C++中如何移动文件指针到下一行？写入记录覆盖问题求解

Let's fix your issues step by step—you're running into two main problems: overwriting existing records, and not getting your fields aligned to the 0, 24, 56 positions per line. Here's how to resolve both:

1. Stop Overwriting Existing Records #

Your current code uses f2.seekp(0, ios::beg) which jumps straight to the start of the file every time you write—this is why old records get overwritten. Instead, you need to either:

• Open the file in append mode when you initialize f2, so new writes automatically go to the end of the file:

  fstream f2("your_filename.txt", ios::out | ios::app);
  

• Or, if you need to keep the file open for multiple writes, explicitly seek to the end before each write:

  f2.seekp(0, ios::end);
  

2. Align Fields to Fixed Positions #

To get customerno starting at 0, customername at 24, and product at 56, each field needs to occupy a fixed width of characters. If your string is shorter than the required width, you'll need to pad it with spaces to fill the gap.

Option 1: Use C++ Stream Manipulators (Simpler)

Include the <iomanip> header first, then use setw() to define field widths and left to align text to the left (adjust to right if you want right-aligned fields):

#include <iomanip> // Required for setw and left/right

void filewrite(fstream &f2) {
    // Ensure we're writing to the end of the file
    f2.seekp(0, ios::end);
    
    // Set left alignment so padding spaces go to the right of the text
    f2 << left;
    // customerno takes 24 characters (positions 0-23)
    f2 << setw(24) << customerno;
    // customername takes 32 characters (positions 24-55, since 56-24=32)
    f2 << setw(32) << customername;
    // product starts at 56, write it followed by a newline to end the record
    f2 << product << endl;
}

Option 2: Manual Padding with write() (If You Prefer C-Style Strings)

If you want to stick with the write() method, calculate how many spaces you need to add to each field to reach the required width:

#include <string> // For std::string to create padding spaces

void filewrite(fstream &f2) {
    // Move to the end of the file first
    f2.seekp(0, ios::end);
    
    // Write customerno + padding to 24 characters
    int no_length = strlen(customerno);
    f2.write(customerno, no_length);
    if (no_length < 24) {
        // Create a string of (24 - no_length) spaces and write it
        f2.write(std::string(24 - no_length, ' ').c_str(), 24 - no_length);
    }
    
    // Write customername + padding to 32 characters (to reach position 56)
    int name_length = strlen(customername);
    f2.write(customername, name_length);
    if (name_length < 32) {
        f2.write(std::string(32 - name_length, ' ').c_str(), 32 - name_length);
    }
    
    // Write product and end the line
    f2.write(product, strlen(product));
    f2 << '\n'; // Use '\n' instead of endl if you don't need to flush the buffer immediately
}

Why Your Original Code Failed #

• seekp(0, ios::beg) forced every write to start at the file's beginning, overwriting old data.
• You didn't pad shorter strings with spaces, so customername would start right after the end of customerno instead of at position 24.

内容的提问来源于stack exchange，提问作者Vinita Kumari