How to throw std::exceptions with variable messages?

Question

This is an example of what I often do when I want to add some information to an exception:

std::stringstream errMsg;
errMsg << \"Could not load config          


        

Answer 1

Maybe this?

throw std::runtime_error(
    (std::ostringstream()
        << "Could not load config file '"
        << configfile
        << "'"
    ).str()
);

It creates a temporary ostringstream, calls the << operators as necessary and then you wrap that in round brackets and call the .str() function on the evaluated result (which is an ostringstream) to pass a temporary std::string to the constructor of runtime_error.

Note: the ostringstream and the string are r-value temporaries and so go out of scope after this line ends. Your exception object's constructor MUST take the input string using either copy or (better) move semantics.

Additional: I don't necessarily consider this approach "best practice", but it does work and can be used at a pinch. One of the biggest issues is that this method requires heap allocations and so the operator << can throw. You probably don't want that happening; however, if your get into that state your probably have way more issues to worry about!