How to Use Function Template Parameter Packs in C++

This updated guide examines How to Use Function Template Parameter Packs in C++ and organizes the essential facts, background, and practical takeaways in clear American English.

Method 1

Using Non-Type Function-Template Parameter Packs

1. Set up themainfunction. No header files are required to construct a variadic template. For demonstration purposes in this article, theiostreamheader file will be included.

   #includeusingnamespacestd;intmain(){}

2. Construct a non-type templated function. The code below shows a function accepting aninttemplate argument.

   #includeusingnamespacestd;templatevoidaddOne(){cout<();// Expected output: 6}

3. Expand the template to a parameter pack. Consider what happens if you want to add more arguments to the function, so that you can call the function with any number of arguments. You could provide default arguments, e.g.

   templatevoidaddOne(){cout<
   
   
   But this is tedious. What's more, this will always output 3 numbers, regardless of the arguments passed. A parameter pack easily resolves this:
   
   
   templatevoidaddOne(){for(inti:{Ns...})cout<
   
   The ellipsis "packs"intvalues intoNs. It is then "unpacked" into an initialiser-list in a for-range loop. The loop will output each value plus one as it iterates.
   
   You can useNsto denote the presence multipleNtemplate parameters.
   
   
   
   

4. Call the function. Pass an arbitrary number ofints to the template and watch as you have successfully constructed and called your first variadic template function.

   intmain(){addOne<5,1,4,-20>();//	Expected Output://	6//	2//	5//	-19return0;}

Method 2

Using Type Function-Template Parameter Packs

1. Set up up themainfunction and construct a simple templated function. Include any necessary header files. The template function used in this example will be a print function, output a variable of any type.

   #includeusingnamespacestd;templatevoidprint(Targ){cout<
   
   Due to type deduction, you don't need to pass any type-arguments to the template.
   
   

2. Expand the template to a parameter pack. Similar to packing non-type template parameters, you use an ellipsis to pack types . The sample code below shows how function arguments can be white-space-delimited along a single line.

   // parameter packtemplatevoidprint(Targ,Ts...args){cout<<" "<
   
   It's impossible to unpackargsinto an initialiser-list unless the values passed intoprintall have the same type. However, you can rely on recursion to handle the job. One argument is deducted on each recursive iteration and printed.
   You can useTsto denote multipleT. Note that theseTscould be of any type.
   You may also choose to unpack the values into anstd::tuple.
   
   

3. Provide a base case. Without the base case, compilers will throw errors about a missingprint()function (with zero arguments). It is important to provide a base case to both terminate the recursion and handle any cleanup (in this case, printing a newline). When the number of arguments inprintreaches 0, the base case will be called. A suitable base case that will terminate the recursion may be:

   // base casevoidprint(){cout<
   

4. Test the function. Ensure that the function works correctly as intended.

   intmain(){print(2019,':',"Happy",14,"th","Birthday","wikiHow!");// Expected Output://	2019: Happy 14 th Birthday wikiHow!}

   • To useprinton STL containers or user-defined classes, you will need to overloadoperator<<.