Every part You Must Know About std::variant from C++17

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Across the time C++17 was being standardized I noticed magical phrases like “discriminated union”, “type-safe union” or “sum sort” floating round. Later it appeared to imply the identical sort: “variant”.

Let’s see how this model new std::variant from C++17 works and the place it may be helpful.

Replace in early 2023 with notes about C++20, C++23 and different smaller refinements/wording.

One thing higher than `union`

However lest first begin with one thing outdated… unions…

Unions are not often used within the consumer code, and more often than not, they need to be averted.

For instance, there’s a “frequent” trick with floating-point operations:

union SuperFloat {
    float f;
    int i;
};

int RawMantissa(SuperFloat f) { 
    return f.i & ((1 << 23) - 1); 
}
int RawExponent(SuperFloat f) {
    return (f.i >> 23) & 0xFF; 
}

Nevertheless, whereas the above code would possibly work in C99, as a consequence of stricter aliasing guidelines it’s undefined behaviour in C++!

There’s an present Core Guideline Rule on that C.183:

C.183: Don’t use a union for sort punning:

It’s undefined behaviour to learn a union member with a distinct sort from the one with which it was written. Such punning is invisible, or at the least more durable to identify than utilizing a named solid. Kind punning utilizing a union is a supply of errors.

There’s additionally further concern with unions: they’re quite simple and crude. You don’t have a option to know what’s the presently used sort and what’s extra they received’t name destructors of the underlying sorts. Right here’s an instance from cppreference/union that clearly illustrate how exhausting it may be:

#embody <iostream>
#embody <string>
#embody <vector>

union S
{
    std::string str;
    std::vector<int> vec;
    ~S() { } // what to delete right here?
};

int principal()
{
    S s = {"Good day, world"};
    // at this level, studying from s.vec is undefined conduct
    std::cout << "s.str = " << s.str << 'n';

    // it's important to name destructor of the contained objects!
    s.str.~basic_string<char>();

    // and a constructor!
    new (&s.vec) std::vector<int>;

    // now, s.vec is the lively member of the union
    s.vec.push_back(10);
    std::cout << s.vec.dimension() << 'n';

    // one other destructor
    s.vec.~vector<int>();
}

Play with the code @Coliru

As you see, the S union wants numerous upkeep out of your facet. It’s important to know which kind is lively and adequately name destructors/constructors earlier than switching to a brand new variant.

That’s the explanation you in all probability received’t see numerous unions that use “superior” sorts reminiscent of vectors, strings, containers, and so forth, and so forth. Union is usually for primary sorts.

What might make unions higher?

the power to make use of complicated sorts
- and the complete assist of their lifetime: should you swap the kind then a correct destructor is named. That means we don’t leak.
a option to know what’s the lively sort

Earlier than C++17 you could possibly use some third-party library…. or use enhance::variant. However now you might have std::variant.

Primary demo of `std::variant`

Right here’s a primary demo of what you are able to do with this new sort:

#embody <string>
#embody <iostream>
#embody <variant>

struct SampleVisitor
{
    void operator()(int i) const { 
        std::cout << "int: " << i << "n"; 
    }
    void operator()(float f) const { 
        std::cout << "float: " << f << "n"; 
    }
    void operator()(const std::string& s) const { 
        std::cout << "string: " << s << "n"; 
    }
};

int principal()
{
    std::variant<int, float, std::string> intFloatString;
    static_assert(std::variant_size_v<decltype(intFloatString)> == 3);

    // default initialized to the primary various, ought to be 0
    std::go to(SampleVisitor{}, intFloatString);

    // index will present the presently used 'sort'
    std::cout << "index = " << intFloatString.index() << std::endl;
    intFloatString = 100.0f;
    std::cout << "index = " << intFloatString.index() << std::endl;
    intFloatString = "hiya tremendous world";
    std::cout << "index = " << intFloatString.index() << std::endl;

    // strive with get_if:
    if (const auto intPtr (std::get_if<int>(&intFloatString)); intPtr) 
        std::cout << "int!" << *intPtr << "n";
    else if (const auto floatPtr (std::get_if<float>(&intFloatString)); floatPtr) 
        std::cout << "float!" << *floatPtr << "n";

    if (std::holds_alternative<int>(intFloatString))
        std::cout << "the variant holds an int!n";
    else if (std::holds_alternative<float>(intFloatString))
        std::cout << "the variant holds a floatn";
    else if (std::holds_alternative<std::string>(intFloatString))
        std::cout << "the variant holds a stringn";  

    // strive/catch and bad_variant_access
    strive 
    {
        auto f = std::get<float>(intFloatString); 
        std::cout << "float! " << f << "n";
    }
    catch (std::bad_variant_access&) 
    {
        std::cout << "our variant would not maintain float at this second...n";
    }

    // go to:
    std::go to(SampleVisitor{}, intFloatString);
    intFloatString = 10;
    std::go to(SampleVisitor{}, intFloatString);
    intFloatString = 10.0f;
    std::go to(SampleVisitor{}, intFloatString);
}

Play with the code @Coliru

the output:

int: 0
index = 0
index = 1
allocating 18 bytes
index = 2
the variant holds a string
our variant would not maintain float at this second...
string: hiya tremendous world
international op delete known as
int: 10
float: 10

We’ve got a number of issues confirmed within the instance above:

You already know what’s the presently used sort by way of index() or test by way of holds_alternative.
You possibly can entry the worth by utilizing get_if or get (however that may throw bad_variant_access exception)
Kind Security – the variant doesn’t enable to get a worth of the kind that’s not lively
In case you don’t initialize a variant with a worth, then the variant is initialized with the primary sort. In that case the primary various sort should have a default constructor.
No further heap allocation occurs
You need to use a customer to invoke some motion on a presently maintain sort.
The variant class calls destructors and constructors of non-trivial sorts, so within the instance, the string object is cleaned up earlier than we swap to new variants.

When to Use

I’d say that except you’re doing a little low-level stuff, presumably solely with easy sorts, then unions would possibly nonetheless be okay. However for all different makes use of circumstances, the place you want variant sorts, std::variant is a option to go!

Some potential makes use of:

All of the locations the place you would possibly get just a few sorts for a single area: so issues like parsing command strains, ini information, language parsers, and so forth, and so forth.
Expressing effectively a number of potential outcomes of a computation: like discovering roots of equations
Error dealing with – for instance you possibly can return variant<Object, ErrorCode>. If the worth is offered, then you definately return Object in any other case you assign some error code (as of C++23 you should use std::anticipated).
State machines
Polymorphism with out vtables and inheritance (due to visiting sample)

A Practical Background

It’s additionally price mentioning that variant sorts (additionally known as a tagged union, a discriminated union, or a sum sort) comes from the practical language world and Kind Principle.

After a little bit demo and introduction, we will now discuss some extra particulars… so learn on.

The Collection

This text is a part of my collection about C++17 Library Utilities. Right here’s the listing of the opposite subjects that I’ll cowl:

Plus different 110+ articles with the C++17 tag @C++Tales.

Assets about C++17 STL:

`std::variant` Creation

There are a number of methods you possibly can create and initialize std::variant:

// default initialization: (sort has to has a default ctor)
std::variant<int, float> intFloat;
std::cout << intFloat.index() << ", worth " << std::get<int>(intFloat) << "n";

// monostate for default initialization:
class NotSimple
{
public:
    NotSimple(int, float) { }
};

// std::variant<NotSimple, int> cannotInit; // error
std::variant<std::monostate, NotSimple, int> okInit;
std::cout << okInit.index() << "n";

// cross a worth:
std::variant<int, float, std::string> intFloatString { 10.5f };
std::cout << intFloatString.index() << ", worth " << std::get<float>(intFloatString) << "n";

// ambiguity
// double would possibly convert to drift or int, so the compiler can not determine

//std::variant<int, float, std::string> intFloatString { 10.5 };

// ambiguity resolved by in_place
std::variant<lengthy, float, std::string> longFloatString { std::in_place_index<1>, 7.6 }; // double!
std::cout << longFloatString.index() << ", worth " << std::get<float>(longFloatString) << "n";

// in_place for complicated sorts
std::variant<std::vector<int>, std::string> vecStr { std::in_place_index<0>, { 0, 1, 2, 3 }};
std::cout << vecStr.index() << ", vector dimension " << std::get<std::vector<int>>(vecStr).dimension() << "n";

// copy-initialize from different variant:
std::variant<int, float> intFloatSecond { intFloat };
std::cout << intFloatSecond.index() << ", worth " << std::get<int>(intFloatSecond) << "n";

Play with the code right here @Compiler Explorer

The output:

0, worth 0
0
1, worth 10.5
1, worth 7.6
0, vector dimension 4
0, worth 0

Notes:

By default, a variant object is initialized with the primary sort,
- if that’s not potential when the kind doesn’t have a default constructor, then you definately’ll get a compiler error
- you should use std::monostate to cross it as the primary sort in that case
You possibly can initialize it with a worth, after which the perfect matching sort is used
- if there’s an ambiguity, then you should use a model std::in_place_index to explicitly point out what sort ought to be used.
std::in_place additionally lets you create extra complicated sorts and cross extra parameters to the constructor

About `std::monostate`

Within the instance you would possibly discover a particular sort known as std::monostate. It’s simply an empty sort that can be utilized with variants to characterize empty state. The kind may be helpful when the primary various doesn’t have a default constructor. In that state of affairs you possibly can place std::monostate as the primary various.

Altering values

There are 4 methods to alter the present worth of the variant:

the project operator
emplace
get after which assign a brand new worth for the presently lively sort
a customer

The vital half is to know that the whole lot is sort protected and likewise the thing lifetime is honoured.

std::variant<int, float, std::string> intFloatString { "Good day" };
    
intFloatString = 10; // we're now an int
    
intFloatString.emplace<2>(std::string("Good day")); // we're now string once more
    
// std::get returns a reference, so you possibly can change the worth:
std::get<std::string>(intFloatString) += std::string(" World");
    
intFloatString = 10.1f;
if (auto pFloat = std::get_if<float>(&intFloatString); pFloat)
    *pFloat *= 2.0f;

See the dwell instance @Coliru

Output:

Good day
10
Good day
Good day World
20.2

Object Lifetime

If you use union, it is advisable to handle the inner state: name constructors or destructors. That is error susceptible and straightforward to shoot your self within the foot. However std::variant handles object lifetime as you count on. That signifies that if it’s about to alter the presently saved sort then a destructor of the underlying sort is named.

std::variant<std::string, int> v { "Good day A Fairly Lengthy String" };
// v allocates some reminiscence for the string
v = 10; // we name destructor for the string!
// no reminiscence leak

Or see this instance with a customized sort:

class MyType
{
public:
    MyType() { std::cout << "MyType::MyTypen"; }
    ~MyType() { std::cout << "MyType::~MyTypen"; }
};

class OtherType
{
public:
    OtherType() { std::cout << "OtherType::OtherTypen"; }
    OtherType(const OtherType&) { 
        std::cout << "OtherType::OtherType(const OtherType&)n"; 
    }
    ~OtherType() { std::cout << "OtherType::~OtherTypen"; }
};

int principal() 
{
    std::variant<MyType, OtherType> v;
    v = OtherType();

    return 0;
}

This may produce the next output:

MyType::MyType
OtherType::OtherType
MyType::~MyType
OtherType::OtherType(const OtherType&)
OtherType::~OtherType
OtherType::~OtherType

Play with the code @Coliru

Firstly, we initialize with a default worth of sort MyType; then we modify the worth with an occasion of OtherType, and earlier than the project, the destructor of MyType is named. Later we destroy the non permanent object and the thing saved within the variant.

Accessing the Saved Worth

From all the examples, you’ve seen to this point you would possibly get an thought the best way to entry the worth. However let’s make a abstract of this vital operation.

To start with, even when you understand what’s the presently lively sort you can not do:

std::variant<int, float, std::string> intFloatString { "Good day" };
std::string s = intFloatString;

// error: conversion from 
// 'std::variant<int, float, std::string>' 
// to non-scalar sort 'std::string' requested
// std::string s = intFloatString;

So it’s important to use helper features to entry the worth.

You’ve gotten std::get<Kind|Index>(variant) which is a non member operate. It returns a reference to the specified sort if it’s lively (You possibly can cross a Kind or Index). If not then you definately’ll get std::bad_variant_access exception.

std::variant<int, float, std::string> intFloatString;
strive 
{
    auto f = std::get<float>(intFloatString); 
    std::cout << "float! " << f << "n";
}
catch (std::bad_variant_access&) 
{
    std::cout << "our variant would not maintain float at this second...n";
}

The following choice is std::get_if. This operate can also be a non-member and received’t throw. It returns a pointer to the lively sort or nullptr. Whereas std::get wants a reference to the variant, std::get_if takes a pointer. I’m undecided why we now have this inconsistency.

if (const auto intPtr = std::get_if<0>(&intFloatString)) 
    std::cout << "int!" << *intPtr << "n";

Nevertheless, in all probability a very powerful option to entry a worth inside a variant is by utilizing guests.

Guests for `std::variant`

With the introduction of std::variant we additionally bought a helpful STL operate known as std::go to.

It will probably name a given “customer” on all handed variants.

Right here’s the declaration:

template <class Customer, class... Variants>  
constexpr go to(Customer&& vis, Variants&&... vars);

And it’ll name vis on the presently lively sort of variants.

In case you cross just one variant, then it’s important to have overloads for the categories from that variant. In case you give two variants, then it’s important to have overloads for all potential pairs of the categories from the variants.

A customer is “a callable that accepts each potential various from each variant “.

Let’s see some examples:

// a generic lambda:
auto PrintVisitor = [](const auto& t) { std::cout << t << "n"; };

std::variant<int, float, std::string> intFloatString { "Good day" };
std::go to(PrintVisitor, intFloatString);

Within the above instance, a generic lambda is used to generate all potential overloads. Since all the sorts within the variant helps << then we will print them.

Within the one other case we will use a customer to alter the worth:

auto PrintVisitor = [](const auto& t) { std::cout << t << "n"; };
auto TwiceMoreVisitor = [](auto& t) { t*= 2; };

std::variant<int, float> intFloat { 20.4f };
std::go to(PrintVisitor, intFloat);
std::go to(TwiceMoreVisitor, intFloat);
std::go to(PrintVisitor, intFloat);

Generic lambdas can work if our sorts share the identical “interface”, however in many of the circumstances, we’d love to do some totally different actions primarily based on an lively sort.

That’s why we will outline a construction with a number of overloads for the operator ():

struct MultiplyVisitor
{
    float mFactor;

    MultiplyVisitor(float issue) : mFactor(issue) { }

    void operator()(int& i) const {
        i *= static_cast<int>(mFactor);
    }

    void operator()(float& f) const {
        f *= mFactor;
    }

    void operator()(std::string& ) const {
        // nothing to do right here...
    }
};

std::go to(MultiplyVisitor(0.5f), intFloat);
std::go to(PrintVisitor, intFloat);

Within the instance, you would possibly discover that I’ve used a state to carry the specified scaling issue worth.

With lambdas, we bought used to declaring issues simply subsequent to its utilization. And when it is advisable to write a separate construction, it is advisable to exit of that native scope. That’s why it may be helpful to make use of overload building.

Overload

With this utility you possibly can write all a number of lambdas for all matching sorts in a single place:

std::go to
(
  overload
  (
    [](const int& i) { PRINT("int: " + i); },
    [](const std::string& s) { PRINT("it is a string: " + s); },
    [](const float& f) { PRINT("float" + f); }
  ),
  yourVariant;
);

At present this helper just isn’t a part of the library (~~it would get into with C++20~~), however the code would possibly appear like that:

template<class... Ts> struct overload : Ts... { utilizing Ts::operator()...; };
template<class... Ts> overload(Ts...) -> overload<Ts...>;

These two strains appear like a little bit of magic 🙂 However all they do is that they create a struct that inherits all given lambdas and makes use of their Ts::operator(). The entire construction could be now handed to std::go to.

For instance:

std::variant<int, float, std::string> intFloatString { "Good day" };
std::go to(overload{
    [](int& i) { i*= 2; },
    [](float& f) { f*= 2.0f; },
    [](std::string& s) { s = s + s; }
}, intFloatString);
std::go to(PrintVisitor, intFloatString);
// prints: "HelloHello"

Play with the code @Coliru

Right here’s a full weblog publish explaining the characteristic: 2 Traces Of Code and three C++17 Options – The overload Sample – C++ Tales.

And different articles:

Different `std::variant` Operations

Only for the sake of completeness:

You possibly can evaluate two variants of the identical sort:
- in the event that they include the identical lively various then the corresponding comparability operator is named.
- If one variant has an “earlier” various then it’s “much less than” the variant with the subsequent lively various.
Variant is a worth sort, so you possibly can transfer it.
std::hash on a variant can also be potential.

Exception Security Ensures

To date the whole lot seems good and clean… however what occurs when there’s an exception in the course of the creation of the choice in a variant?

For instance

class ThrowingClass
{
public:
    specific ThrowingClass(int i) { if (i == 0) throw int (10); }
    operator int () { throw int(10); }
};

int principal(int argc, char** argv) 
{
    std::variant<int, ThrowingClass> v;

    // change the worth:
    strive
    {
        v = ThrowingClass(0);
    }
    catch (...)
    {
        std::cout << "catch(...)n";
        // we preserve the outdated state!
        std::cout << v.valueless_by_exception() << "n";   
        std::cout << std::get<int>(v) << "n"; 
    }

    // inside emplace
    strive
    {
        v.emplace<0>(ThrowingClass(10)); // calls the operator int
    }
    catch (...)
    {
        std::cout << "catch(...)n";
        // the outdated state was destroyed, so we're not in invalid state!
        std::cout << v.valueless_by_exception() << "n";   
    }

    return 0;
}

Play with the code @Coliru

The output:

catch(...)
0
0
catch(...)
1

Within the first case – with the project operator – the exception is thrown within the constructor of the kind. This occurs earlier than the outdated worth is changed within the variant, so the variant state is unchanged. As you possibly can see we will nonetheless entry int and print it.

Nevertheless, within the second case – emplace – the exception is thrown after the outdated state of the variant is destroyed. Emplace calls operator int to switch the worth, however that throws. After that, the variant is in a fallacious state, as we can not get well.

Additionally observe {that a} variant that’s “worthless by exception” is in an invalid state. Accessing a worth from such variant just isn’t potential. That’s why variant::index returns variant_npos, and std::get and std::go to will throw bad_variant_access.

Efficiency & Reminiscence Issues

std::variant makes use of the reminiscence in an identical option to union: so it is going to take the max dimension of the underlying sorts. However since we’d like one thing that may know what’s the presently lively various, then we have to add some more room.

Plus the whole lot must honour the alignment guidelines.

Listed below are some primary sizes:

std::cout << "sizeof string: " 
          << sizeof(std::string) << "n";

std::cout << "sizeof variant<int, string>: " 
          << sizeof(std::variant<int, std::string>) << "n";

std::cout << "sizeof variant<int, float>: " 
          << sizeof(std::variant<int, float>) << "n";

std::cout << "sizeof variant<int, double>: " 
          << sizeof(std::variant<int, double>) << "n";

On GCC 8.1, 32 bit I’ve:

sizeof string: 32
sizeof variant<int, string>: 40
sizeof variant<int, float>: 8
sizeof variant<int, double>: 16

Play with the code @Coliru

What’s extra fascinating is that std::variant received’t allocate anyextra area! No dynamic allocation occurs to carry variants. and the discriminator.

Whilst you pay some further area for all of the type-safe performance, it shouldn’t value you relating to runtime efficiency.

Migration From `enhance::variant`

Increase Variant was launched across the 12 months 2004, so it was 13 years of expertise earlier than std::variant was added into the Customary. The STL sort takes from the expertise of the enhance model and improves it.

Listed below are the primary adjustments:

Additional reminiscence allocation	Attainable on project, see Design Overview – By no means Empty	No
visiting	apply_visitor	std::go to
get by index	no	sure
recursive variant	sure, see make_recursive_variant	no
duplicated entries	no	sure
empty various	`enhance::clean`	`std::monostate`

You can even see the slides from Variants – Previous, Current, and Future – David Sankel – CppCon 2016 The place there’s extra dialogue concerning the adjustments and the proposal.

Or the video @Youtube

Examples of `std::variant`

After we realized many of the std::variant particulars, we will now discover just a few examples. To date, the code I used was a bit synthetic, however on this part, I attempted to search for some real-life examples.

Error Dealing with

The fundamental thought is to wrap the potential return sort with some ErrorCode, and that means enable to output extra details about the errors. With out utilizing exceptions or output parameters. That is just like what std::anticipated may be in C++23..

enum class ErrorCode
{
    Okay,
    SystemError,
    IoError,
    NetworkError
};

std::variant<std::string, ErrorCode> FetchNameFromNetwork(int i)
{
    if (i == 0)
        return ErrorCode::SystemError;

    if (i == 1)
        return ErrorCode::NetworkError;

    return std::string("Good day World!");
}

int principal() 
{
    auto response = FetchNameFromNetwork(0);
    if (std::holds_alternative<std::string>(response))
        std::cout << std::get<std::string>(response) << "n";
    else
        std::cout << "Error!n";

    response = FetchNameFromNetwork(10);
    if (std::holds_alternative<std::string>(response))
        std::cout << std::get<std::string>(response) << "n";
    else
        std::cout << "Error!n";

    return 0;
}

Play with the instance @Coliru

Within the instance, I’m returning ErrorCode or a sound sort – on this case, a string.

In C++23 you should use std::anticipated. See at std::anticipated – cppreference.com.

Computing Roots of an Equation

Generally the computation would possibly give us a number of choices, for instance, actual roots of the equation. With variant, we will wrap all of the obtainable choices and specific clearly what number of roots can we discover.

utilizing DoublePair = std::pair<double, double>
utilizing EquationRoots = std::variant<DoublePair, double, std::monostate>; 

EquationRoots FindRoots(double a, double b, double c)
{
    if (a == 0.0)
        return std::nan("");

    auto d = b*b-4*a*c; // discriminant
    if (d > 0.0)
    {
        double root1 = (-b + std::sqrt(d)) / (2*a);
        double root2 = (-b - std::sqrt(d)) / (2*a);
        return std::make_pair(root1, root2);
    }
    else if (d == 0.0)
        return (-1*b)/(2*a);
    
    return std::monostate();
}

struct RootPrinterVisitor
{
    void operator()(const DoublePair& arg) 
    { 
        std::cout << "2 roots: " << arg.first << " " << arg.second << 'n'; 
    }
    void operator()(double arg) 
    { 
        if (std::isnan(arg))
            std::cout << "not a quadratic equation!" << arg << 'n'; 
        else
            std::cout << "1 root discovered: " << arg << 'n'; 
    }
    void operator()(std::monostate) 
    { 
        std::cout << "No actual roots discovered.n"; 
    }
};

int principal() 
{
    std::go to(RootPrinterVisitor{}, FindRoots(10,0,-2));
    std::go to(RootPrinterVisitor{}, FindRoots(2,0,-1));
    std::go to(RootPrinterVisitor{}, FindRoots(1,2,1));
    std::go to(RootPrinterVisitor{}, FindRoots(1,2,3));
    std::go to(RootPrinterVisitor{}, FindRoots(0,2,3));
}

Play with the code @Compiler Explorer.

The output:

2 roots discovered: 0.447214 -0.447214
2 roots discovered: 0.707107 -0.707107
1 root discovered: -1
No actual roots discovered.
not a quadratic equation! nan

Parsing a Command Line

Command line would possibly include textual content arguments that may be interpreted in just a few methods:

as integer
as boolean flag
as a string (not parsed)
…

We will construct a variant that may maintain all of the potential choices.

Right here’s a easy model with intand string:

class CmdLine
{
public:
    utilizing Arg = std::variant<int, std::string>;

non-public:
    std::map<std::string, Arg> mParsedArgs;

public:
    specific CmdLine(int argc, char** argv) { ParseArgs(argc, argv); }

    // ...
};

And the parsing code:

CmdLine::Arg TryParseString(char* arg)
{
    // strive with int first
    int iResult = 0;
    auto res = std::from_chars(arg, arg+strlen(arg), iResult);
    if (res.ec == std::errc::invalid_argument)
    {
        // if not potential, then simply assume it is a string
        return std::string(arg);
    }

    return iResult;
}

void CmdLine::ParseArgs(int argc, char** argv)
{
    // the shape: -argName worth -argName worth
    // unnamed? later...
    for (int i = 1; i < argc; i+=2)
    {
        if (argv[i][0] != '-') // tremendous superior sample matching! :)
            throw std::runtime_error("fallacious command identify");

        mParsedArgs[argv[i]+1] = TryParseString(argv[i+1]);
    }
}

For the time being of writing, std::from_chars in GCC solely helps integers, in MSVC floating level assist is on the best way. However the thought of the TryParseString is to strive with parsing the enter string to the perfect matching sort. So if it seems like an integer, then we attempt to fetch integer. In any other case, we’ll return an unparsed string. After all, we will prolong this method.

Instance how we will use it:

strive
{
    CmdLine cmdLine(argc, argv);

    auto arg = cmdLine.Discover("paramInt");
    if (arg && std::holds_alternative<int>(*arg))
        std::cout << "paramInt is " 
                  << std::get<int>(*arg) << "n";

    arg = cmdLine.Discover("textParam");
    if (arg && std::holds_alternative<std::string>(*arg))
        std::cout << "textParam is " 
                  << std::get<std::string>(*arg) << "n";    
}
catch (std::runtime_error &err)
{
    std::cout << err.what() << "n";
}

Play with the code @Coliru

Parsing a Config File

I don’t have a code for that, however the thought comes from the earlier instance of a command line. Within the case of a configuration file, we often work with pairs of <Identify, Worth>. The place Worth may be a distinct sort: string, int, array, bool, float, and so forth.

In my expertise I’ve seen examples the place even void* was used to carry such unknown sort so we might enhance the design by utilizing std::variant if we all know all of the potential sorts, or leverage std::any.

State Machines

How about modelling a state machine? For instance door’s state:

We will use several types of states and the use guests as occasions:

struct DoorState 
{
    struct DoorOpened {};
    struct DoorClosed {};
    struct DoorLocked {};

    utilizing State = std::variant<DoorOpened, DoorClosed, DoorLocked>;

    void open()
    {
        m_state = std::go to(OpenEvent{}, m_state);
    }

    void shut()
    {
        m_state = std::go to(CloseEvent{}, m_state);
    }

    void lock()
    {
        m_state = std::go to(LockEvent{}, m_state);
    }

    void unlock()
    {
        m_state = std::go to(UnlockEvent{}, m_state);
    }

    State m_state;
};

And listed here are the occasions:

struct OpenEvent 
{
    State operator()(const DoorOpened&){ return DoorOpened(); }
    State operator()(const DoorClosed&){ return DoorOpened(); }
    // can not open locked doorways
    State operator()(const DoorLocked&){ return DoorLocked(); } 
};

struct CloseEvent 
{
    State operator()(const DoorOpened&){ return DoorClosed(); }
    State operator()(const DoorClosed&){ return DoorClosed(); }
    State operator()(const DoorLocked&){ return DoorLocked(); }
};

struct LockEvent 
{
    // can not lock opened doorways
    State operator()(const DoorOpened&){ return DoorOpened(); }
    State operator()(const DoorClosed&){ return DoorLocked(); }
    State operator()(const DoorLocked&){ return DoorLocked(); }
};

struct UnlockEvent 
{
    // can not unlock opened doorways
    State operator()(const DoorOpened&){ return DoorOpened(); }
    State operator()(const DoorClosed&){ return DoorClosed(); }
    // unlock
    State operator()(const DoorLocked&){ return DoorClosed(); } 
};

Play with the code utilizing the next instance: @Coliru

The concept is predicated on the weblog posts:

Polymorphism

More often than not in C++ we will safely use runtime polymorphism primarily based on v-table method. You’ve gotten a group of associated sorts – that shares the identical interface, and you’ve got a properly outlined digital technique that may be invoked.

However what in case you have “unrelated” sorts that don’t share the identical base class? What should you’d prefer to rapidly add new performance with out altering the code of the supported sorts?

In such conditions, we now have a helpful sample of Customer. I’ve even described in my older publish.

With std::variant and std::go to we will construct the next instance:

class Triangle
{
public:
    void Render()  { std::cout << "Drawing a triangle!n"; }
};

class Polygon
{
public:
    void Render() { std::cout << "Drawing a polygon!n"; }
};

class Sphere
{
public:
    void Render() { std::cout << "Drawing a sphere!n"; }
};

int principal() 
{
    std::vector<std::variant<Triangle, Polygon, Sphere>> objects { 
        Polygon(), 
        Triangle(),
        Sphere(),
        Triangle()
    };

    auto CallRender = [](auto& obj) { obj.Render(); };

    for (auto& obj : objects)
        std::go to(CallRender, obj);

}

Play with the code: @Coliru

The output:

Drawing a polygon!
Drawing a triangle!
Drawing a sphere!
Drawing a triangle!

Within the above instance, I’ve proven solely the primary case of invoking a technique from unrelated sorts. I wrap all of the potential form sorts right into a single variant after which use a customer to dispatch the decision to the right sort.

In case you’d like, for instance, to type objects, then we will write one other customer, that holds some state. And that means you enable to have extra performance with out altering the categories.

I even have one other article the place I clarify this sort of polymorphism intimately: Runtime Polymorphism with std::variant and std::go to – C++ Tales.

You possibly can discover extra about this sample and its benefits in:

Different Makes use of

There are numerous many extra instance, see this tweet:

do you might have any real-life examples of
std::variant?#cpp
#cpp17

— Bartlomiej Filipek (@fenbf) 24 maja
2018

You possibly can open this tweet and comply with the dialogue.

C++20 & C++23 enhancements

After a few years with C++20 and C++23 std::variant bought just a few updates. For instance due to P2231 applied in opposition to C++20, we will use std::variant in fixed expressions:

#embody <iostream>
#embody <variant>

constexpr std::variant<int, double> foo(int x) {
    std::variant<int, double> oi { x * 0.1 };
    if (x > 10)
        oi = x;
    return oi;
}

int principal() {
   constexpr auto dv = foo(1);
   static_assert(std::holds_alternative<double>(dv));
   constexpr auto iv = foo(100);
   static_assert(std::holds_alternative<int>(iv));
}

See at Compiler Explorer.

Moreover there was additionally an vital repair for circumstances like:

variant<string, bool> x = "abc";             // holds bool !!

Now with A sane std::variant changing constructor you possibly can count on that "abc" string literal will converst into std::string.

And you may take a look at different options like: std::go to() for courses derived from std::variant and std::variant and std::elective ought to propagate copy/transfer triviality.

Wrap Up

After studying this publish, you have to be outfitted with all of the information required to make use of std::variant in your initiatives!

Whereas an identical sort has been obtainable for years – within the type of enhance.variant – I’m comfortable to see the official STL model. That means we will count on increasingly code that makes use of this helpful wrapper sort.

Listed below are the issues to recollect about std::variant:

It holds one among a number of options in a type-safe means
No further reminiscence allocation is required. The variant wants the dimensions of the max of the sizes of the options, plus some little further area for figuring out the presently lively worth.
By default, it initializes with the default worth of the primary various
You possibly can assess the worth by utilizing std::get, std::get_if or by utilizing a type of a customer.
To test the presently lively sort you should use std::holds_alternative or std::variant::index
std::go to is a option to invoke an operation on the presently lively sort within the variant. It’s a callable object with overloads for all of the potential sorts within the variant(s).
Not often std::variant would possibly get into invalid state, you possibly can test it by way of valueless_by_exception

I’d prefer to thank Patrice Roy (@PatriceRoy1), Mandar Kulkarni (@mjkcool) for locating time to do a evaluate of this text!

See additionally another posts about std::variant:

Again to you

Have you ever tried std::variant?
What’s the commonest use case for you?

Tell us within the feedback under.

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One thing higher than union

Primary demo of std::variant

When to Use

A Practical Background

The Collection

std::variant Creation

About std::monostate

Altering values

Object Lifetime

Accessing the Saved Worth

Guests for std::variant

Overload

Different std::variant Operations

Exception Security Ensures

Efficiency & Reminiscence Issues

Migration From enhance::variant

Examples of std::variant

Error Dealing with

Computing Roots of an Equation

Parsing a Command Line

Parsing a Config File

State Machines

Polymorphism

Different Makes use of

C++20 & C++23 enhancements

Wrap Up

Again to you

Leave a Comment

One thing higher than `union`

Primary demo of `std::variant`

`std::variant` Creation

About `std::monostate`

Guests for `std::variant`

Different `std::variant` Operations

Migration From `enhance::variant`

Examples of `std::variant`