C++ ofstream 与 C++ cout 通过管道传送到文件
我正在编写一组单元测试，将计算值写入文件。每个测试都会生成一个方阵，其中包含 50,000 到 500,000 个双精度数，我总共有 128 个测试用例组合。
编写 cout 语句然后将输出通过管道传输到文件是否涉及任何重大开销，或者我最好使用 ofstream 直接写入文件吗？
I'm writing a set of unit tests that write calculated values out to files. Each test produces a square matrix that holds anywhere from 50,000 to 500,000 doubles, and I have a total of 128 combinations of test cases.
Is there any significant overhead involved in writing cout statements and then piping that output to files, or would I be better off writing directly to the file using an ofstream?
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这将取决于您的系统和环境。这可能差别很小，但只有一种方法可以确定：尝试两种方法并进行测量。
This is going to be dependent on your system and environment. This likely to be very little difference, but there is only one way to be sure: try both approaches and measure them.
由于涉及的维度太大，我假设这些文件不适合人类阅读？只需确保将它们写为二进制而不是人类可读的文本，因为这将比使用 ofstream 或管道 cout 之间的差异大得多。
我不知道这是否意味着您必须使用ofstream。我从来没有写过二进制文件，所以我不能说这是否可能......
Since the dimensions involved are so large I'm assuming that these files are not meant to be read by a human being? Just make sure you write them out as binary and not human-readable text because that will make so much more difference than the difference between using ofstream or piping cout.
Whether this means you have to use ofstream or not I don't know. I've never written binary to cout so I can't say whether that's possible...
正如 Charles Bailey 所说，它取决于实现；接下来的内容主要是针对使用 gnu 工具链的 Linux 实现，但我很难想象它在其他操作系统中会有很大不同。
在 libstdc++ 4.4.2 中：
fstream 包含底层 stdio_filebuf，它是 basic_filebuf。这个 basic_filebuf 通过继承 basic_streambuf 包含它自己的缓冲区，并且实际上包含一个 __basic_file，它本身包含一个底层纯 C stdio 抽象（FILE* 或 std::__c_file*），在其中刷新缓冲区。
cout 是一个 ostream，使用 stdio_sync_filebuf 进行初始化，stdio_sync_filebuf 本身使用 C 文件抽象 stdout 进行初始化。 stdio_sync_filebuf 调用纯 C stdio 函数。
仅考虑 C++，由于有两层缓冲区，fstream 似乎可能更高效。
仅考虑 C，如果使用在文件中重定向的 stdout 文件描述符分叉进程，则写入新打开的文件（fstream 最后执行的操作）或写入 stdout 之间应该没有区别，因为 fd 无论如何都指向文件（cout 最后做了什么）。
如果我是你，我会使用 fstream，因为这是你的意图。
As Charles Bailey said, it's implementation dependent; what follows is mostly for linux implementation with gnu toolchain, but I hardly imagine it being very different in other os.
In libstdc++ 4.4.2:
An fstream contain an underlying stdio_filebuf which is a basic_filebuf. This basic_filebuf contain it's own buffer by inheriting basic_streambuf, and actually contain a __basic_file, itself containing an underlying plain C stdio abstraction (FILE* or std::__c_file*), in which it flush the buffer.
cout, which is an ostream is initialized with a stdio_sync_filebuf itself initialized with the C file abstraction stdout. stdio_sync_filebuf call plain C stdio functions.
Considering only C++, it appear that an fstream may be more efficient thanks to two layers of buffer.
Considering C only, if the process is forked with the stdout file descriptor redirected in a file, there should be no difference between writing to a new opened file (what fstream does at the end) or to stdout since the fd point to a file anyway (what cout does at the end).
If I were you, I would use an fstream since it's your intent.