c++ - 如何在C++中将n行从一个文件替换到另一个文件？

Question

I want to replace the last n lines of first.txt with the first n lines of second.txt where n is input by the user. Here's my code:

ofstream myFirst("first.txt", ios:: binary);
ifstream mySecond("second.txt", ios::binary);
int firstFileLine = lineCountFirst;
int count = 0;
while(getline(myFirst, line)){
    count++;
    if(count >= firstFileLine){
        getline(mySecond, line2); // I don't know if it can be written like this
        line = line2;            // here to replace line to line2
    }
}
myFirst.close();
mySecond.close();

The two lines of code with comments are where I am confused about. Any ideas about how to replace the file?

Answer 1

I want to replace the last n lines of first.txt with the first n lines of second.txt where n is input by the user.

Read first.txt in a std::vector<std::string> line by line. Remove the last n elements from the vector. Write the elements of the vector to a temporary file. Read n lines from second.txt and append them to the temporary file. Unlink first.txt and rename the temporary file you created.