std :: transform和move语义 [英] std::transform and move semantics
问题描述
我使用Boost.Filesystem在目录中创建文件列表。我使用 boost :: filesystem :: recursive_directory_iterator
和 std :: copy
将每个路径放入std :: vector作为 boost :: filesystem :: directory_entry
对象。我希望作为std :: strings输出到文件,所以我做了以下(\\\
,以避免使用<<):
I'm using Boost.Filesystem to create a listing of files in a directory. I use boost::filesystem::recursive_directory_iterator
and std::copy
to put each path into a std::vector as a boost::filesystem::directory_entry
object. I wish to output to the file as std::strings though, so I did the following (\n to avoid the use of <<):
std::vector<boost::filesystem::directory_entry> buffer; //filled with paths
...
std::vector<std::string> buffer_native(buffer.size());
//transform directory_entry into std::string, and add a \n, so output is formatted without use of <<
std::transform(buffer.begin(),buffer.end(),buffer_native.begin(), [](boost::filesystem::directory_entry de)->std::string
{
std::string temp=de.path().string();
temp+="\n";
return temp;
}
buffer.clear();
std::copy(buffer_native.begin(),buffer_native.end(),std::ostream_iterator<std::string>(out_file));
但是,这里的问题是创建两个向量,原来的向量立即被清除,因为它不需要,这听起来像一个完美的地方对于移动语义,但是n3242只提供与C ++ 98中相同的两个重载变换。是否可以使用 std :: transform
实现移动语义?
However the problem with this is it creates two vectors, the original of which is immediately cleared because it is not needed. This sounds like a perfect place for move semantics, but n3242 only provides the same two overloads of transform as in C++98. Is it possible to implement move semantics with std::transform
? If it is not, would writing a custom loop be better?
我在Windows XP上使用GCC 4.5.2(MinGW)。
I'm using GCC 4.5.2 (MinGW) on Windows XP.
推荐答案
这看起来像是 make_move_iterator
的工作:
std::transform(make_move_iterator(buffer.begin()),
make_move_iterator(buffer.end()), buffer_native.begin(),
[](boost::filesystem::directory_entry&& de) -> std::string
{
// still makes copy :/ perhaps native() would work better, I don't know
std::string temp = de.path().string();
temp += "\n";
return temp;
}
移动迭代器只是一个移动其解引用结果的迭代器。注意,类需要支持移动语义,这甚至会产生差异;我不知道Boost FS是否有效。
A move iterator is simply an iterator which moves its dereference result. Note that the class needs to support move semantics for this to even make a difference; I don't know if Boost FS does.
注意,如果你的目标是在不同的行输出它们,重做错了。格式化打印不应要求输入数据采用某种格式,这会违背目的。为数据添加换行符只是为了使其具有换行符是令人讨厌的。它由 ostream_iterator
为您处理:
Note if your goal is to output them on separate lines, you're doing it wrong. Formatted printing shouldn't require the input data be in a certain format, that defeats the purpose. Adding newlines to your data just to format it to have newlines is nasty. It's handled for you by ostream_iterator
anyway:
std::copy(buffer.begin(), buffer.end(), // vvvv
std::ostream_iterator<std::string>(out_file, "\n"));
任何更复杂的,请不要事先修改您的数据。
Anything more complex, make a lambda in for printing; don't modify your data beforehand.
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