The explicit converter as in

int i = boost::convert<int>("123", converter).value();

provides considerable flexibility, configurability and efficiency. However, in certain contexts that might be not that important or even counter-productive if, for example, an application relies on certain consistent behavior associated with one particular converter type and configuration. To accommodate such a scenario Boost.Convert introduces the concept of the default converter implemented as boost::cnv::by_default.

	Important
	There is no default converter set by default.

Consequently, without additional configuration steps the following call will fail to compile:

int i = boost::convert<int>("123").value(); // No converter provided

However, after boost::cnv::by_default is defined simply as:

struct boost::cnv::by_default : boost::cnv::cstream {};

or potentially configured with additional formatting:

struct boost::cnv::by_default : boost::cnv::cstream
{
    by_default() { (*this)(std::uppercase)(std::hex); }
};

the code compiles and deploys boost::cnv::cstream when boost::convert() is called without an explicitly supplied converter:

// No explicit converter provided. boost::cnv::by_default is used.
int         i = boost::convert<int>("F").value_or(-1);
std::string s = boost::convert<std::string>(255).value_or("bad");

// 'i' and 's' are converted using boost::cnv::cstream
// with std::uppercase and std::hex formatting applied.

BOOST_TEST(i == 15);   // 15(10) = F(16)
BOOST_TEST(s == "FF"); // 255(10) = FF(16)

The trade-off for the convenience is the rigid converter configuration (which in certain contexts might be the desired behavior) and a potential performance impact. When a converter is not provided explicitly, the default converter is created, potentially configured, deployed and destroyed for every boost::convert() call. Consequently, if efficiency of this particular component is important, then the implementation of boost::cnv::by_default will need to take that into account and to make sure those operations are cheap.