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;
}