As it was indicated previously boost::optional is the actual type returned by the boost::convert() main interface:

boost::optional<TypeOut> boost::convert(TypeIn const&, Converter const&);

The signature is functionally-complete and routinely elided during compilation. Still, the following alternative (and arguably more traditional) interface might be potentially more suitable for certain deployment scenarios (or due to personal preferences):

TypeOut convert(TypeIn const&, Converter const&, TypeOut const& fallback_value);
TypeOut convert(TypeIn const&, Converter const&, Functor const& fallback_func);
TypeOut convert(TypeIn const&, Converter const&, boost::throw_on_failure);

The interface still provides unambiguous behavior and readability, full support for various program flows and various degrees of conversion-failure detection and processing. It can be deployed in a similar fashion as follows:

struct fallback_func
{
    int operator()() const { log("Failed to convert"); return 42; }
};

// Error-processing behavior are specified unambiguously and uniformly.
// a) i1: Returns the provided fallback value;
// b) i2: Calls the provided failure-processing function;
// c) i3: Throws an exception.

int i1 = convert<int>(str, cnv, fallback_value);
int i2 = convert<int>(str, cnv, fallback_func());

try
{
    // Throwing behavior specified explicitly rather than implied.
    int i3 = convert<int>(str, cnv, boost::throw_on_failure);
}
catch (boost::bad_optional_access const&)
{
  // Handle failed conversion.
}

Still, the described interfaces are convenience wrappers around the main interface which provides the described behavior with:

int m1 = convert<int>(str, cnv).value_or(fallback_value);
int m2 = convert<int>(str, cnv).value_or_eval(fallback_func());
int m3 = convert<int>(str, cnv).value();