Question

Suppose I have a text file with this content: abcdefghk

I want to write a text at position index of 3 with a new text: xyz

In such a way that I will have a new text file: abcxyzghk

How can I achieve this in native C++?

Answer 1

Just use fseek to position and rewrite. If you need to insert, you should use another approach. First, open for appending (“a+t”), set position, and write.

if (FILE* f = fopen("", "a+t"))
{
    const char* line = "xyz";
    const long int offset = 3;
    fseek(f, offset, SEEK_SET);
    fputs(line, f);
    fclose(f);
}

Note this is C++ code, and I put FILE* f under if scope to avoid accidental use f after fclose. Take care about possible I/O exceptions (fseek outside the EOF).