如何直接读取一大块内存到std :: vector？ [英] How to directly read a huge chunk of memory into std::vector?
问题描述
我有一个巨大的连续数组 x ，我从文件
I have a huge contiguous array x that I fread from a file.
如何将这个块放入 std :: vector<> ？换句话说，我喜欢的结果是在 std :: vector<> 而不是数组，但我想要的结果C ++代码是一样高效的
How do I drop this chunk into a std::vector<>? In other words, I prefer to have the result to be in std::vector<> rather than the array, but I want the resultant C++ code to be as efficient as this plain C-version which drops the chunk right into the array.
通过搜索，我想我可能需要在某种形式中使用placement-new， m不确定呼叫和所有权问题的顺序。
From searching around, I think I may have to use placement-new in some form, but I'm uncertain about the sequence of calls and ownership issues. Also, do I need to worry about alignment issues?
我使用 T = unsigned 进行测试，但我需要担心对齐问题吗？预期一个合理的解决方案适用于任何POD结构。
I am testing for with T = unsigned, but I expect a reasonable solution to work for any POD struct.
using T = unsigned;
FILE* fp = fopen( outfile.c_str(), "r" );
T* x = new T[big_n];
fread( x, sizeof(T), big_n, fp );
// how do I get x into std::vector<T> v
// without calling a gazillion push_backs() or copies ?!?
delete[] x;
fclose( fp );
推荐答案
使用 std： ：vector 构造函数，用于设置向量，并使用 std :: vector :: data 以获取指向已分配内存的指针。
You use the std::vector constructor which sets the size of the vector, and use std::vector::data to get a pointer to allocated memory.
使用 fread ：
std::vector<T> x(big_n);
fread(x.data(), sizeof(T), big_n, fp);
正如其他人所说，使用 fread 如果类型 T 不是 POD类型很可能不工作。然后，您可以使用C ++流和 std :: istreambuf_iterator 将文件读入向量。但是，这有一个缺点，它循环文件中的所有项目，如果 big_n 大到听起来那么这可能是一个性能问题。
As noted by others, using fread if the type T is not a POD type will most likely not work. You can then use C++ streams and std::istreambuf_iterator to read the file into the vector. However this have the drawback that it loops over all items in the file, and if big_n is as big as it sounds then this might be a performance problem.
但是，如果文件真的很大，我建议使用内存映射来读取文件。
However, if the file truly is big, I rather recommend using memory mapping to read the file.
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