Part 1: Functions as Objects

First, let’s create a simple C++ program that calculates the sum of two numbers. Start up a text editor (e.g. nano) and use it to create a file called sum.cpp. Copy into this file the below code;

#include <iostream>

int sum(int x, int y)
{
    return x + y;
}

int main(int argc, char **argv)
{
    int result = sum(3, 7);
    std::cout << result << std::endl;
    return 0;
}

Compile and run this program using the command;

g++ sum.cpp -o sum
./sum

You should see output the number 10.

Functional programming is based on treating a function in the same way as you would a variable or object. So, in this first program, we have created a function. This is a simple function that just adds together two numbers.

Calling the function via

int result = sum(3, 7);

resulted in the value 10.

In functional programming, a function is treated in exactly the same way as a variable or an object. This means that a function can be assigned to a variable. For example, amend the main function of your sum.cpp file to read;

int main(int argc, char **argv)
{
    auto a = sum;
    auto result = a(3, 7);
    std::cout << result << std::endl;
    return 0;
}

Compile and run your program using;

g++ --std=c++14 sum.cpp -o sum
./sum

Note here that we have told the compiler to use C++ 2014, as this has allowed us to use the C++-14 auto keyword.

This should print 10 again. Here, we have assigned the function sum to the variable a. So how does this work?

For variables, you should be comfortable with the idea that a variable is a container for a piece of data. For example,

auto b = 10;

would create a piece of data (the integer 10) and will place it into the container (the variable b). When we type

auto a = b;

we are copying the data from b and placing it into the variable a. Now both a and b contain the same data.

For functional programming, the code of a function is also treated like a piece of data. The code

int sum(int x, int y)
{
    return x + y;
}

creates a new piece of data (the code to sum together x and y), and places that code into a container (named sum). When we then typed

auto a = sum;

we copied the code data from sum and placed it into the variable a. Now both a and sum contain (or point to) the same data, i.e. the same code that sums together the two arguments (e.g. sum(3,7) and a(3,7) will call the same code, and give the same result).

This means that “code of a function” is a type, in the same way that “integer”, “string” and “floating point number” are types.

You may be interested to know what the C++ type of a function is? It depends on the types of the arguments and type of the return value. In the case of sum, the C++ type is int (*)(int, int). This is painful to type and remember. Fortunately, C++ 2011 introduced the auto keyword, which tells the compiler to automatically work out the type of the data. The auto keyword significantly cuts down on the amount of typing and mistakes, and will be used heavily in this workshop.

Functions as Arguments

As well as assigning functions to variables, you can also pass functions as arguments. Create a new C++ file called funcargs.cpp, and copy in the below code;

#include <iostream>

int sum(int x, int y)
{
    return x + y;
}

template<class FUNC, class ARG1, class ARG2>
auto call_function(FUNC func, ARG1 arg1, ARG2 arg2)
{
    auto result = func(arg1, arg2);
    return result;
}

int main(int argc, char **argv)
{
    auto result = call_function( sum, 3, 7 );
    std::cout << result << std::endl;
    return 0;
}

Compile and run this program by typing;

g++ --std=c++14 funcargs.cpp -o funcargs
./funcargs

This should print out 10. Can you see why?

The function call_function takes three arguments. The first is the function to be called. The second two arguments are the arguments that will be passed to that function. The code in call_function simply calls func using the arguments arg1 and arg2. So far, so useless…

However, let us now create another function, called difference. Please edit you funcargs.cpp file so that it contains the following code

#include <iostream>

int sum(int x, int y)
{
    return x + y;
}

int difference(int x, int y)
{
    return x - y;
}

template<class FUNC, class ARG1, class ARG2>
auto call_function(FUNC func, ARG1 arg1, ARG2 arg2)
{
    auto result = func(arg1, arg2);
    return result;
}

int main(int argc, char **argv)
{
    auto result = call_function( difference, 9, 2 );
    std::cout << result << std::endl;
    return 0;
}

Compile and run using;

g++ --std=c++14 funcargs.cpp -o funcargs
./funcargs

What do you now see? What has happened here?

Now we have passed difference to call_function, and so func(arg1,arg2) has used the code contained in diff, e.g. calculating the difference of the two numbers. The result, 7, should be printed.

You are probably now wondering how has this helped? Well, let us now change call_function. Please edit your funcargs.cpp file so that it contains the following;

#include <iostream>

int sum(int x, int y)
{
    return x + y;
}

int difference(int x, int y)
{
    return x - y;
}

template<class FUNC, class ARG1, class ARG2>
auto call_function(FUNC func, ARG1 arg1, ARG2 arg2)
{
    std::cout << "Calling a function with arguments " << arg1
              << " and " << arg2;

    auto result = func(arg1,arg2);

    std::cout << ". The result is " << result << std::endl; 

    return result;
}

int main(int argc, char **argv)
{
    auto result = call_function( difference, 9, 2 );
    std::cout << result << std::endl;

    result = call_function( sum, 3, 7 );
    std::cout << result << std::endl;

    return 0;
}

Compile and run using

g++ --std=c++14 funcargs.cpp -o funcargs
./funcargs

You should now see printed to the screen;

Calling a function with arguments 9 and 2. The result is 7
7
Calling a function with arguments 3 and 7. The result is 10
10

The new call_function is now doing something useful. It is printing out extra information about our functions, and can do that for any function (which accepts two arguments) that we pass. For example, add in an extra function called multiply, e.g.

int multiply(int x, int y)
{
    return x * y;
}

and add a call to this in the main function, e.g.

int main(int argc, char **argv)
{
    auto result = call_function( difference, 9, 2 );
    std::cout << result << std::endl;

    result = call_function( sum, 3, 7 );
    std::cout << result << std::endl;

    result = call_function( multiply, 4, 5 );
    std::cout << result << std::endl;

    return 0;
}

You should see printed to the screen when you compile and run your updated program.

Calling a function with arguments 4 and 5. The result is 20
20

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