C++ ofstream vs. C++ cout piped to file

I'm writing a set of unit tests that write calculated values out to files. Each test produces 开发者_如何学Pythona 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?

---

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.

---

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...

---

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.