C++11自动变量
C++11自动变量
提问于 2015-07-10 12:05:24
在现代C++风格的要点(视频)中，Herb倡导：
auto varname = Constructor{ ... };而不是
Constructor varname( ... );我试着做：
auto log = fstream{"log.txt", fstream::out};但是从g++4.8 -std=c++11收到了一条关于已删除函数的错误消息(不管我是使用大括号还是括号)。
是我的错还是g++的错？
fstream log{"log.txt", fstream::out};效果很好。
MCVE：
#include <fstream>
using namespace std;
int main(int argc, char **argv)
{
auto log = fstream{"log.txt", fstream::out};
//fstream log{"log.txt", fstream::out};
log << "hello world" << endl;
}错误：
g++ -std=c++11 1.cc -o 1
1.cc: In function ‘int main(int, char**)’:
1.cc:6:47: error: use of deleted function ‘std::basic_fstream<char>::basic_fstream(const std::basic_fstream<char>&)’
auto log = fstream{"log.txt", fstream::out};
^
In file included from 1.cc:1:0:
/usr/include/c++/4.8/fstream:776:11: note: ‘std::basic_fstream<char>::basic_fstream(const std::basic_fstream<char>&)’ is implicitly deleted because the default definition would be ill-formed:
class basic_fstream : public basic_iostream<_CharT, _Traits>
^
/usr/include/c++/4.8/fstream:776:11: error: use of deleted function ‘std::basic_iostream<char>::basic_iostream(const std::basic_iostream<char>&)’
In file included from /usr/include/c++/4.8/fstream:38:0,
from 1.cc:1:
/usr/include/c++/4.8/istream:795:11: note: ‘std::basic_iostream<char>::basic_iostream(const std::basic_iostream<char>&)’ is implicitly deleted because the default definition would be ill-formed:
class basic_iostream
^
/usr/include/c++/4.8/istream:795:11: error: use of deleted function ‘std::basic_istream<char>::basic_istream(const std::basic_istream<char>&)’
/usr/include/c++/4.8/istream:58:11: note: ‘std::basic_istream<char>::basic_istream(const std::basic_istream<char>&)’ is implicitly deleted because the default definition would be ill-formed:
class basic_istream : virtual public basic_ios<_CharT, _Traits>
^
/usr/include/c++/4.8/istream:58:11: error: use of deleted function ‘std::basic_ios<char>::basic_ios(const std::basic_ios<char>&)’
In file included from /usr/include/c++/4.8/ios:44:0,
from /usr/include/c++/4.8/istream:38,
from /usr/include/c++/4.8/fstream:38,
from 1.cc:1:
/usr/include/c++/4.8/bits/basic_ios.h:66:11: note: ‘std::basic_ios<char>::basic_ios(const std::basic_ios<char>&)’ is implicitly deleted because the default definition would be ill-formed:
class basic_ios : public ios_base
^
In file included from /usr/include/c++/4.8/ios:42:0,
from /usr/include/c++/4.8/istream:38,
from /usr/include/c++/4.8/fstream:38,
from 1.cc:1:
/usr/include/c++/4.8/bits/ios_base.h:786:5: error: ‘std::ios_base::ios_base(const std::ios_base&)’ is private
ios_base(const ios_base&);
^
In file included from /usr/include/c++/4.8/ios:44:0,
from /usr/include/c++/4.8/istream:38,
from /usr/include/c++/4.8/fstream:38,
from 1.cc:1:
/usr/include/c++/4.8/bits/basic_ios.h:66:11: error: within this context
class basic_ios : public ios_base
^
In file included from /usr/include/c++/4.8/fstream:38:0,
from 1.cc:1:
/usr/include/c++/4.8/istream:795:11: error: use of deleted function ‘std::basic_ostream<char>::basic_ostream(const std::basic_ostream<char>&)’
class basic_iostream
^
In file included from /usr/include/c++/4.8/istream:39:0,
from /usr/include/c++/4.8/fstream:38,
from 1.cc:1:
/usr/include/c++/4.8/ostream:58:11: note: ‘std::basic_ostream<char>::basic_ostream(const std::basic_ostream<char>&)’ is implicitly deleted because the default definition would be ill-formed:
class basic_ostream : virtual public basic_ios<_CharT, _Traits>
^
/usr/include/c++/4.8/ostream:58:11: error: use of deleted function ‘std::basic_ios<char>::basic_ios(const std::basic_ios<char>&)’
In file included from /usr/include/c++/4.8/fstream:38:0,
from 1.cc:1:
/usr/include/c++/4.8/istream:795:11: error: use of deleted function ‘std::basic_ios<char>::basic_ios(const std::basic_ios<char>&)’
class basic_iostream
^
In file included from 1.cc:1:0:
/usr/include/c++/4.8/fstream:776:11: error: use of deleted function ‘std::basic_ios<char>::basic_ios(const std::basic_ios<char>&)’
class basic_fstream : public basic_iostream<_CharT, _Traits>
^
/usr/include/c++/4.8/fstream:776:11: error: use of deleted function ‘std::basic_filebuf<char>::basic_filebuf(const std::basic_filebuf<char>&)’
/usr/include/c++/4.8/fstream:72:11: note: ‘std::basic_filebuf<char>::basic_filebuf(const std::basic_filebuf<char>&)’ is implicitly deleted because the default definition would be ill-formed:
class basic_filebuf : public basic_streambuf<_CharT, _Traits>
^
In file included from /usr/include/c++/4.8/ios:43:0,
from /usr/include/c++/4.8/istream:38,
from /usr/include/c++/4.8/fstream:38,
from 1.cc:1:
/usr/include/c++/4.8/streambuf:802:7: error: ‘std::basic_streambuf<_CharT, _Traits>::basic_streambuf(const std::basic_streambuf<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits<char>]’ is private
basic_streambuf(const basic_streambuf& __sb)
^
In file included from 1.cc:1:0:
/usr/include/c++/4.8/fstream:72:11: error: within this context
class basic_filebuf : public basic_streambuf<_CharT, _Traits>
^
make: *** [1] Error 1回答 2
Stack Overflow用户
回答已采纳
发布于 2015-07-10 12:21:23
auto var = Type{...};语法要求Type必须是可移动的，但是gcc直到GCC 5.1才实现可移动的fstreams。你的代码用GCC 5.1编译。
Stack Overflow用户
发布于 2015-07-10 12:13:27
如果我没有错的话，这个建议来自赫伯·萨特。
显然，问题在于你不能复制构造std::fstream (std::fstream的复制构造函数被标记为delete)，然而，直觉地说，由于右边有一个临时构造函数，所以应该使用移动构造函数而不是复制构造函数。
实际上，以下内容在clang上编译得很好(clang++ -std=c++11，OS )：
#include <iostream>
#include <fstream>
int main() {
auto log = std::fstream{"test.cpp", std::fstream::out};
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/31340345
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