How to edit a file in C++

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

std::fstream& GotoLine(std::fstream& file, unsigned int num){
    file.seekg(std::ios::beg);
    for(unsigned int i=0; i < num - 1; ++i){
        file.ignore(std::numeric_limits<std::streamsize>::max(),'\n');
    }
    return file;
}

std::fstream& GotoLineWrite(std::fstream& file, unsigned int num){
    file.seekg(std::ios::beg);
    for(unsigned int i=0; i < num - 1; ++i){
        file.ignore(std::numeric_limits<std::streamsize>::max(),'\n');
    }
    long len=0;
    len=file.tellg();
    file.seekp(len,std::ios::beg);
    return file;
}


int main(){
    using namespace std;
    fstream file("bla.txt",ios::in|ios::out);
    GotoLine(file, 4);
    int line=0;
    file >> line;
    cout<<line<<endl;
    line=line+14;
    GotoLineWrite(file,5);
    file<<line;
    cout<<line;
    return 0;
}