When does boost::lexical_cast to std::string fail?

Question

I'm writing unit tests and trying to have all my code covered.

I have in my code something like this:

template<typename ValueType>
std::string ConvertToStringUsingBoost(ValueType const& v)
{
    try 
    {
        return boost::lexical_cast<std::string, ValueType>(v);
    }
    catch(boost::bad_lexical_cast const& e)
    {
        LOG_ERR("Fail to cast %s to string", e.source_type().name);
        return std::string();
    }
}

I was reading these docs and couldn't find any information about when boost::lexical_cast to std::string can throw an exception.

Can you please help me with that?

If it's impossible I'll simply delete this try-catch. If it's possible, I'd prefer to cover this in unit testing.

Answer 1

It can fail for example if a user-defined conversion throws:

enum class MyType {};

std::ostream& operator<<( std::ostream&, MyType const& )
{
    throw "error";
}

int main()
{
    try 
    {
        boost::lexical_cast< std::string >( MyType{} );
    }
    catch(...)
    {
        std::cout << "lexical_cast exception";
    }
}

As you have no control about the type of exceptions thrown by user-defined conversions, catching boost::bad_lexical_cast won't even be enough. Your unit test has to catch all exceptions.

Live Demo

Answer 2

I can't think of any reason for lexical cast to string to throw bad_lexical_cast, except with user-defined types. If the ValueType stream insertion operator can set an error flag on the stream then that's going to result in a bad_lexical_cast. Otherwise, not.

Personally I'd keep the catch in, even if you're just converting built-ins like ints; it doesn't hurt, and may catch bugs if you change the lexical_cast in some manner, or if there's some edge case that neither you nor I has considered; if you're not handling the resulting exception, you'll get an abort at runtime!

If you're concerned about the overhead of an exception, you can use try_lexical_cast instead and check that it returns true rather than catching. However, if the ValueType stream insertion operator can throw then you'd still need to be able to catch that exception anyway.

Answer 3

The only safe and futureproof (eg. no nasty surprises after an update of boost) is to impair your code with something (ugly) like this:

template<typename ValueType>
std::string ConvertToStringUsingBoost(ValueType const& v)
{
    try 
    {

#ifdef UNITTEST
      if (unittest == case_fail) {
        throw boost::bad_lexical_cast();
      }
#endif
        return boost::lexical_cast<std::string, ValueType>(v);
    }
    catch(boost::bad_lexical_cast const& e)
    {
        LOG_ERR("Fail to cast %s to string", e.source_type().name);
        return std::string();
    }
}

Now you should be able to get to that ~100% code coverage !