Smart Tricks with Parameter Packs and Fold Expressions
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.
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.
?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.
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.
?
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Base on the last poll, I've created the next pdf bundle.
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?The pdf bundle includes all
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