"error! value was " + std::to_string(actualValue) + " but I expected " + std::to_string(expectedValue)

#include <sstream>
std::stringstream ss;
ss << "Hello world!" << std::endl;
throw std::runtime_error(ss.str());

#include <stdexcept>
throw std::runtime_error(
    fmt::format("Error has been detected with code {} while {}",
        0x42, "copying"));