std::variant and std::visit: Type-Safe Tagged Unions

Advanced13 min read•Updated: 2026-10-22

🎭 std::variant and std::visit: Type-Safe Tagged Unions

Introduction

C-style union structures are unsafe: they do not call destructors on complex types and lack runtime awareness of which type is currently active. C++17's std::variant is a type-safe tagged union capable of holding one value from a predefined set of types at any time.

🏛️ `std::variant` with `std::visit` Pattern

1#include <iostream>
2#include <variant>
3#include <string>
4 
5// A payment payload can be an Integer (UPI ID), String (Card Mask), or Double (Cash amount)
6using PaymentInfo = std::variant<int, std::string, double>;
7 
8// Visitor struct using overloaded lambdas
9struct PaymentVisitor {
10    void operator()(int upiCode) const {
11        std::cout << "Processing UPI Pin Code: " << upiCode << "\n";
12    }
13    void operator()(const std::string& card) const {
14        std::cout << "Processing Credit Card ending in: " << card << "\n";
15    }
16    void operator()(double cash) const {
17        std::cout << "Processing Cash Payment: ₹" << cash << "\n";
18    }
19};
20 
21int main() {
22    PaymentInfo payment1 = 12345;
23    PaymentInfo payment2 = std::string("**** 9042");
24    PaymentInfo payment3 = 3499.50;
25 
26    // Inspect active type index:
27    std::cout << "payment1 holds type index: " << payment1.index() << "\n";
28 
29    // Visit invokes the matching overloaded operator():
30    std::visit(PaymentVisitor{}, payment1);
31    std::visit(PaymentVisitor{}, payment2);
32    std::visit(PaymentVisitor{}, payment3);
33 
34    // Checking specific type directly:
35    if (std::holds_alternative<double>(payment3)) {
36        std::cout << "Direct Cash extracted: ₹" << std::get<double>(payment3) << "\n";
37    }
38 
39    return 0;
40}

Next Lesson

Structured Bindings (auto [a, b] = pair)

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
std::optional: Safe Representation of Nullable Values	Structured Bindings (auto [a, b] = pair)

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

Previous Next

PreviousPrevNext

MSK Institute • Shikohabad

Loading Page Content...

Preparing courses, live labs, and learning resources.सामग्री लोड हो रही है, कृपया प्रतीक्षा करें...

Connecting serverPlease wait...

💻 Practical Labs🚀 Free Demo📜 Verified Certificates

Structured Bindings (auto [a, b] = pair)

std::variant and std::visit: Type-Safe Tagged Unions

Advanced13 min read•Updated: 2026-10-22

Study Materials

🎭 std::variant and std::visit: Type-Safe Tagged Unions

Introduction

🏛️ `std::variant` with `std::visit` Pattern

1#include <iostream>
2#include <variant>
3#include <string>
4 
5// A payment payload can be an Integer (UPI ID), String (Card Mask), or Double (Cash amount)
6using PaymentInfo = std::variant<int, std::string, double>;
7 
8// Visitor struct using overloaded lambdas
9struct PaymentVisitor {
10    void operator()(int upiCode) const {
11        std::cout << "Processing UPI Pin Code: " << upiCode << "\n";
12    }
13    void operator()(const std::string& card) const {
14        std::cout << "Processing Credit Card ending in: " << card << "\n";
15    }
16    void operator()(double cash) const {
17        std::cout << "Processing Cash Payment: ₹" << cash << "\n";
18    }
19};
20 
21int main() {
22    PaymentInfo payment1 = 12345;
23    PaymentInfo payment2 = std::string("**** 9042");
24    PaymentInfo payment3 = 3499.50;
25 
26    // Inspect active type index:
27    std::cout << "payment1 holds type index: " << payment1.index() << "\n";
28 
29    // Visit invokes the matching overloaded operator():
30    std::visit(PaymentVisitor{}, payment1);
31    std::visit(PaymentVisitor{}, payment2);
32    std::visit(PaymentVisitor{}, payment3);
33 
34    // Checking specific type directly:
35    if (std::holds_alternative<double>(payment3)) {
36        std::cout << "Direct Cash extracted: ₹" << std::get<double>(payment3) << "\n";
37    }
38 
39    return 0;
40}

Next Lesson

Structured Bindings (auto [a, b] = pair)

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
std::optional: Safe Representation of Nullable Values	Structured Bindings (auto [a, b] = pair)

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

Previous Next

🎭 std::variant and std::visit: Type-Safe Tagged Unions

Introduction

🏛️ std::variant with std::visit Pattern

Structured Bindings (auto [a, b] = pair)

Related Lessons

Practice Quiz

Loading Page Content...

🎭 std::variant and std::visit: Type-Safe Tagged Unions

Introduction

🏛️ std::variant with std::visit Pattern

Structured Bindings (auto [a, b] = pair)

Related Lessons

Practice Quiz

🏛️ `std::variant` with `std::visit` Pattern

🏛️ `std::variant` with `std::visit` Pattern