Add project StringInterning

Author: bblanchon

.gitignore

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+/tools
+/libraries
+/hardware

README.md

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+# C++ for Arduino - cpp4arduino.com
+
+This repository contains the code samples for the articles on [cpp4arduino.com](https://cpp4arduino.com/).
+

StringInterning/README.md

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+# String interning in action
+
+This program displays the addresses of string so you can see the effect of string interning.
+
+Read the article on [cpp4arduino.com](https://cpp4arduino.com/2018/10/23/what-is-string-interning.html)

StringInterning/StringInterning.ino

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+// String interning in action
+//
+// This program displays the addresses of string so you can see the effect of
+// string interning
+//
+// cpp4arduino.com
+
+const char *a1 = "hello";
+const char *a2 = "hello";
+
+const char b1[] = "hello";
+const char b2[] = "hello";
+
+const char c1[] PROGMEM = "hello";
+const char c2[] PROGMEM = "hello";
+
+String d1 = "hello";
+String d2 = "hello";
+
+void showAddresses(const char *title, const void *s1, const void *s2) {
+  Serial.println(title);
+  Serial.print(" s1 = ");
+  Serial.println((intptr_t)s1);
+  Serial.print(" s2 = ");
+  Serial.println((intptr_t)s2);
+}
+
+void setup() {
+  Serial.begin(9600);
+  while (!Serial)
+    continue;
+
+  showAddresses("const char*", a1, a2);
+  showAddresses("char[]", b1, b2);
+  showAddresses("char[] PROGMEM", c1, c2);
+  showAddresses("F()", F("hello"), F("hello"));
+  showAddresses("String", d1.c_str(), d2.c_str());
+}
+
+void loop() {}

StringInterning/results/ATmega328P.txt

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+const char*
+ s1 = 309
+ s2 = 309
+char[]
+ s1 = 268
+ s2 = 262
+char[] PROGMEM
+ s1 = 122
+ s2 = 116
+F()
+ s1 = 110
+ s2 = 104
+String
+ s1 = 544
+ s2 = 552

StringInterning/results/ESP8266.txt

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+const char*
+ s1 = 1073644664
+ s2 = 1073644664
+char[]
+ s1 = 1073644680
+ s2 = 1073644672
+char[] PROGMEM
+ s1 = 1076029988
+ s2 = 1076029980
+F()
+ s1 = 1076029972
+ s2 = 1076029964
+String
+ s1 = 1073676940
+ s2 = 1073676964