**Understanding std::variant: A Modern C++ Polymorphism Replacement**

std::variant is a type-safe union that can hold one of several specified types. Since C++17 it offers a powerful alternative to classical inheritance for representing a value that may be one of a set of alternatives. In this article we will explore why std::variant is useful, how it works, and demonstrate common patterns and pitfalls through code examples.

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1. Why Use std::variant?

Traditional Polymorphism	std::variant
Requires a common base class	No common base needed
Virtual function overhead	Zero runtime cost beyond type tagging
Subclass proliferation	Simple to add or remove types
Potential for slicing	Guarantees correct type on access

When you have a value that can be one of several discrete types, especially when the types are unrelated, std::variant keeps the type system explicit and compile‑time safe. It also eliminates the indirection and dynamic dispatch costs of virtual functions.

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2. Basic Syntax

#include <variant>
#include <string>
#include <iostream>
#include <vector>

using Value = std::variant<int, double, std::string>;

int main() {
    Value v = 42;                 // holds an int
    v = 3.14;                     // holds a double
    v = std::string{"Hello"};     // holds a string

    std::visit([](auto&& arg){
        std::cout << arg << '\n';
    }, v);
}

• Definition: using Value = std::variant<int, double, std::string>;
• Construction: Implicit conversion from any alternative type.
• Access: `std::get (v)` or `std::visit`.

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3. Common Operations

Operation	Function	Example
Check type	`std::holds_alternative
(v)|if (std::holds_alternative(v)) {}`
Get value	`std::get
(v)|int i = std::get(v);`
Visit	std::visit(fn, v)	See example above
Index	v.index()	Returns 0‑based index of current alternative
Swap	std::swap(v1, v2)	Swaps contents safely

Error Handling
`std::get

(v)` throws `std::bad_variant_access` if the stored type is not `T`. Use `std::get_if(&v)` for a null‑terminated pointer when the type may be absent. — ### 4. Example: A JSON‑Like Value “`cpp #include #include #include #include