Smart Tricks with Parameter Packs and Fold Expressions

Smart Tricks with Parameter Packs and Fold Expressions

Rainer Grimm Rainer Grimm

Rainer Grimm

Trainer at Modernes C++

Published Sep 20, 2021
+ Follow

This post is a cross-post from www.ModernesCpp.com.

To complete my post about variadic templates and fold expressions, I present in this post smart tricks using parameter packs and fold expressions.

Fold expressions enable it to reduce a parameter pack with a binary operator. Thanks to them, you can write concise expressions for repeated operations. This repeated operation can be a print function or a push_back function to push elements onto a vector. Let me start with the print function.

// printFoldExpressions.cpp

#include <iostream>
#include <string>

template<typename ... Args>
void printMe(Args&& ... args) {
    (std::cout <<  ... <<  std::forward<Args>(args)) << '\n';
}

int main() {

    std::cout << '\n';

    std::cout << std::boolalpha;

    printMe();
    printMe("Rainer ", "Grimm");
    printMe(true, " ", "+", " ",false, " = ", true + false);
    
    std::cout << '\n';

}

The printMe function can accept an arbitrary number of arguments. In the concrete function, this means no argument, two C-strings, and a few strings and numbers. The printMe function automatically deduces their types and displays them. Three powerful C++ techniques are involved.

Finally, here is the output of the program.

No alt text provided for this image

Thanks to fold expressions, you can also push an arbitrary number of arguments onto a vector.

// pushBackFoldExpressions.cpp

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

using namespace std;
 
template<typename T, typename... Args>
void myPushBack(vector<T>& v, Args&&... args) {
    (v.push_back(args), ...);                    // (1)
}

int main() {

    std::cout << '\n';

    std::vector<int> myIntVec;
	myPushBack(myIntVec, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
	for (auto v : myIntVec) std::cout << v << ' ';

    std::cout << "\n\n";

    std::vector myDoubleVec{1.1, 2.2, 3.3};      // (2)
    myPushBack(myDoubleVec, 4.4, 5.5, 6.6);
    for (auto v: myDoubleVec) std::cout << v << ' ';

    std::cout << "\n\n";

}

The lines (1) and (2) are the most interesting ones. (2) pushes the three doubles onto the vector. With C++17, the compiler can automatically deduce the types of the arguments. The expression (v.push_back(args),...) pushes the elements from the right using the binary comma operator (,). Alternatively, I could also push from the left (..., v.push_back(args)), because the comma operator is associative. Honestly, this looks weird. Therefore, I prefer the first variant.

The following screenshot shows the output of the program.

No alt text provided for this image

 Now, I want to go one stack back from fold expressions to variadic templates and present the overload pattern. The overload pattern is a clever way to wrap multiple lambdas into an overload set.

Overload Pattern

I want to make it short. Here is the overload pattern implemented with C++20:

template<typename ... Ts> struct Overload : Ts ... { using Ts::operator() ... ; };

What? Sorry, my mistake. I should layout it properly.

template<typename ... Ts> 
struct Overload : Ts ... { 
    using Ts::operator() ... ; 
};

 The struct Overload can have arbitrary many base classes (Ts ...). It derives from each class public and brings the call operator (Ts::operator...) of each base class into its scope.

There is more to explain about these four magic lines of code. Before I do that in my next post, let me use the overload pattern to display the types of integral literals. The following program requires a C++20 compiler.

// overloadPattern.cpp

#include <iostream>

template<typename ... Ts> 
struct Overload : Ts ... { 
    using Ts::operator() ...;
};


int main() {

    std::cout << '\n';

    auto TypeOfIntegral = Overload {
        [](int) { return "  int"; },
        [](unsigned int) { return " unsigned int"; },
        [](long int) { return " long int"; },
        [](long long int) { return "long long int"; },
        [](auto) { return "unknown type"; },
    };

    std::cout << "TypeOfIntegral(5): " << TypeOfIntegral(5) << '\n';
    std::cout << "TypeOfIntegral(5u): " << TypeOfIntegral(5u) << '\n';
    std::cout << "TypeOfIntegral(5U): " << TypeOfIntegral(5U) << '\n';
    std::cout << "TypeOfIntegral(5l): " << TypeOfIntegral(5l) << '\n';
    std::cout << "TypeOfIntegral(5L): " << TypeOfIntegral(5L) << '\n';
    std::cout << "TypeOfIntegral(5ll): " << TypeOfIntegral(5ll) << '\n';
    std::cout << "TypeOfIntegral(5LL): " << TypeOfIntegral(5LL) << '\n';

    std::cout << '\n';

    std::cout << "TypeOfIntegral(5ul): " << TypeOfIntegral(5ul) << '\n';
    std::cout << "TypeOfIntegral(5.5): " << TypeOfIntegral(5.5) << '\n';

    std::cout << '\n'; 

}

In the program overloadPattern.cpp, the overload set consists of lambda expressions accepting an int, an unsigned int, a long int, a long long int, and auto. auto is the fallback that is used for example if the overload set is invoked with an unknown type. This happens when I invoke TypeOfIntegral with an unsigned long or a double value.

No alt text provided for this image

What's next?

Typically, you use the overload pattern for a std::variant. std::variant is a type-safe union. An instance var of std::variant (C++17) has one value from one of its types. std::visit allows you to apply a visitor to var. Exactly here comes the overload pattern very handy into play. Read more about std::variant, std::visit, and the overload pattern in my next post.

 

Pdf Bundle: C++20 Modules

Base on the last poll, I've created the next pdf bundle.

No alt text provided for this image

 The pdf bundle includes all

  • posts.
  • source code files to these posts.

Here is more info on how to get the pdf bundle: The New pdf Bundle is Ready: C++20 Modules



Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, Marko, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, espkk, Louis St-Amour, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Neil Wang, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Tobi Heideman, Daniel Hufschläger, Red Trip, Alexander Schwarz, Tornike Porchxidze, Alessandro Pezzato, Evangelos Denaxas, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Dimitrov Tsvetomir, Leo Goodstadt, Eduardo Velasquez, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, and Robin Furness.

Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, and Said Mert Turkal.

My special thanks to Embarcadero

Seminars

I'm happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.

Bookable (Online)

German

Standard Seminars (English/German)

Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.

New

Contact Me

Modernes C++


 

 

 

 

 

 

To view or add a comment, sign in

More articles by this author

See all

Others also viewed

Explore topics