note: no known conversion for argument 1 from 'std::__cxx11::wstring' {aka 'std::__cxx11::basic_string<wchar_t>'} to 'const char*' D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:871:7: note: candidate: 'void std::basic_ofstream<_CharT, _Traits>::open(const string&, std::ios_base::openmode) [with _CharT = wchar_t; _Traits = std::char_traits<wchar_t>; std::__cxx11::string = std::__cxx11::basic_string<char>; std::ios_base::openmode = std::_Ios_Openmode]' open(const std::string& __s, ios_base::openmode __mode = ios_base::out) ^~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:871:7: note: no known conversion for argument 1 from 'std::__cxx11::wstring' {aka 'std::__cxx11::basic_string<wchar_t>'} to 'const string&' {aka 'const std::__cxx11::basic_string<char>&'} F:/S/screensaver_clock.cpp: In function 'void SetTitle(HWND)': F:/S/screensaver_clock.cpp:32:54: error: no matching function for call to 'std::basic_ofstream<wchar_t>::open(std::__cxx11::wstring&, const openmode&)' logFile.open(g_runtimeFilePath, std::ios::app); ^ In file included from F:/S/screensaver_clock.cpp:2: D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:851:7: note: candidate: 'void std::basic_ofstream<_CharT, _Traits>::open(const char*, std::ios_base::openmode) [with _CharT = wchar_t; _Traits = std::char_traits<wchar_t>; std::ios_base::openmode = std::_Ios_Openmode]' open(const char* __s, ios_base::openmode __mode = ios_base::out) ^~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:851:7: note: no known conversion for argument 1 from 'std::__cxx11::wstring' {aka 'std::__cxx11::basic_string<wchar_t>'} to 'const char*' D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:871:7: note: candidate: 'void std::basic_ofstream<_CharT, _Traits>::open(const string&, std::ios_base::openmode) [with _CharT = wchar_t; _Traits = std::char_traits<wchar_t>; std::__cxx11::string = std::__cxx11::basic_string<char>; std::ios_base::openmode = std::_Ios_Openmode]' open(const std::string& __s, ios_base::openmode __mode = ios_base::out) ^~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:871:7: note: no known conversion for argument 1 from 'std::__cxx11::wstring' {aka 'std::__cxx11::basic_string<wchar_t>'} to 'const string&' {aka 'const std::__cxx11::basic_string<char>&'} F:/S/screensaver_clock.cpp: In function 'void SetCountdownDate(HWND)': F:/S/screensaver_clock.cpp:43:54: error: no matching function for call to 'std::basic_ofstream<wchar_t>::open(std::__cxx11::wstring&, const openmode&)' logFile.open(g_runtimeFilePath, std::ios::app); ^ In file included from F:/S/screensaver_clock.cpp:2: D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:851:7: note: candidate: 'void std::basic_ofstream<_CharT, _Traits>::open(const char*, std::ios_base::openmode) [with _CharT = wchar_t; _Traits = std::char_traits<wchar_t>; std::ios_base::openmode = std::_Ios_Openmode]' open(const char* __s, ios_base::openmode __mode = ios_base::out) ^~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:851:7: note: no known conversion for argument 1 from 'std::__cxx11::wstring' {aka 'std::__cxx11::basic_string<wchar_t>'} to 'const char*' D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:871:7: note: candidate: 'void std::basic_ofstream<_CharT, _Traits>::open(const string&, std::ios_base::openmode) [with _CharT = wchar_t; _Traits = std::char_traits<wchar_t>; std::__cxx11::string = std::__cxx11::basic_string<char>; std::ios_base::openmode = std::_Ios_Openmode]' open(const std::string& __s, ios_base::openmode __mode = ios_base::out) ^~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:871:7: note: no known conversion for argument 1 from 'std::__cxx11::wstring' {aka 'std::__cxx11::basic_string<wchar_t>'} to 'const string&' {aka 'const std::__cxx11::basic_string<char>&'}报错,优化代码,代码全输出
05-13
[ERROR] /workspace/topic/v2/98/cpp/main.cpp: In function ‘int solution(std::vector<int>)’: /workspace/topic/v2/98/cpp/main.cpp:8:12: error: no match for ‘operator^=’ (operand types are ‘int’ and ‘__gnu_cxx::__normal_iterator<int*, std::vector<int> >’) 8 | unique ^= i; | ~~~~~~~^~~~ In file included from /usr/include/c++/10/ios:42, from /usr/include/c++/10/ostream:38, from /usr/include/c++/10/iostream:39, from /workspace/topic/v2/98/cpp/main.cpp:2: /usr/include/c++/10/bits/ios_base.h:107:3: note: candidate: ‘const std::_Ios_Fmtflags& std::operator^=(std::_Ios_Fmtflags&, std::_Ios_Fmtflags)’ 107 | operator^=(_Ios_Fmtflags& __a, _Ios_Fmtflags __b) | ^~~~~~~~ /usr/include/c++/10/bits/ios_base.h:107:48: note: no known conversion for argument 2 from ‘__gnu_cxx::__normal_iterator<int*, std::vector<int> >’ to ‘std::_Ios_Fmtflags’ 107 | operator^=(_Ios_Fmtflags& __a, _Ios_Fmtflags __b) | ~~~~~~~~~~~~~~^~~ /usr/include/c++/10/bits/ios_base.h:149:3: note: candidate: ‘const std::_Ios_Openmode& std::operator^=(std::_Ios_Openmode&, std::_Ios_Openmode)’ 149 | operator^=(_Ios_Openmode& __a, _Ios_Openmode __b) | ^~~~~~~~ /usr/include/c++/10/bits/ios_base.h:149:48: note: no known conversion for argument 2 from ‘__gnu_cxx::__normal_iterator<int*, std::vector<int> >’ to ‘std::_Ios_Openmode’ 149 | operator^=(_Ios_Openmode& __a, _Ios_Openmode __b) | ~~~~~~~~~~~~~~^~~ /usr/include/c++/10/bits/ios_base.h:189:3: note: candidate: ‘const std::_Ios_Iostate& std::operator^=(std::_Ios_Iostate&, std::_Ios_Iostate)’ 189 | operator^=(_Ios_Iostate& __a, _Ios_Iostate __b) | ^~~~~~~~ /usr/include/c++/10/bits/ios_base.h:189:46: note: no known conversion for argument 2 from ‘__gnu_cxx::__normal_iterator<int*, std::vector<int> >’ to ‘std::_Ios_Iostate’ 189 | operator^=(_Ios_Iostate& __a, _Ios_Iostate __b)
03-24
49725@DESKTOP-CB4LRLP /cygdrive/f/S $ g++ -mwindows screensaver_clock.cpp -o screensaver_ -lcomctl32 -lgdi32 -luser32 screensaver_clock.cpp: In function 'void LoadLearningTime()': screensaver_clock.cpp:36:32: error: no matching function for call to 'std::basic_ifstream<char>::basic_ifstream(const wchar_t*&)' std::ifstream file(LOG_FILE); ^ In file included from screensaver_clock.cpp:5: D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:554:7: note: candidate: 'std::basic_ifstream<_CharT, _Traits>::basic_ifstream(std::basic_ifstream<_CharT, _Traits>&&) [with _CharT = char; _Traits = std::char_traits<char>]' basic_ifstream(basic_ifstream&& __rhs) ^~~~~~~~~~~~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:554:7: note: no known conversion for argument 1 from 'const wchar_t*' to 'std::basic_ifstream<char>&&' D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:529:7: note: candidate: 'std::basic_ifstream<_CharT, _Traits>::basic_ifstream(const string&, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_traits<char>; std::__cxx11::string = std::__cxx11::basic_string<char>; std::ios_base::openmode = std::_Ios_Openmode]' basic_ifstream(const std::string& __s, ^~~~~~~~~~~~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:529:7: note: no known conversion for argument 1 from 'const wchar_t*' to 'const string&' {aka 'const std::__cxx11::basic_string<char>&'} D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:513:7: note: candidate: 'std::basic_ifstream<_CharT, _Traits>::basic_ifstream(const char*, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_traits<char>; std::ios_base::openmode = std::_Ios_Openmode]' basic_ifstream(const char* __s, ios_base::openmode __mode = ios_base::in) ^~~~~~~~~~~~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:513:7: note: no known conversion for argument 1 from 'const wchar_t*' to 'const char*' D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:502:7: note: candidate: 'std::basic_ifstream<_CharT, _Traits>::basic_ifstream() [with _CharT = char; _Traits = std::char_traits<char>]' basic_ifstream() : __istream_type(), _M_filebuf() ^~~~~~~~~~~~~~ D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:502:7: note: candidate expects 0 arguments, 1 provided screensaver_clock.cpp: In function 'void SaveLearningTime()': screensaver_clock.cpp:53:32: error: no matching function for call to 'std::basic_ofstream<char>::basic_ofstream(const wchar_t*&)' std::ofstream file(LOG_FILE); ^ In file included from screensaver_clock.cpp:5: D:/wingcc/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/fstream:780:7: note: candidate: 'std::basic_ofstream<_CharT, _Traits>::basic_ofstream(std::basic_ofstream<_CharT, _Traits>&&) [with _CharT = char; _Traits = std::char_traits<char>]' basic_ofstream(basic_ofstream&& _优化代码,全代码输出
05-13
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