From 9e2c89ea1405906ccb679424a679bbd19dd92f90 Mon Sep 17 00:00:00 2001
From: Marcus Mohr <marcus.mohr@lmu.de>
Date: Tue, 30 Nov 2021 16:33:08 +0100
Subject: [PATCH] Starts new notebook on functors, lambda functions and
std::function
---
notebooks/08_Functors+Lambdas.ipynb | 338 ++++++++++++++++++++++++++++
1 file changed, 338 insertions(+)
create mode 100644 notebooks/08_Functors+Lambdas.ipynb
diff --git a/notebooks/08_Functors+Lambdas.ipynb b/notebooks/08_Functors+Lambdas.ipynb
new file mode 100644
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@@ -0,0 +1,338 @@
+{
+ "cells": [
+ {
+ "cell_type": "markdown",
+ "id": "789d7a72",
+ "metadata": {},
+ "source": [
+ "# Functors, Lambdas and std::function"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "43efc656",
+ "metadata": {},
+ "source": [
+ " \n",
+ " "
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "008aa425",
+ "metadata": {},
+ "source": [
+ "#### Function Pointers\n",
+ "In a previous example we had implemented a function to compare to objects of type ```myPair```."
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 17,
+ "id": "96f5ab84",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "class myPair;\n",
+ "bool cmp( const myPair& left, const myPair& right );"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "ea6fcef6",
+ "metadata": {},
+ "source": [
+ "We needed that function to be able to create a set of ```myPair```'s.\n",
+ "\n",
+ " std::set< myPair, decltype( cmp )* > pairs ( cmp );\n",
+ " \n",
+ "What we pass to std::set's constructor is a **function pointer** of type\n",
+ "\n",
+ " bool (*) ( const myPair&, const myPair& )"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "dc79e822",
+ "metadata": {},
+ "source": [
+ "***\n",
+ "- This way to work with functions is something that C++ inherited from C.\n",
+ "- Not only in the STL, but also in scientific programming we often have the case that we implement something, but want to leave some details to the user: \n",
+ " - Consider e.g. implementing an algorithm to approximate the integral\n",
+ "$$I = \\int_a^b f(x)\\, dx$$\n",
+ " - When we code that we would like to leave the details of the integrand to be specified flexibly by the user. \n",
+ " - As with std::set we can let the user provide a function that we then call inside our code.\n",
+ " - This is known as a **callback**.\n",
+ "- The main downside to using function (pointers) in this approach is that a free function is **stateless**. In order to influence its behaviour we have to pass arguments through its interface. That does not work nicely with the callback idea.\n",
+ " - Note: We can add *state* to a function by making some of its local variables ```static```. See the ```accumulate``` examples.\n",
+ " - However, a function is a global entity. Thus, there can always only be one state for it."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "9ce08962",
+ "metadata": {},
+ "source": [
+ "#### Functors\n",
+ "In C++ we can as one alternative use a **function object**, a.k.a. **functor**. This is an object of a class that overloads the function call operator ```operator()```."
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 20,
+ "id": "9bf146ec",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "#include <iostream>\n",
+ "\n",
+ "class greeter{\n",
+ "public:\n",
+ " void operator() ( const std::string& str ) {\n",
+ " std::cout << \"Greetings \" << str << std::endl;\n",
+ " }\n",
+ "};"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 21,
+ "id": "b13aa200",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "int main() {\n",
+ " greeter hello;\n",
+ " hello( \"Class\" );\n",
+ "}"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 22,
+ "id": "f7a3701a",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Greetings Class\n"
+ ]
+ }
+ ],
+ "source": [
+ "main();"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "58a187c4",
+ "metadata": {},
+ "source": [
+ "***\n",
+ "Now that was a simple functor and did only demonstrate **how** it works and not **why** that approach can be advantageous.\n",
+ "\n",
+ "**Example 2:** \n",
+ "In our second example we will use a stateful functor that allows to perform different binary operations. The type of operation will be passed via its constructor."
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 23,
+ "id": "8c0f30c5",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "#include <iostream>\n",
+ "\n",
+ "class BinaryOperator {\n",
+ "\n",
+ "public:\n",
+ "\n",
+ " typedef enum { ADD, MULT } type;\n",
+ "\n",
+ " BinaryOperator() = delete;\n",
+ " BinaryOperator( BinaryOperator::type op ) : whatToDo( op ) {};\n",
+ "\n",
+ " int operator() ( int a, int b ) {\n",
+ " switch( whatToDo ) {\n",
+ " case ADD:\n",
+ " return a+b;\n",
+ " case MULT:\n",
+ " return a*b;\n",
+ " }\n",
+ " }\n",
+ "\n",
+ "private:\n",
+ " type whatToDo;\n",
+ "\n",
+ "};"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "6adaab27",
+ "metadata": {},
+ "source": [
+ "**Example 3:** \n",
+ "So how would our set example for myPair look like with a functor?"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 25,
+ "id": "856ee747",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Writing functor.cpp\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%file functor.cpp\n",
+ "\n",
+ "#include <iostream>\n",
+ "#include <set>\n",
+ "\n",
+ "struct myPair {\n",
+ " myPair( int fir, int sec ) : fir_( fir ), sec_( sec ) {\n",
+ " std::cout << \"(\" << fir_ << \",\" << sec_ << \") constructed\" << std::endl;\n",
+ " };\n",
+ " int fir_;\n",
+ " int sec_;\n",
+ "};\n",
+ "\n",
+ "struct myCompare {\n",
+ " bool operator() ( const myPair& left, const myPair& right ) const {\n",
+ " int val1 = left.fir_ * left.fir_ + left.sec_ * left.sec_;\n",
+ " int val2 = right.fir_ * right.fir_ + right.sec_ * right.sec_;\n",
+ " return val1 < val2;\n",
+ " }\n",
+ "};\n",
+ "\n",
+ "int main() {\n",
+ "\n",
+ " myCompare cmp;\n",
+ " std::set< myPair, myCompare > pairs( cmp );\n",
+ "\n",
+ " myPair p1( 1, 2 );\n",
+ " myPair p2( 3, 4 );\n",
+ "\n",
+ " std::cout << \"p1 < p2 is \" << cmp( p1, p2 ) << std::endl;\n",
+ " std::cout << \"p2 < p1 is \" << cmp( p2, p1 ) << std::endl;\n",
+ "\n",
+ " pairs.insert( p1 );\n",
+ " pairs.insert( p2 );\n",
+ "\n",
+ " pairs.emplace( 3, 4 );\n",
+ " pairs.emplace( 5, 6 );\n",
+ "}"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 29,
+ "id": "4adb7f0b",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "!g++ -Wall -Wextra functor.cpp"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 30,
+ "id": "f9bfde7f",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "(1,2) constructed\n",
+ "(3,4) constructed\n",
+ "p1 < p2 is 1\n",
+ "p2 < p1 is 0\n",
+ "(3,4) constructed\n",
+ "(5,6) constructed\n"
+ ]
+ }
+ ],
+ "source": [
+ "!./a.out"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 24,
+ "id": "fe907464",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "5\n",
+ "6\n"
+ ]
+ }
+ ],
+ "source": [
+ "int main() {\n",
+ "\n",
+ " int a = 2;\n",
+ " int b = 3;\n",
+ "\n",
+ " BinaryOperator adder( BinaryOperator::ADD );\n",
+ " BinaryOperator multiplier( BinaryOperator::MULT );\n",
+ "\n",
+ " std::cout << adder( a, b ) << std::endl;\n",
+ " std::cout << multiplier( a, b ) << std::endl;\n",
+ "}\n",
+ "\n",
+ "main();"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "9d2c9715",
+ "metadata": {},
+ "source": [
+ "#### Lambdas\n",
+ "Using *function pointers* or *functors* implies some significant coding overhead.\n",
+ "\n",
+ "C++11 introduced **lambda (functions)**, also known as **closures** as a leight-weight alternative, that can be written inline in the source code."
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": null,
+ "id": "8729d298",
+ "metadata": {},
+ "outputs": [],
+ "source": []
+ }
+ ],
+ "metadata": {
+ "kernelspec": {
+ "display_name": "C++14",
+ "language": "C++14",
+ "name": "xcpp14"
+ },
+ "language_info": {
+ "codemirror_mode": "text/x-c++src",
+ "file_extension": ".cpp",
+ "mimetype": "text/x-c++src",
+ "name": "c++",
+ "version": "14"
+ }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}
--
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