std::variant

What problem does this section solve?

Sometimes a variable needs to store a value that could be an int, a string, or a double. The traditional approach is union (in C), but it's not type-safe — you don't know which type is currently stored, and if you access it incorrectly, it crashes.

std::variant is a type-safe union that can store one of multiple types and knows which type it currently holds.

What is this feature?

std::variant<T1, T2, ...> is a type-safe union introduced in C++17. At any given time, it stores a value of only one of its contained types. When you access it, the compiler performs checks to ensure you don't accidentally retrieve a value of the wrong type.

C++ standard version

C++17

Required header files

#include <variant>

Basic Syntax

std::variant<int, double, std::string> v;

v = 42;                      // 存 int
v = 3.14;                    // 存 double
v = std::string("hello");   // 存 string

// 访问方式 1：std::get<T>(v) —— 类型不对抛异常
int n = std::get<int>(v);

// 访问方式 2：std::get_if<T>(&v) —— 类型不对返回 nullptr
if (auto* p = std::get_if<int>(&v)) { ... }

// 访问方式 3：std::visit —— 用 visitor 模式处理所有可能的类型
std::visit([](auto&& val) { ... }, v);

// 查询当前存储的类型的索引
size_t idx = v.index();  // 0-based

Common Usage

Example code

Example 1: variant basic usage—storing different types of values

#include <iostream>
#include <variant>
#include <string>
#include <type_traits>

int main()
{
    // v 可以存 int、double 或 string
    std::variant<int, double, std::string> v;

    v = 42;
    std::cout << "int: " << std::get<int>(v) << "\n";

    v = 3.14;
    std::cout << "double: " << std::get<double>(v) << "\n";

    v = std::string("hello");
    std::cout << "string: " << std::get<std::string>(v) << "\n";

    // 查看当前类型索引
    std::cout << "current index: " << v.index() << "\n";  // 2 (string)

    return 0;
}

Results：

int: 42
double: 3.14
string: hello
current index: 2

Example 2: Building on Example 1, using get_if for safe access

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

void print_value(const std::variant<int, double, std::string>& v)
{
    // 安全方式：逐个尝试，get_if 返回指针
    if (auto* p = std::get_if<int>(&v))
    {
        std::cout << "int: " << *p << "\n";
    }
    else if (auto* p = std::get_if<double>(&v))
    {
        std::cout << "double: " << *p << "\n";
    }
    else if (auto* p = std::get_if<std::string>(&v))
    {
        std::cout << "string: " << *p << "\n";
    }
}

int main()
{
    std::variant<int, double, std::string> v;

    v = 42;
    print_value(v);

    v = 3.14159;
    print_value(v);

    v = std::string("C++17");
    print_value(v);

    return 0;
}

Results：

int: 42
double: 3.14159
string: C++17

Example 3：Building on Example 2, use std::visit to handle all types

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

int main()
{
    std::variant<int, double, std::string> v;

    // std::visit 配合泛型 lambda 优雅处理所有类型
    auto printer = [](const auto& val) {
        std::cout << "value: " << val << "\n";
    };

    v = 42;
    std::visit(printer, v);

    v = 2.718;
    std::visit(printer, v);

    v = std::string("hello variant");
    std::visit(printer, v);

    // 也可以返回不同类型的值
    auto to_double = [](const auto& val) -> double {
        if constexpr (std::is_same_v<std::decay_t<decltype(val)>, std::string>)
        {
            return 0.0;  // string 不能转 double
        }
        else
        {
            return static_cast<double>(val);
        }
    };

    v = 10;
    std::cout << "to_double: " << std::visit(to_double, v) << "\n";

    return 0;
}

Results：

value: 42
value: 2.718
value: hello variant
to_double: 10

Example 4: Building on Example 3, using variant to represent message types

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

// 定义消息类型
struct TextMessage { std::string text; };
struct NumberMessage { int number; };
struct QuitMessage {};

using Message = std::variant<TextMessage, NumberMessage, QuitMessage>;

// 处理消息的 visitor
struct MessageHandler
{
    void operator()(const TextMessage& msg) const
    {
        std::cout << "Text: " << msg.text << "\n";
    }
    void operator()(const NumberMessage& msg) const
    {
        std::cout << "Number: " << msg.number << "\n";
    }
    void operator()(const QuitMessage&) const
    {
        std::cout << "Quit!\n";
    }
};

int main()
{
    Message msg;

    msg = TextMessage{"Hello World"};
    std::visit(MessageHandler{}, msg);

    msg = NumberMessage{42};
    std::visit(MessageHandler{}, msg);

    msg = QuitMessage{};
    std::visit(MessageHandler{}, msg);

    return 0;
}

Results：

Text: Hello World
Number: 42
Quit!

runtime results

See the "running results" for each example above.

Key syntax explanation in the example

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Variant is suitable for "one of a limited number of types."

variant isn't meant to replace all inheritance and polymorphism. It's best suited for scenarios where you have a limited set of type kinds and you want the compiler to remind you to handle all the cases.

Example 5: Using variant to represent a state machine

#include <iostream>
#include <string>
#include <type_traits>
#include <variant>

struct Idle {};
struct Running
{
    int task_id;
};
struct Error
{
    std::string message;
};

// variant 表示一个变量可以在多个候选类型中保存其中一种。
using State = std::variant<Idle, Running, Error>;

void print_state(const State& state)
{
    // visit 会根据 variant 当前保存的类型调用对应处理逻辑。
    std::visit([](const auto& s) {
        using T = std::decay_t<decltype(s)>;

        if constexpr (std::is_same_v<T, Idle>)
        {
            std::cout << "state: idle\n";
        }
        else if constexpr (std::is_same_v<T, Running>)
        {
            std::cout << "state: running task " << s.task_id << "\n";
        }
        else if constexpr (std::is_same_v<T, Error>)
        {
            std::cout << "state: error " << s.message << "\n";
        }
    }, state);
}

int main()
{
    // 程序从 main 函数开始执行，下面的语句会按顺序运行。
    State state = Idle{};
    print_state(state);

    state = Running{42};
    print_state(state);

    state = Error{"motor timeout"};
    print_state(state);

    return 0;
}

Results：

state: idle
state: running task 42
state: error motor timeout

Here, the state can only ever be one of three. Rather than adding extra fields with int state_code, variant can place the data needed for each state within its corresponding type, reducing issues like "reading the running field while in an error state."

Common Errors

Error 1: Incorrect type used with get, causing exceptions to be thrown.

std::variant<int, double> v = 42;
std::cout << std::get<double>(v);  // ❌ 抛出 std::bad_variant_access！

Correct approach: First use std::holds_alternative<double>(v) to check, or use std::get_if.

Error 2: Default construction when variant has no default type

std::variant<int, std::string> v;  // 默认构造第一个类型的默认值（int = 0）

This situation is valid, but if the first type lacks a default constructor, compilation fails.

Error 3: Visitor of visit has not covered all types

struct Visitor {
    void operator()(int) {}
    // 缺少 double 和 string 的 operator()
};
std::variant<int, double, std::string> v;
std::visit(Visitor{}, v);  // ❌ 编译错误！

Correct approach: The visitor for visit must provide operator() for all types in the variant, or use a generic lambda.

使用建议

• 明确目标：在开始前确定您的具体需求，以便选择最合适的工具或教程。
• 充分利用资源：参考官方文档、教程和博客，这些资料能帮助您快速上手并解决问题。
• 实践应用：通过动手操作项目或编写代码来巩固学习成果，提升实际操作能力。
• 问题解决：遇到困难时，查阅参考资料或寻求社区支持，逐步培养独立解决问题的能力。
• 分享经验：完成项目后，可以撰写文章或博客分享心得，帮助其他学习者。

如果需要针对特定领域（如单片机、机器人或环境搭建）的进一步建议，请提供更多信息，我将为您细化内容。

1. 替代 union：type-safe variant, knows what it currently holds.
2. Using std::visit + generic lambda is the most concise way to access.
3. When you need to "know the current type": return a pointer, safely and efficiently.
4. Implementing message/event dispatching with variant + visit: A rudimentary form of pattern matching.
5. The size of a variant is the largest among all types + the index field: Avoid storing many large types.
6. variant is clearer when type kinds are limited: If types need to be extended with plugins, inheritance and virtual functions are usually more appropriate.

Summary

• std::variant<T1, T2, ...> is a type-safe union.
• std::get<T>(v) direct access (unsafe), std::get_if<T>(&v) returns pointer (safe).
• std::visit(visitor, v) is the most recommended approach to force coverage for all types.
• Suitable for scenarios like message distribution, optional configuration, and state machines.