boost::capy::MatchCondition

Concept for callables that detect delimiters in streamed data.

Synopsis

Declared in <boost/capy/concept/match_condition.hpp>

template<class F>
concept MatchCondition = requires(F f, std::string_view data, std::size_t* hint) {
    { f(data, hint) } ‐> std::convertible_to<std::size_t>;
};

Description

A type satisfies MatchCondition if it is callable with std::string_view and a std::size_t* hint parameter, returning the position after a match or npos if not found. Used by read_until to scan accumulated data for delimiters.

Syntactic Requirements

`f(data, hint)` must be a valid expression where: ‐ `data` is `std::string_view` ‐ `hint` is `std::size_t*` (may be null)
The return type must be convertible to `std::size_t`

Semantic Requirements

The callable scans data for a delimiter or pattern:

On match: returns position after the match (bytes to consume)
On no match: returns `std::string_view::npos`

The hint parameter enables efficient cross‐boundary searching:

If `hint` is null, the matcher ignores it
If `hint` is non‐null and no match is found, the matcher may write how many bytes from the end might be part of a partial match (e.g., 3 for "\r\n\r" when searching for "\r\n\r\n")
The hint allows the caller to retain only necessary bytes when the buffer must be compacted

Conforming Signatures

std::size_t operator()( std::string_view data, std::size_t* hint ) const;

Example

    // Simple line matcher (ignores hint)
    auto line_matcher = []( std::string_view data, std::size_t* ) {
        auto pos = data.find( "\r\n" );
        return pos != std::string_view::npos ? pos + 2 : pos;
    };

    // HTTP header end matcher with overlap hint
    struct http_header_matcher
    {
        std::size_t operator()(
            std::string_view data,
            std::size_t* hint ) const noexcept
        {
            auto pos = data.find( "\r\n\r\n" );
            if( pos != std::string_view::npos )
                return pos + 4;
            if( hint )
hint = 3;  // "\r\n\r" might span reads
            return std::string_view::npos;
        }
    };

    static_assert( MatchCondition<http_header_matcher> );

Template Parameters

Name	Description
F	The callable type.