Newer standards of C++ (starting with C++11) let us enhance templates with variable-length lists of types. We can create simpler method declarations with less copy-pasting.
Simple example
We all know how to sum multiple numbers, huh? A possible approach:
The most interesting part is typename... Ts in the template definition. It says the compiler that Ts is a parameter pack (a list of types). In this function, we take the first argument and sum it with that of the rest. The only thing to remember is that operator+ must be defined for all the types we used as an argument.
It’s just templates
You may think: “Hey, it’s just a recursion!”. It’s not. All those methods are generated at compile time and will be different ones in the sense of memory address etc. Also, it’s not possible to modify this variadic template function to receive e.g. vector contents. Vector allocates memory for contents in runtime after all the methods are generated. For parameter packs, the number of elements has to be known during compilation.
Look at the example of moving average (of last three items) below:
The output would be:
Example 2: moving average AVG(4, 16, 78) = 32.6667 AVG(16, 78, 0) = 31.3333 AVG(78, 0, 15) = 31 AVG(0, 15, -300) = -95 AVG(15, -300, 680) = 131.667
This approach won’t work for less than 3 elements, because there is no required template and compiler would not know what method to generate.
Size of
You can easily count contents of a parameter pack using sizeof...(ts). With parameter packs, it just says how many elements are there; you will get no information about memory size, etc.
Output is:
Example 3: Output Message 1 (3 more messages) Message 2 (2 more messages) Message 3 (1 more messages) Message 4
This one was pretty straightforward, I think.
Simple logger
That’s enough to make use of variadic templates. As an example, here’s a simple function working as a logger – it prepends message with current time. The declaration is the same as for printf (I copied it from the reference in the beginning):
Sample output:
[Sun Mar 05 14:27:29 2017] Example 4: Simple logger [Sun Mar 05 14:27:29 2017] Test message 1 [Sun Mar 05 14:27:29 2017] Multiple elements 10, 152.398 [Sun Mar 05 14:27:29 2017] IO Error: Could not load file hello_variadic.txt
Further reading
Variadic templates work nicely with std::tuple and STL containers. There are also new features coming in C++17 like fold expressions. But that’s the topic for another article. You can find more here:
- Variadic Templates are Funadic, great talk by Andrei Alexandrescu
- C++11 variadic templates, for the doubters by Luis Brandy
- Using Variadic Templates cleanly by Florian Weber
