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// Copyright (C) 2014-2019,2021 Internet Systems Consortium, Inc. ("ISC")
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.
#ifndef OPTIONAL_H
#define OPTIONAL_H
#include <exceptions/exceptions.h>
#include <ostream>
#include <string>
namespace isc {
namespace util {
/// @brief A template representing an optional value.
///
/// This template class encapsulates an optional value. The default implementation
/// encapsulates numeric values, but additional specializations are defined
/// as necessary to support other types od data.
///
/// This class includes a boolean flag which indicates if the encapsulated
/// value is specified or unspecified. For example, a configuration parser
/// for the DHCP server may use this class to represent a value of the
/// configuration parameter which may appear in the configuration file, but
/// is not mandatory. The value of the @c Optional may be initialized to
/// "unspecified" initially. When the configuration parser finds that the
/// particular parameter exists in the configuration file, the default value
/// can be overridden and the value may be marked as "specified". If the
/// parameter is not found, the value remains "unspecified" and the appropriate
/// actions may be taken, e.g. the default value may be used.
///
/// @tparam Type of the encapsulated value.
template<typename T>
class Optional {
public:
/// @brief Type of the encapsulated value.
typedef T ValueType;
/// @brief Assigns a new value value and marks it "specified".
///
/// @tparam A Type of the value to be assigned. Typically this is @c T, but
/// may also be a type that can be cast to @c T.
/// @param other new actual value.
template<typename A>
Optional<T>& operator=(A other) {
default_ = other;
unspecified_ = false;
return (*this);
}
/// @brief Type cast operator.
///
/// This operator converts the optional value to the actual value being
/// encapsulated.
///
/// @return Encapsulated value.
operator T() const {
return (default_);
}
/// @brief Equality operator.
///
/// @param other value to be compared.
bool operator==(const T& other) const {
return (default_ == other);
}
/// @brief Inequality operator.
///
/// @param other value to be compared.
bool operator!=(const T& other) const {
return (default_ != other);
}
/// @brief Default constructor.
///
/// Sets the encapsulated value to 0 and marks it as "unspecified".
///
/// The caller must ensure that the constructor of the class @c T
/// creates a valid object when invoked with 0 as an argument.
/// For example, a @c std::string(0) compiles but will crash at
/// runtime as 0 is not a valid pointer for the
/// @c std::string(const char*) constructor. Therefore, the
/// specialization of the @c Optional template for @c std::string
/// is provided below. It uses @c std::string default constructor.
///
/// For any other type used with this template which doesn't come
/// with an appropriate constructor, the caller must create a
/// template specialization similar to the one provided for
/// @c std::string below.
Optional()
: default_(T(0)), unspecified_(true) {
}
/// @brief Constructor
///
/// Sets an explicit value and marks it as "specified".
///
/// @tparam A Type of the value to be assigned. Typically this is @c T, but
/// may also be a type that can be cast to @c T.
/// @param value value to be assigned.
/// @param unspecified initial state. Default is "unspecified".
template<typename A>
Optional(A value, const bool unspecified = false)
: default_(value), unspecified_(unspecified) {
}
/// @brief Retrieves the encapsulated value.
T get() const {
return (default_);
}
/// @brief Modifies the flag that indicates whether the value is specified
/// or unspecified.
///
/// @param unspecified new value of the flag. If it is @c true, the
/// value is marked as unspecified, otherwise it is marked as specified.
void unspecified(bool unspecified) {
unspecified_ = unspecified;
}
/// @brief Checks if the value has been specified or unspecified.
///
/// @return true if the value hasn't been specified, false otherwise.
bool unspecified() const {
return (unspecified_);
}
/// @brief Checks if the encapsulated value is empty.
///
/// This method can be overloaded in the template specializations that
/// are dedicated to strings, vectors etc.
///
/// @throw isc::InvalidOperation.
bool empty() const {
isc_throw(isc::InvalidOperation, "call to empty() not supported");
}
protected:
T default_; ///< Encapsulated value.
bool unspecified_; ///< Flag which indicates if the value is specified.
};
/// @brief Specialization of the default @c Optional constructor for
/// strings.
///
/// It calls default string object constructor.
template<>
inline Optional<std::string>::Optional()
: default_(), unspecified_(true) {
}
/// @brief Specialization of the @c Optional::empty method for strings.
///
/// @return true if the value is empty, false otherwise.
template<>
inline bool Optional<std::string>::empty() const {
return (default_.empty());
}
/// @brief Inserts an optional value to a stream.
///
/// This function overloads the global operator<< to behave as in
/// @c ostream::operator<< but applied to @c Optional objects.
///
/// @param os A @c std::ostream object to which the value is inserted.
/// @param optional_value An @c Optional object to be inserted into
/// a stream.
/// @tparam Type of the value encapsulated by the @c Optional object.
///
/// @return A reference to the stream after insertion.
template<typename T>
std::ostream&
operator<<(std::ostream& os, const Optional<T>& optional_value) {
os << optional_value.get();
return (os);
}
} // end of namespace isc::util
} // end of namespace isc
#endif // OPTIONAL_VALUE_H
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