Smart Tricks with Parameter Packs and Fold Expressions

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.

Variadic templates ( ... ): accepts an arbitrary number of arguments. Read more here: “Variadic Templates or the Power of Three Dots” and “More about Variadic Templates“.
Perfect forwarding (std::forward): forwards the arguments without changing their value category. Read more here: Perfect Forwarding.
Fold expressions (std::cout << ... << std::forward<Args>(args)): reduces the parameter pack from the left using the binary operator << and the initial value std::cout. Read more here: From Variadic Templates to Fold Expressions.

Finally, here is the output of the program.

Thanks to fold expressions, you can 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";

}

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 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.

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The following screenshot shows the program output.

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.

Johnathan O’Connor called my attention to the fact that the article Nifty Fold Expressions Tricks by Jonathan Müller provides more fold tricks.

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 lay it out properly.

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

The struct Overload can have arbitrarily 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 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.

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 convenient into play. Read more about std::variant, std::visit, and the overload pattern in my next post.

Pdf Bundle: C++20 Modules

Based on the last poll, I’ve created the next pdf bundle.

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

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