What is a C++ Standard Library Variant Library?

Table of Contents

• Introduction
• Using std::variant
  • Declaring a std::variant
  • Accessing the Value with std::get and std::visit
  • Using std::holds_alternative for Type Checking
• Practical Examples
  • Example 1: Handling Multiple Return Types
  • Example 2: Event Handling with Variants
• Conclusion

Introduction

The C++ Standard Library introduces std::variant in C++17, a type-safe union that can store one of several types at any given time. It provides a way to handle multiple types in a single variable while ensuring type safety at compile time. Unlike std::any, which can hold any type, std::variant is constrained to a predefined list of types. This library is useful when you know in advance the types that will be handled but want to avoid manual type checking.

Using std::variant

Declaring a std::variant

A std::variant is defined by listing the possible types it can hold. At any moment, it holds exactly one of those types, and the currently held type can be queried or manipulated using specific functions like std::get and std::visit.

Example:

In this example, the std::variant can hold an int, float, or std::string, and we use std::get to retrieve the currently held type.

Accessing the Value with std::get and std::visit

std::get is used to retrieve the value stored in a variant by specifying the type or index. If the wrong type is requested, it throws a std::bad_variant_access exception. For more dynamic handling, std::visit is used to apply a function to the current value, making it ideal for handling variant data without manually querying the type.

Example:

In this example, std::visit allows us to handle the different types within the std::variant without needing explicit type checks. The lambda function automatically matches the type of the value stored in the variant.

Using std::holds_alternative for Type Checking

Sometimes, you need to check if a std::variant holds a specific type before accessing it. This can be done using std::holds_alternative, which returns true if the variant currently holds the specified type.

Example:

Here, std::holds_alternative checks if the variant holds a particular type before accessing it using std::get.

Practical Examples

Example 1: Handling Multiple Return Types

Consider a function that can return different types based on conditions. Instead of relying on overloading or multiple return values, std::variant allows for returning multiple types from a single function.

In this example, compute returns either an int or a float based on the input, and std::visit is used to handle the returned value regardless of type.

Example 2: Event Handling with Variants

When managing multiple event types in a program, a std::variant can be used to encapsulate different event structures and process them in a unified way.

Here, std::variant is used to store either a MouseEvent or a KeyEvent, and std::visit ensures that the correct type is processed when an event occurs.

Conclusion

The C++ Standard Library's std::variant provides a type-safe way to manage multiple types in a single variable. Unlike std::any, which can store any type dynamically, std::variant ensures that the types it can store are known at compile time, offering better safety and control. Using tools like std::visit and std::holds_alternative, you can effectively manage and process variant data in a type-safe manner, making it ideal for applications that require handling a small set of alternative types.