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##### url::persist

Return a shared, persistent copy of the url.

###### Synopsis

    std::shared_ptr< url_view const >
    persist() const;

###### Description

This function returns a read-only copy of the url, with shared lifetime. The returned value owns (persists) the underlying string. The algorithm used to create the value minimizes the number of individual memory allocations, making it more efficient than when using direct standard library functions.

###### Example

    std::shared_ptr< url_view const > sp;
    {
        std::string s( "http://example.com" );
        url_view u( s );                        // u references characters in s

        assert( u.data() == s.data() );         // same buffer

        sp = u.persist();

        assert( sp->data() != s.data() );       // different buffer
        assert( sp->buffer() == s);             // same contents

        // s is destroyed and thus u
        // becomes invalid, but sp remains valid.
    }

###### Complexity

Linear in `this->size()`.

###### Exception Safety

Calls to allocate may throw.