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Variant.cpp
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Variant.cpp
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// =====================================================================================
// Variant.cpp // std::variant
// =====================================================================================
module modern_cpp:variant;
namespace VariantDemo {
static void test_01() {
std::variant<int, double, std::string> var{ 123 };
size_t index{ var.index() };
int n{ std::get<int>(var) }; // std::get using type
std::cout << index << ", value: " << n << std::endl;
// -------------------------------------------------
n = std::get<0>(var); // std::get using index
std::cout << index << ", value: " << n << std::endl;
// -------------------------------------------------
var = std::string{ "Hello" }; // we're now a std::string
index = var.index();
std::string s = std::get<std::string>(var);
std::cout << index << ", value: " << s << std::endl;
// -------------------------------------------------
var = 123.456; // we're now a double
index = var.index();
double d = std::get<double>(var);
std::cout << index << ", value: " << d << std::endl;
// -------------------------------------------------
var.emplace<2>("Hello again"); // we're now a std::string again
index = var.index();
s = std::get<std::string>(var);
std::cout << index << ", value: " << s << std::endl;
// -------------------------------------------------
// std::get returns a reference, so you can change the value:
std::get<std::string>(var) += std::string{ " World" };
std::cout
<< var.index()
<< ", value: "
<< std::get<std::string>(var)
<< std::endl;
}
// -------------------------------------------------------------------
static void test_02() {
// accessing a variant
std::variant<int, double, std::string> var{ std::string{ "Hello" } };
std::cout
<< var.index()
<< ", value "
<< std::get<std::string>(var)
<< std::endl;
try
{
auto f = std::get<double>(var);
std::cout << "double! " << f << "\n";
}
catch (std::bad_variant_access&)
{
std::cout << "Variant doesn't hold float at this moment ..." << std::endl;
}
var = 123;
std::cout
<< var.index()
<< ", value "
<< std::get<int>(var)
<< std::endl;
}
// -------------------------------------------------------------------
static void test_03() {
std::variant<int, double, std::string> var{ 123 };
// using a generic visitor (matching all types in the variant)
auto visitor = [](const auto& elem) {
std::cout << elem << std::endl;
};
std::visit(visitor, var);
var = 123.456;
std::visit(visitor, var);
var = std::string{ "Hello" };
std::visit(visitor, var);
}
// -------------------------------------------------------------------
class Visitor
{
public:
void operator() (int n) {
std::cout << "int: " << n << std::endl;
}
void operator() (double f) {
std::cout << "double: " << f << std::endl;
}
void operator() (std::string s) {
std::cout << "std::string: " << s << std::endl;
}
};
static void test_04() {
std::variant<int, double, std::string> var{ 123 };
Visitor visitor{};
std::visit(visitor, var);
var = 123.456;
std::visit(visitor, var);
var = std::string{ "Hello" };
std::visit(visitor, var);
}
// -------------------------------------------------------------------
static void test_05() {
std::vector<std::variant<int, long, long long, float, double>>
vec = { 100, 200l, 300ll, 400.5f, 500.5 };
// display each value
std::cout << "Values: ";
for (const auto& var : vec) {
std::visit([](const auto& n) { std::cout << n << " "; }, var);
}
std::cout << std::endl;
// display each type
std::cout << "Types: ";
for (const auto& var : vec) {
std::visit([](const auto& arg) { std::cout << typeid(arg).name() << " "; }, var);
}
std::cout << std::endl;
// get the sum
std::common_type<int, long, long long, float, double>::type res{};
std::cout << "Type of Sum: " << typeid(res).name() << std::endl;
for (const auto& var : vec) {
std::visit([&res](const auto& arg) { res += arg; }, var);
}
std::cout << "Sum: " << res << std::endl;
// double each value of the vector
std::cout << "Values: ";
for (auto& var : vec) {
std::visit([](auto& arg) { arg *= 2; }, var);
std::visit([](const auto& arg) { std::cout << arg << " "; }, var);
}
std::cout << std::endl;
}
// -------------------------------------------------------------------
template<class... Ts>
struct Overload : Ts... { using Ts::operator()...; };
template<class... Ts> Overload(Ts...) -> Overload<Ts...>;
static void test_06() {
std::variant<int, double, std::string> intFloatString{ "Hello" };
std::visit(Overload{
[](const int& i) { std::cout << "int: " << i << std::endl; },
[](const double& f) { std::cout << "double: " << f << std::endl; },
[](const std::string& s) { std::cout << "string: " << s << std::endl; }
},
intFloatString
);
intFloatString = 123;
std::visit(Overload{
[](const int& i) { std::cout << "int: " << i << std::endl; },
[](const double& f) { std::cout << "double: " << f << std::endl; },
[](const std::string& s) { std::cout << "string: " << s << std::endl; }
},
intFloatString
);
}
// -------------------------------------------------------------------
static void test_07() {
std::variant<int, double, std::string> intFloatString{ "Hello" };
Overload overloadSet {
[](const int& i) { std::cout << "int: " << i << std::endl; },
[](const double& f) { std::cout << "double: " << f << std::endl; },
[](const std::string& s) { std::cout << "string: " << s << std::endl; }
};
std::visit(overloadSet, intFloatString);
intFloatString = 123;
std::visit(overloadSet, intFloatString);
}
}
void main_variant()
{
using namespace VariantDemo;
test_01();
test_02();
test_03();
test_04();
test_05();
test_06();
test_07();
}
// =====================================================================================
// End-of-File
// =====================================================================================