Good day guys, this is my first time posting, so if there is anything I should know about this particular forum, please do let me know!

I have tried searching on the internet, but do you guys know what does this line do?

istringstream is(Expr);

if(Expr.at(i)=='(') p++;

return Value_Of_Expr(Expr.substr(0,i))+Value_Of_Expr(Expr .substr(i+1,Expr.length()-i-1));

These three lines have something I don't understand, such as :

istringstream is(XX) - What does it do?

if(Exr.at(i)=='(') p++;

return Value_Of_Expr(Expr.substr(0,i))+Value_Of_Expr(Expr.substr(i+1,Expr.length()-i-1));