Sum data type: Difference between revisions
C++ entry
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Float f = 3.14159 , address of f = 0x7ffd71122354
Character c = C , address of c = 0x7ffd71122354
</pre>
=={{header|C++}}==
<lang cpp>#include <iostream>
#include <optional>
#include <string>
#include <variant>
#include <vector>
// A variant is a sum type, it can hold exaclty one type at a time
std::variant<int, std::string, bool, int> myVariant{"Ukraine"};
struct Tree
{
// Variants can be used in recusive data types to define structures like
// trees. Here the node of a tree is represented by a variant of either
// an int or a vector of sub-trees.
std::variant<std::vector<Tree>, int> Nodes;
};
Tree tree1; // empty tree
Tree tree2{2}; // a tree with a single value
// a bigger tree
Tree tree3{std::vector{Tree{3}, Tree{std::vector{Tree{2}, Tree{7}}}, Tree{8}}};
// optional is a special case of a sum type between a value and nothing
std::optional<int> maybeInt1; // empty optional
std::optional<int> maybeInt2{2}; // optional containing 2
// In practice pointers are often used as sum types between a valid value and null
int* intPtr1 = nullptr; // a null int pointer
int value = 3;
int* intPtr2 = &value; // a pointer to a valid object
// Print a tree
void PrintTree(const Tree& tree)
{
std::cout << "(";
if(holds_alternative<int>(tree.Nodes))
{
std::cout << get<1>(tree.Nodes);
}
else
{
for(const auto& subtree : get<0>(tree.Nodes)) PrintTree(subtree);
}
std::cout <<")";
}
int main()
{
std::cout << "myVariant: " << get<std::string>(myVariant) << "\n";
PrintTree(tree1); std::cout << "\n";
PrintTree(tree2); std::cout << "\n";
PrintTree(tree3); std::cout << "\n";
std::cout << "*maybeInt2: " << *maybeInt2 << "\n";
std::cout << "intPtr1: " << intPtr1 << "\n";
std::cout << "*intPtr2: " << *intPtr2 << "\n";
}
</lang>
{{out}}
<pre>
()
(2)
((3)((2)(7))(8))
*maybeInt2: 2
intPtr1: 0
*intPtr2: 3
</pre>
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