使用stringstream逐行读取文件两次

Question

I Need to read a file line-by-line twice. The file content is expected to fit into memory. So, I would normally read the whole file into a buffer and work with that buffer afterwards.

However, since I would like to use std::getline , I need to work with a std::basic_istream . So, I thought it would be a good idea to write

std::ifstream file(filepath);
std::stringstream ss;
ss << file.rdbuf();

for (std::string line; std::getline(ss, line);)
{
}

However, I'm not sure what exactly is happening here. I guess ss << file.rdbuf(); does not read the file into any internal buffer of ss . Actual file access should occure only at std::getline(ss, line); .

So, with a second for-loop of the provided form, I should end in reading the whole file once again. That's inefficient.

Am I correct and hence need to come up with an other approach?

Answer 1

I guess ss << file.rdbuf(); does not read the file into any internal buffer of ss . Actual file access should occure only at std::getline(ss, line); .

This is incorrect. cppreference.com has this to say about that operator<< overload:

 basic_ostream& operator<<( std::basic_streambuf<CharT, Traits>* sb); (9) 

9) Behaves as an UnformattedOutputFunction. After constructing and checking the sentry object, checks if sb is a null pointer. If it is, executes setstate(badbit) and exits. Otherwise, extracts characters from the input sequence controlled by sb and inserts them into *this until one of the following conditions are met:

• end-of-file occurs on the input sequence;
• inserting in the output sequence fails (in which case the character to be inserted is not extracted);
• an exception occurs (in which case the exception is caught).

If no characters were inserted, executes setstate(failbit) . If an exception was thrown while extracting, sets failbit and, if failbit is set in exceptions() , rethrows the exception.

So your assumption is incorrect. The entire contents of file is copied to the buffer controlled by ss , so reading from ss does not access the filesystem. You can freely read through ss and seek back to the beginning as many times as you like without incurring the overhead of re-reading the file each time.

Answer 2

After the first loop, clear the EOF and fail bits and go back to the beginning of the stringstream with:

ss.clear();
ss.seekg(0, std::ios::beg);

Answer 3

Am I correct and hence need to come up with an other approach?

You're not correct. The "hense" is unwarranted also. There's not enough info in the question, but I suspect the problem has nothing to do with using a stream buffer.

Without knowing what that first "garbage" character is, I cannot say for sure, but I suspect the file is in a wide-character unicode format, and you are using access operations that do not work on wide characters. If that is the case, buffering the file has nothing to do with the problem.

As an experiment, try the following. Mind the w's.

    std::wifstream file(filepath);
    std::wstringstream ss;
    ss << file.rdbuf();

    for (int i = 0; i < 42; ++i) {
        wchar_t ch;
        ss >> ch;
        std::cout << static_cast<unsigned>(ch) << ' ';
    }

It would not surprise me if the first four numbers are 255 254 92 0, or 255 254 47 0.

This might help: Problem using getline with unicode files