Create an object/Native demonstration: Difference between revisions
Create an object/Native demonstration (view source)
Revision as of 03:22, 31 January 2021
, 3 years agoAdded C++ implementation
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If the language supports '''Magic Methods''', then show how these work.
=={{header|C++}}==
<lang cpp>#include <iostream>
#include <map>
#include <utility>
using namespace std;
template<typename T>
class FixedMap : private T
{
// Two standard maps are used to implement FixedMap. One as a private
// base class that will allow the values (but not the keys) to be modified.
// Members of a private base class are not exposed to the derived class which will
// prevent keys from being added or deleted. Another map will hold copies of
// the initial values.
T m_defaultValues;
public:
FixedMap(T map)
: T(map), m_defaultValues(move(map)){}
// Expose members of the base class that are not able to add or remove keys so
// that FixedMap will behave like a standard container.
using T::begin;
using T::cbegin;
using T::end;
using T::cend;
using T::at;
using T::empty;
using T::find;
using T::size;
// The [] operator will normally add a new key if the key is not already in the
// map. Instead, throw an error if the key is missing.
auto& operator[](typename T::key_type key)
{
auto iterator = this->find(key);
if(iterator == this->end()) throw out_of_range("key not found");
return iterator->second;
}
// Reset the value of key to its initial value. Throws an error
// if the key is missing.
void reset(typename T::key_type key)
{
T::operator[](key) = m_defaultValues.at(key);
}
};
// Print the contents of a map
auto PrintMap = [](const auto &map)
{
for(auto &[key, value] : map)
{
cout << "{" << key << " : " << value << "} ";
}
cout << "\n";
};
int main(void)
{
// Create a fixed map based on the standard map
FixedMap<map<const char *, int>> fixedMap ({
{"a", 1},
{"b", 2}});
PrintMap(fixedMap);
// Change the values of the keys
fixedMap["a"] = 55;
fixedMap["b"] = 56;
PrintMap(fixedMap);
// Reset a key
fixedMap.reset("a");
PrintMap(fixedMap);
try
{
// Adding or retrieving a missing key is a run time error
fixedMap["newKey"] = 99;
}
catch (exception &ex)
{
cout << "error: " << ex.what();
}
}
</lang>
{{out}}
<pre>{a : 1} {b : 2}
{a : 55} {b : 56}
{a : 1} {b : 56}
error: key not found</pre>
=={{header|D}}==
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