Making templates

Overview

Teaching: 10 min
Exercises: 5 min

Questions

• How do you use templates to create generic functions that work with any type?

• What is a template parameter?

Objectives

• Learn about the template keyword and template parameters.

• Create our own template function that works with any type that the << operator is overloaded to support.

In the last episode we created a function to display a std::vector<int> however one of the nice things about std:vector is that it can be used with different data types, however our function can only be used with the int type, can we use templates to create our own function template? Yes we can, so lets do that.

To create a template you use the template keyword followed by an open and close angle bracket <>. Inside those angle brackets we need to specify one or more template parameters. These parameters can be thought of like the macro defines and how they can be used to replace some text by the preprocessor before compilation for example:

#define MAX_SIZE 10
int array[MAX_SIZE]

where MAX_SIZE is replaced by 10 by the preprocessor.

$ cp stl_vector.cpp template.cpp
$ nano template.cpp

#include <iostream>
#include <vector>

template <typename T>
void displayVector(std::vector<T>& vec){
  for(int i=0;i<vec.size();++i){
    std::cout<<vec[i]<<" ";
  }
  std::cout<<"\n";
};

int main(){
  std::vector<int> a;
  a.push_back(0);
  a.push_back(1);
  a.push_back(2);
  displayVector(a);
  
  std::vector<double> b;
  b.push_back(0.1);
  b.push_back(1.1);
  b.push_back(2.1);
  displayVector(b);
}

In our case we are using T as a parameter for a datatype within our template function displayVector. We can replace any location we needed to specify a type with a T. We could have even specified more template parameters if we wanted separating them with commas like template<typename T, typename U>.

In our example we are making use of the fact that the << operator on an output stream knows how to output both an int and a double.

$ g++ template.cpp -o template
$ ./template

0 1 2
0.1 1.1 2.1

Key Points

• Create a new template with the template keyword.

• Template parameters are specified inside <> brackets after the typename keyword.

• Template parameters can be used in place of types and will be replace with actual types as needed at compile time.