saving the changes in file - c++

Question

Ojective: To open config file, look for a particular line which has tag and change its value
Example:
config .txt
bla bla
bla bla
SYSTEM_A_EXTERNAL_IP=192.168.0.57 // need to change ip address
bla bla
bla bla
What I have done so far: My code open the file and look for the particular line which has tag and change its value .
Problem: I failed to see those changes in the actual file. I am aware that ofstream will write into line but if I do ofstream instead of ifstream getline function gets an error.
Code1:
bool SetIpAddr(string & iAddr)
{
bool check= false;
// Open file
ifstream iFile(IpConfigFile.c_str(), ios_base::out|ios::app | ios::binary);
if(iFile.is_open())
{
check= true;
}
// Read content of ipconf file searching for the value of Head Id Name
if(true == check)
{
string TargetName = SystemAIdValue + ExternalIpNamePostfix+ ValueAssignmentCharacter;
string outbuf;
while( !iFile.eof() )
{
getline(iFile, outbuf);
// find the matching string
if( 0 == outbuf.compare(0,TargetName.size(),TargetName) )
{
outbuf.erase(outbuf.begin()+TargetName.size(), outbuf.end());
outbuf.insert(0+TargetName.size(), iAddr);
std::cout<< " after insertion " << outbuf << std::endl;
break;
}
}
}
// Close (input) file
if( iFile.is_open() )
{
iFile.close();
}
return check;
}
Code2: Also doesnt work.
bool SetIpAddr(string & aIpAddr)
{
bool lResult = false;
std::fstream ifile("platform_ip_config");
string lTargetIp = HeadAIdValue + ExternalIpNamePostfix+ ValueAssignmentCharacter;
std::deque<std::string>lines;
std::string inbuf;
while( std::getline(ifile, inbuf));
{
std::cout<<" we are in while"<<std::endl;
std::cout<<" getline =="<< inbuf<< std::endl;
std::cout<<" ip =="<<lTargetIp<< std::endl;
if( 0 == inbuf.compare(0,lTargetIp.size(),lTargetIp) )
{
std::cout<<" we are in matching"<<std::endl;
std::cout<< inbuf << std::endl;
inbuf.erase(inbuf.begin()+lTargetIp.size(), inbuf.end());
std::cout<<" after erase " << inbuf << std::endl;
inbuf.insert(0+lTargetIp.size(), aIpAddr);
std::cout<< " after insertinon " << inbuf << std::endl;
}
lines.push_back(inbuf);
}
ifile.seekg (0, ios::beg );
std::copy(lines.begin(), lines.end(),
std::ostream_iterator<std::string>(ifile, "\n"));
}
lines.push_back(inbuf);
}
ifile.seekg (0, ios::beg );
std::copy(lines.begin(), lines.end(),
std::ostream_iterator<std::string>(ifile, "\n"));
}

Answer 1

What you are doing is inherently tricky, writing to the middle of a file hoping the result will remain in place.
for example
192.168.120.111
uses up a lot more characters than
10.10.10.10
the way i would do this is read the whole files into lines.
std::fstream file(file_name);
std::deque<std::string> lines;
std::string temp;
while(std::getline(file, temp))
{
//detect that this the line you want
if(this_is_my_line(temp))
{
//do what you need to do to the string
modify(temp);
}
lines.push_back(temp);
}
file.seekg (0, ios::beg
//write back to file
std::copy(lines.begin(), lines.end(),
std::ostream_iterator<std::string>(file, "\n"));
Please note you do not need to explicitly close a file in C++, the RAII semantics will close it for you, by doing it your self your increasing the opportunity for bugs to creep into your code.
THIS WORKS ON MY SYSTEM
#include <iostream>
#include <fstream>
#include <string>
#include <deque>
#include <iterator>
int main()
{
std::string line;
const std::string token("SYSTEM_A_EXTERNAL_IP=");
std::deque<std::string> lines;
{
std::ifstream file("config.txt");
while(std::getline(file, line))
{
if(std::equal(token.begin(), token.end(), line.begin()))
{
line=token+"10.10.10.5"; //or whatever
}
lines.push_back(line);
}
}
{
std::ofstream file("config.txt");
std::copy(lines.begin(), lines.end(),
std::ostream_iterator<std::string>(file,"\n"));
}
return 0;
}

Answer 2

By using an ifstream you are opening the file for reading, if you use the fstream base class instead you should be able modify the file
Another approach is to read the entire file into a buffer, make changes to the buffer and overwrite the original file using the ofstream class