how to reuse istringstream in cplusplus

I am using c++11.

Codes like below:

std::string line;
int leftnode, rightnode, timestep;

int simmulationtimes; double InitialIDensity;

// deploy configuration file
std::ifstream config(configfile);
if (!config.is_open())
{
       return false;
}
std::getline(config, line);
std::istringstream sline(line);
std::string sInitialIDensity;
while(std::getline(sline, sInitialIDensity, '=')); 
InitialIDensity = std::stod(sInitialIDensity);

std::getline(config, line);
std::string ssimmulationtimes;
sline.str("");  sline.clear();
sline = std::istringstream(line);
while (std::getline(sline, ssimmulationtimes, '='));
simmulationtimes = std::stoi(ssimmulationtimes);

configuration file should be:

IDensity=0.5
times=5

Error:

error: no match for ‘operator=’ (operand types are ‘std::istringstream {aka std::basic_istringstream<char>}’ and ‘std::string {aka std::basic_string<char>}’)
  sline = std::istringstream(line);

Solutions on stackoverflow about reusing stringstream not work for me. Any ideas about reusing istringstream? Thanks for your consideration.