How to throw std::exceptions with variable messages?

The standard exceptions can be constructed from a std::string:

#include <stdexcept>

char const * configfile = "hardcode.cfg";
std::string const anotherfile = get_file();

throw std::runtime_error(std::string("Failed: ") + configfile);
throw std::runtime_error("Error: " + anotherfile);

Note that the base class std::exception can not be constructed thus; you have to use one of the concrete, derived classes.

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