Default Converter

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## Default Converter

The explicitly provided 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 : public boost::cnv::cstream {};

or potentially configured with additional formatting:

    struct boost::cnv::by_default : public 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 potentially 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.