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Add project EfficientString
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‎EfficientString/EfficientString.ino‎

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#include "Polyfills.h" // adds move()
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#include "WString.h" // adds the instrumented String class
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#define RESOURCE "outdoor_temp"
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#define API_KEY "0123456789"
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// We must work in the namespace cpp4arduino to use the instrumented String
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namespace cpp4arduino {
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// A function that takes a C-style string.
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// Calling this function doesn't makes a copy
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void passByPointer(const char *) {
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// (empty)
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}
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// A function that takes a string by value.
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// Calling this function makes a copy of the String
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void passByValue(String) {
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// (empty)
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}
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// A function that takes a string by referece.
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// Calling this function doesn't makes a copy
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void passByConstReference(const String &) {
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// (empty)
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}
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void example1() {
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Serial.println("#1: Initialize from RAM");
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String s = "example";
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}
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void example2() {
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Serial.println("#2: Initialize from Flash");
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String s = F("example");
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}
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void example3() {
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Serial.println(F("#3: toCharArray()"));
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String s = "example";
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char tmp[32];
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s.toCharArray(tmp, sizeof(tmp));
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passByPointer(tmp);
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}
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void example4() {
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Serial.println(F("#4: c_str()"));
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String s = "example";
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passByPointer(s.c_str());
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}
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void example5() {
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Serial.println(F("#5: Pass by value"));
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String s = "example";
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passByValue(s);
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}
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void example6() {
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Serial.println(F("#6: Pass by const reference"));
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String s = "example";
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passByConstReference(s);
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}
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void example7() {
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Serial.println(F("#7: Move instance"));
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String s = "example";
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passByValue(move(s));
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}
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void example8() {
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Serial.println(F("#8: Append temporaries"));
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// Construct "/api/outdoor_temp?key=0123456789"
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String path = String("/api/") + RESOURCE + "?key=" + API_KEY;
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passByConstReference(path);
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}
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void example9() {
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Serial.println(F("#9: Mutate instance"));
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// Construct "/api/outdoor_temp?key=0123456789"
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String path("/api/");
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path += RESOURCE;
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path += "?key=";
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path += API_KEY;
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passByConstReference(path);
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}
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void example10() {
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Serial.println(F("#10: Call reserve"));
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// Construct "/api/outdoor_temp?key=0123456789"
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String path;
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path.reserve(64);
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path += "/api/";
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path += RESOURCE;
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path += "?key=";
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path += API_KEY;
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passByConstReference(path);
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}
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void example11() {
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Serial.println(F("#11: Pass a null string to the constructor"));
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// Construct "/api/outdoor_temp?key=0123456789"
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String path((char *)0);
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path.reserve(64);
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path += "/api/";
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path += RESOURCE;
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path += "?key=";
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path += API_KEY;
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passByConstReference(path);
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}
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void example12() {
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Serial.println(F("#12: Concat at compile time"));
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// Construct "/api/outdoor_temp?key=0123456789"
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String path = "/api/" RESOURCE "?key=" API_KEY;
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passByConstReference(path);
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}
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} // namespace cpp4arduino
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void setup() {
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Serial.begin(9600);
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while (!Serial)
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continue;
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cpp4arduino::example1();
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Serial.println();
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cpp4arduino::example2();
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Serial.println();
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cpp4arduino::example3();
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Serial.println();
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cpp4arduino::example4();
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Serial.println();
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cpp4arduino::example5();
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Serial.println();
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cpp4arduino::example6();
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Serial.println();
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cpp4arduino::example7();
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Serial.println();
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cpp4arduino::example8();
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Serial.println();
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cpp4arduino::example9();
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Serial.println();
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cpp4arduino::example10();
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Serial.println();
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cpp4arduino::example11();
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Serial.println();
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cpp4arduino::example12();
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}
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void loop() {}

‎EfficientString/LoggingProxies.h‎

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// Wrappers on top of the standard functions to log what happens.
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#include <Arduino.h>
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namespace cpp4arduino {
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template <typename... Args>
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inline void log(const __FlashStringHelper *fmt, Args... args) {
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char buffer[64];
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sprintf_P(buffer, (PGM_P)fmt, args...);
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Serial.println(buffer);
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}
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inline void *malloc(size_t n) {
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void *result = ::malloc(n);
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log(F("- malloc(%u) -> %p"), n, result);
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return result;
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}
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inline void *realloc(void *p, size_t n) {
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void *result = ::realloc(p, n);
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if (p == 0)
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log(F("- malloc(%u) -> %p"), n, result);
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else if (n == 0)
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log(F("- free(%p)"), p);
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else if (n && p)
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log(F("- realloc(%p, %u) -> %p"), p, n, result);
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return result;
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}
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inline void free(void *p) {
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::free(p);
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log(F("- free(%p)"), p);
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}
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inline void memcpy(char *dst, const char *src, size_t n) {
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::memcpy(dst, src, n);
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log(F("- memcpy(%p, %p, %u)"), dst, src, n);
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}
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inline void memmove(char *dst, const char *src, size_t n) {
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::memmove(dst, src, n);
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log(F("- memmove(%p, %p, %u)"), dst, src, n);
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}
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inline void strcpy(char *dst, const char *src) {
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::strcpy(dst, src);
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log(F("- strcpy(%p, %p) - %u bytes"), dst, src, ::strlen(src));
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}
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inline void strncpy(char *dst, const char *src, size_t n) {
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::strncpy(dst, src, n);
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log(F("- strncpy(%p, %p, %u) - %u bytes"), dst, src, n,
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min(::strlen(src), n));
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}
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inline void strcpy_P(char *dst, const char *src) {
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::strcpy_P(dst, src);
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log(F("- strcpy_P(%p, %p) - %u bytes"), dst, src, ::strlen_P(src));
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}
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} // namespace cpp4arduino

‎EfficientString/Polyfills.h‎

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// Standard C++ functions missing in Arduino
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// cpp4arduino.com
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#pragma once
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namespace cpp4arduino {
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template <class T>
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struct remove_reference {
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typedef T type;
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};
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template <class T>
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struct remove_reference<T &> {
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typedef T type;
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};
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template <class T>
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struct remove_reference<T &&> {
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typedef T type;
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};
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template <typename T>
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constexpr typename remove_reference<T>::type &&move(T &&x) noexcept {
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return static_cast<typename remove_reference<T>::type &&>(x);
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}
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} // namespace cpp4arduino

‎EfficientString/README.md‎

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How to use the `String` class efficiently
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=========================================
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This program demonstrate how you can improve the efficient of your code when using the `String` class:
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* reduce copying
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* reduce allocations
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* reduce fragmentation
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To show the effect of each calls, I modified the original `String` class from the [Arduino Core for AVR](https://github.com/arduino/ArduinoCore-avr/blob/2663be17272e19f00c55f3f2d8f1ebfac47158d6/cores/arduino/WString.h):
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* I added calls to `Serial.println()`, so that we can see what happens in the Serial Monitor.
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* I removed the platform dependent features, so that we can run this program on other platforms.
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The modified class is in a different namespace, to avoid the name clash.
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Results
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-------
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Here is the serial output of this program with the Arduino Core for AVR version 1.6.22:
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```
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#1: Initialize from RAM
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- malloc(8) -> 0x23c
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- strcpy(0x23c, 0x133) - 7 bytes
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- free(0x23c)
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#2: Initialize from Flash
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- malloc(8) -> 0x23c
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- strcpy_P(0x23c, 0xe3) - 7 bytes
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- free(0x23c)
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#3: toCharArray()
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- malloc(8) -> 0x23c
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- strcpy(0x23c, 0x133) - 7 bytes
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- strncpy(0x8d6, 0x23c, 7) - 7 bytes
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- free(0x23c)
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#4: c_str()
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- malloc(8) -> 0x23c
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- strcpy(0x23c, 0x133) - 7 bytes
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- free(0x23c)
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#5: Pass by value
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- malloc(8) -> 0x23c
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- strcpy(0x23c, 0x133) - 7 bytes
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- malloc(8) -> 0x246
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- strcpy(0x246, 0x23c) - 7 bytes
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- free(0x246)
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- free(0x23c)
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#6: Pass by const reference
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- malloc(8) -> 0x23c
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- strcpy(0x23c, 0x133) - 7 bytes
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- free(0x23c)
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#7: Move instance
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- malloc(8) -> 0x23c
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- strcpy(0x23c, 0x133) - 7 bytes
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- free(0x23c)
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- free(0)
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#8: Append temporaries
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- malloc(6) -> 0x23c
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- strcpy(0x23c, 0x155) - 5 bytes
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- malloc(6) -> 0x244
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- strcpy(0x244, 0x23c) - 5 bytes
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- realloc(0x244, 18) -> 0x244
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- strcpy(0x249, 0x15b) - 12 bytes
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- realloc(0x244, 23) -> 0x244
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- strcpy(0x255, 0x168) - 5 bytes
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- realloc(0x244, 33) -> 0x244
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- strcpy(0x25a, 0x184) - 10 bytes
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- malloc(33) -> 0x267
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- strcpy(0x267, 0x244) - 32 bytes
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- free(0x244)
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- free(0x23c)
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- free(0x267)
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#9: Mutate instance
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- malloc(6) -> 0x23c
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- strcpy(0x23c, 0x155) - 5 bytes
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- realloc(0x23c, 18) -> 0x23c
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- strcpy(0x241, 0x15b) - 12 bytes
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- realloc(0x23c, 23) -> 0x23c
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- strcpy(0x24d, 0x168) - 5 bytes
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- realloc(0x23c, 33) -> 0x23c
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- strcpy(0x252, 0x184) - 10 bytes
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- free(0x23c)
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#10: Call reserve
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- malloc(1) -> 0x23c
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- strcpy(0x23c, 0x11a) - 0 bytes
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- realloc(0x23c, 65) -> 0x23c
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- strcpy(0x23c, 0x155) - 5 bytes
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- strcpy(0x241, 0x15b) - 12 bytes
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- strcpy(0x24d, 0x168) - 5 bytes
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- strcpy(0x252, 0x184) - 10 bytes
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- free(0x23c)
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#11: Pass a null string to the constructor
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- malloc(65) -> 0x23c
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- strcpy(0x23c, 0x155) - 5 bytes
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- strcpy(0x241, 0x15b) - 12 bytes
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- strcpy(0x24d, 0x168) - 5 bytes
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- strcpy(0x252, 0x184) - 10 bytes
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- free(0x23c)
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#12: Concat at compile time
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- malloc(33) -> 0x23c
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- strcpy(0x23c, 0x16e) - 32 bytes
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- free(0x23c)
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```

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