fstream的，寫，然後讀，然後寫，但最後寫入失敗，我不知道原因

Question

fstream fs("f.txt", fstream::in | fstream::out | fstream::trunc); 
if(fs) 
{ 
    string str = "45464748"; 
    fs << str; 

    fs.seekg(0, ios::beg); 

    int i = -1; 
    fs >> i; 
    cout << i << endl; 

    fs.seekp(0, ios::beg); 

    i = 0x41424344; 
    fs << i; 

    fs.close(); 
} 

f.txt內容是「45464748」，但我應該瞭解它的內容是「1094861636 」。我沒有理由，請幫助我。

Answer 1

流狀態的eof位由前一次讀取設置，所以寫入無效。寫之前清除流狀態。

void ftest() 
{ 
    std::fstream fs("f.txt", std::fstream::in | std::fstream::out | std::fstream::trunc); 
    if(fs) 
    { 
    std::cout << "A: " << (fs.eof() ? "eof" : "neof") << std::endl; 
    std::string str = "45464748"; 
    fs << str; 
    std::cout << "B: " << (fs.eof() ? "eof" : "neof") << std::endl; 
    fs.seekg(0, std::ios::beg); 
    std::cout << "C: " << (fs.eof() ? "eof" : "neof") << std::endl; 
    int i = -1; 

    // THIS read sets the EOF bit. 
    fs >> i; 

    std::cout << "D: " << (fs.eof() ? "eof" : "neof") << std::endl; 
    std::cout << i << std::endl; 
    fs.seekp(0, std::ios::beg); 
    std::cout << "E: " << (fs.eof() ? "eof" : "neof") << std::endl; 
    i = 0x41424344; 
    std::cout << "F: " << (fs.eof() ? "eof" : "neof") << std::endl; 
    fs << "not written"; 
    fs.clear(); 
    std::cout << "G: " << (fs.eof() ? "eof" : "neof") << std::endl; 
    fs << i; 
    fs.close(); 
    } 
} 

輸出：

A: neof 
B: neof 
C: neof 
D: eof 
45464748 
E: eof 
F: eof 
G: neof 

文件內容：

1094861636