StringKey.h

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/***********************************************************************************\
* (c) Copyright 1998-2025 CERN for the benefit of the LHCb and ATLAS collaborations *
*                                                                                   *
* This software is distributed under the terms of the Apache version 2 licence,     *
* copied verbatim in the file "LICENSE".                                            *
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#pragma once
 
#include <GaudiKernel/Kernel.h>
#include <GaudiKernel/StatusCode.h>
#include <iosfwd>
#include <string>
#include <string_view>
#include <vector>
 
namespace Gaudi {

  struct StringKeyHash {
    using is_transparent = void;
    size_t operator()( std::string_view s ) const { return std::hash<std::string_view>{}( s ); }
  };

  class GAUDI_API StringKey {
  public:
    StringKey() : StringKey( std::string_view{} ) {}
    StringKey( const char* key ) : StringKey( std::string_view{ key } ) {}
    StringKey( std::string_view key ) : m_hash{ StringKeyHash()( key ) }, m_str( key ) {}
    StringKey( std::string key ) : m_hash{ StringKeyHash()( key ) }, m_str( std::move( key ) ) {}

    const std::string& str() const { return m_str; }
    operator const std::string&() const { return m_str; }
    operator std::string_view() const { return m_str; }
    bool empty() const { return m_str.empty(); }
    bool operator!() const { return empty(); }

    bool operator==( const StringKey& ) const = default;
    auto operator<=>( const StringKey& ) const = default;

    bool operator==( const char* rhs ) const { return m_str == rhs; }
    bool operator==( std::string_view rhs ) const { return m_str == rhs; }
    bool operator==( const std::string& rhs ) const { return m_str == rhs; }

    std::size_t __hash__() const { return m_hash; }
    const std::string& __str__() const; // the representation of the object
    std::string __repr__() const; // the representation of the object
    bool __eq__( const StringKey& right ) const;
    bool __eq__( std::string_view right ) const;
    bool __neq__( const StringKey& right ) const;
    bool __neq__( const std::string_view right ) const;

    std::string toString() const; // string representation (for properties)
 
    // interoperability with std::string and const char*
    friend std::string operator+( const std::string& lhs, const Gaudi::StringKey& rhs ) { return lhs + rhs.str(); }
    friend std::string operator+( const char* lhs, const Gaudi::StringKey& rhs ) { return lhs + rhs.str(); }
    friend std::string operator+( const Gaudi::StringKey& lhs, const std::string& rhs ) { return lhs.str() + rhs; }
    friend std::string operator+( const Gaudi::StringKey& lhs, const char* rhs ) { return lhs.str() + rhs; }
 
  private:
    // !!!
    // Do not change the order of these two members. The compiler-generated
    // comparison operators will first compare the hash (fast) and then the full string.
    // !!!

    std::size_t m_hash;

    std::string m_str;
  };

  inline std::size_t hash_value( const Gaudi::StringKey& key ) { return key.__hash__(); }
} // namespace Gaudi
 
// Streaming  value -> string
namespace Gaudi {
  namespace Utils {
    GAUDI_API std::ostream& toStream( const Gaudi::StringKey& key, std::ostream& s );
  } // namespace Utils
  inline std::ostream& operator<<( std::ostream& o, const Gaudi::StringKey& key ) { return o << key.str(); }
} // namespace Gaudi
 
// Parsing : string -> value
namespace Gaudi {
  namespace Parsers {
    GAUDI_API StatusCode parse( Gaudi::StringKey& result, std::string_view input );
    GAUDI_API StatusCode parse( std::vector<Gaudi::StringKey>& result, std::string_view input );
  } // namespace Parsers
} // namespace Gaudi
 
namespace std {
  template <>
  struct hash<Gaudi::StringKey> {
    std::size_t operator()( Gaudi::StringKey const& s ) const { return hash_value( s ); }
  };
} // namespace std