Concatenating two QStrings with an integer

(EDIT: this is an answer to the question before the edit that mentioned QString. For QString, see the newer answer)

This can be done as a very similar one-liner using C++11:

int i = 5;
std::string directory = ":/karim/pic" + std::to_string(i) + ".jpg";

Test: https://ideone.com/jIAxE

With older compilers, it can be substituted with Boost:

int i = 5;
std::string directory = ":/karim/pic" + boost::lexical_cast<std::string>(i) + ".jpg";

Test: https://ideone.com/LFtt7

But the classic way to do it is with a string stream object.

int i = 5;
std::ostringstream oss;
oss << ":/karim/pic" << i << ".jpg";
std::string directory = oss.str();

Test: https://ideone.com/6QVPv