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.
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.
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.
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.
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.
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):
[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
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: