c++ - 在写作之前搜索？

Question

我想用单个文件流读写一个二进制文件。下面的代码尝试读取文件的第一部分，并用它来覆盖文件的第二部分。但我发现我必须使用“seekp(pos [,ios_base::begin]);”在写作之前。此外，“seekp”实际上并没有改变我代码中的位置，但这是必要的!任何人都可以解释一下吗？最好是按照c++标准。非常感谢!

#include <iostream>
#include <fstream>
using namespace std;

int main(){
    fstream flib ("tmp.txt", ios::in | ios::out |ios::binary | ios::trunc);
    if(!flib){
        cerr << "file open failed!" << endl;
        return 1;
    }
    int tmp;

    for(int i = 0; i<2 ; i++){//write 2 numbers
        flib.write((char*)&i, sizeof(tmp));
    }
    flib.seekg(0);
    while(flib.read((char*)&tmp, sizeof(tmp))){//read file contents
        cout <<tmp<<endl; 
    }
    flib.clear();
    flib.seekg(0);
    flib.read((char*)&tmp, sizeof(tmp));
    flib.seekp(sizeof(tmp)); //work
    //flib.seekp(sizeof(tmp), ios_base::beg); //work
    //flib.seekp(0, ios_base::cur); //not work
    //flib.seekp(sizeof(tmp), ios_base::end); //not work
    //flib.seekp(-sizeof(tmp), ios_base::end); //not work
    flib.write((char*)&tmp, sizeof(tmp));
    flib.clear();
    flib.seekg(0);
    while(flib.read((char*)&tmp, sizeof(tmp))){//read file contents
        cout <<tmp<<endl; 
    }

    return 0;
 }

评论:我发现如果我使用 flib.seekp(some_number, ios_base::cur);使用非零 some_number，它可以工作。而且我用的是vs2012 express编译器，是bug吗？

Answer 1

文件流使用 basic_filebuf<>用于流缓冲区。 C++03 标准对 class basic_filebuf<charT,traits> 有这样的说法:

27.8.1.1 Class tempate basic_filebuf

The class basic_filebuf associates both the input sequence and the output sequence with a file.

The restrictions on reading and writing a sequence controlled by an object of class basic_filebuf are the same as for reading and writing with the Standard C library FILEs.

In particular: - If the file is not open for reading the input sequence cannot be read. - If the file is not open for writing the output sequence cannot be written. - A joint file position is maintained for both the input sequence and the output sequence.

不幸的是，在对 FILE 进行读写转换时，它并没有指出这一点。使用标准 C 库的对象，您必须执行文件定位调用(或从写操作转换为读操作时的 fflush())。参见 https://stackoverflow.com/a/14879076/12711 .