ifstream.open

ifstream.open打开文件，如果文件不存在等情况并没有错误提示

一般定义变量直接打开文件，可以正确读文件，但是错误没有反馈

#include      // std::cerr
#include       // std::ifstream

int main() {
    std::ifstream file;
    file.exceptions(std::ifstream::failbit | std::ifstream::badbit);
    try {
        file.open("test.txt");
        if (file.eof()) {
            std::cout << "eof\n";
        }
        while (!file.eof()) {
            std::cout << "get ..";
            file.get();
        }
        //while (!file.eof()) file.get();
        file.close();
    }
    catch (std::ifstream::failure e) {
        std::cerr << "Exception opening/reading/closing file\n" << e.what();
    }

    return 0;
}
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正确做法在这里：

method readeof() that does the same of read() but doesn’t set failbit on EOF. Also real read failures have been tested, like an interrupted transfer by hard removal of a USB stick or link drop in a network share access. It has been tested on Windows 7 with VS2010 and VS2013 and on linux with gcc 4.8.1. On linux only USB stick removal has been tried.
https://stackoverflow.com/questions/6781545/why-is-failbit-set-when-eof-is-found-on-read

#include 
#include 
#include 

using namespace std;

streamsize readeof(istream& stream, char* buffer, streamsize count)
{
    if (count == 0 || stream.eof())
        return 0;

    streamsize offset = 0;
    streamsize reads;
    do
    {
        // This consistently fails on gcc (linux) 4.8.1 with failbit set on read
        // failure. This apparently never fails on VS2010 and VS2013 (Windows 7)
        reads = stream.rdbuf()->sgetn(buffer + offset, count);

        // This rarely sets failbit on VS2010 and VS2013 (Windows 7) on read
        // failure of the previous sgetn()
        (void)stream.rdstate();

        // On gcc (linux) 4.8.1 and VS2010/VS2013 (Windows 7) this consistently
        // sets eofbit when stream is EOF for the conseguences  of sgetn(). It
        // should also throw if exceptions are set, or return on the contrary,
        // and previous rdstate() restored a failbit on Windows. On Windows most
        // of the times it sets eofbit even on real read failure
        (void)stream.peek();

        if (stream.fail())
            throw runtime_error("Stream I/O error while reading");

        offset += reads;
        count -= reads;
    } while (count != 0 && !stream.eof());

    return offset;
}

#define BIGGER_BUFFER_SIZE 200000000

int main(int argc, char* argv[])
{
    ifstream stream;
    stream.exceptions(ifstream::badbit | ifstream::failbit);
    stream.open("test.txt", ios::binary);

    char* buffer = new char[BIGGER_BUFFER_SIZE];

    streamsize reads = readeof(stream, buffer, BIGGER_BUFFER_SIZE);

    if (stream.eof())
        cout << "eof:reads::" << reads << "\n" << buffer << endl << flush;

    delete buffer;

    return 0;
}
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