Short Introduction to Parameter Packs and Fold Expressions

I’m new to this topic since I literally just learned about the fold expressions today, and parameter packs days before that, so it will be a short post where I’ll explain the basics.

Let’s jump straight to the code:

template<typename... Args>
int sum(Args&&... args)
{
    return (args + ...);
}

• Line 1 tells us there will be zero or more parameters.
• Line 2 declares a function with said variable number of parameters.
• Line 4 is where the magic happens. The fold expression states that for every parameter in the args parameter pack, combine it with the next one using operator+. So it becomes args0 + args1 + ... + argsN.

Another example:

template<typename... Ts>
inline void trace(Ts&&... args)
{
    (cout << ... << args) << endl;
}

Here in line 4, the fold expression takes the form of “(init op … op pack)” as described by syntax #4 here. It expands to cout << args0 << args1 << ... << argsN.

Another way of doing the same is:

template<typename... Ts>
inline void trace(Ts&&... args)
{
    ((cout << args << " "),...) << endl;
}

Here, the cout << args << " " is expanded N times, separated by the comma operator, so it’s less efficient but we get a space between each printed argument.

That’s it for now. 🙂 I’ll post more as I learn more!

Complete Listing

#include <iostream>
using namespace std;
 
template<typename... Args>
int sum(Args&&... args)
{
    return (args + ...);
}
 
template<typename... Ts>
inline void trace(Ts&&... args)
{
    //(cout << ... << args) << endl;
    ((cout << args << " "),...) << endl;
}
 
int main(int argc, char** argv)
{
    trace(sum(1));
    trace(sum(1,2,3,4,5));
    trace(sum(1,1,2,3,5,8,13,21,34,55));
    trace(1, 2, 3);
 
    return 1;
}