std::string var = "sometext" + somevar + "sometext" + somevar;
This doesn't work because the additions are performed left-to-right and "sometext" (the first one) is just a const char *. It has no operator+ to call. The simplest fix is this:
std::string var = std::string("sometext") + somevar + "sometext" + somevar;
Now, the first parameter in the left-to-right list of + operations is a std::string, which has an operator+(const char *). That operator produces a string, which makes the rest of the chain work.
You can also make all the operations be on var, which is a std::string and so has all the necessary operators:
var = "sometext";
var += somevar;
var += "sometext";
var += somevar;