Call to non-constexpr function 'std::map...' - c++

Question

I'm attempting to create a pseudo-media center thing on an Arduino and a breadboard, not all that complicated but this is my first project with C++ that amounted to more than 'Hello world!' or a calculator.
Digressing to my point though, I started rewriting almost the entire original program, and in an effort to clean things up, started using a map to hold the keys and values (IR codes), and to further organize it, put the assignments inside of a function (IR_generateMap()), but I'm getting an error when I compile it.
C:\Users\trevo\Documents\Arduino\improvedIRRemote\improvedIRRemote.ino: In function 'void IRLoop(bool)':
improvedIRRemote:78: error: call to non-constexpr function 'std::map<Key, T, Compare, Allocator>::reference std::map<Key, T, Compare, Allocator>::operator[](const key_type&) [with Key = std::basic_string<char>; T = int; Compare = std::less<std::basic_string<char> >; Allocator = std::allocator<int>; std::map<Key, T, Compare, Allocator>::reference = int&; std::map<Key, T, Compare, Allocator>::key_type = std::basic_string<char>]'
case IRCodes["Power Button"]:
^
improvedIRRemote:78: error: call to non-constexpr function 'std::map<Key, T, Compare, Allocator>::reference std::map<Key, T, Compare, Allocator>::operator[](const key_type&) [with Key = std::basic_string<char>; T = int; Compare = std::less<std::basic_string<char> >; Allocator = std::allocator<int>; std::map<Key, T, Compare, Allocator>::reference = int&; std::map<Key, T, Compare, Allocator>::key_type = std::basic_string<char>]'
exit status 1
call to non-constexpr function 'std::map<Key, T, Compare, Allocator>::reference std::map<Key, T, Compare, Allocator>::operator[](const key_type&) [with Key = std::basic_string<char>; T = int; Compare = std::less<std::basic_string<char> >; Allocator = std::allocator<int>; std::map<Key, T, Compare, Allocator>::reference = int&; std::map<Key, T, Compare, Allocator>::key_type = std::basic_string<char>]'
I've looked up and attempted to understand errors with non-constexpr calls, but to be perfectly honest, I don't understand how what I'm trying to do with the map relates to the problems being described in the posts that deal with said error.
#include <IRremote.h>
#include <StandardCplusplus.h>
#include <map>
//Variables responsible for recieving and decoding IR signals/codes.
IRrecv receiver(10); //IR receiver initialization.
decode_results results; //Object that is responsible for decoding the IR signal.
//Variables responsible for tracking channel changing.
int channel = 1; //The active channel.
int desiredChannel = ""; //The string that is used to concatenate the numbers pressed on the remote keypad together.
int channelEnterDelay = 4000; //The delay between the last time a number was pressed on the remote keypad and when the desiredChannel is cleared.
unsigned long channelEnterStartTime = 0; //The time in which that last number was pressed on the remote keypad.
String intConverter; //The string responsible for casting an integer to a string.
//Variables responsible for tracking volume changing.
int volume = 50; //The current volume.
int lastVolume = 50; //Holds the value of the volume previous to the volume being muted.
bool isMuted = false; //Whether or not the sound has been muted.
//Map responsible for the setting and getting of IR codes and their respective keys.
const std::map<std::string, int> IRCodes;
//Miscellanious Stuff - mostly waiting for depreciation.
int IRFlasherPin = 8;
int mode = 0;
int IRFlasherBlinkRate = 10;
//Debug mode shows channel switching info when entering from the remote keypad, turn it off to declutter the serial feed if it is not being used.
bool DEBUG_MODE = false;
void setup() {
receiver.enableIRIn(); //Tells the receiver to start listening for IR communication.
pinMode(IRFlasherPin, OUTPUT); //Configuring the pin that flashes upon recieving the 'excess communication' IR code.
Serial.begin(9600); //Begins serial communication at 9600 baud.
IR_generateMap(); //Sets all of the values for the map of IR keys and values.
//Ready message.
Serial.println("The Arduino is ON and ready for communication.");
if (DEBUG_MODE) { Serial.println("Also please note that debug mode has been preemptively enabled. The serial monitor can be expected to be much more cluttered than normal. To disable this, press the 'U/SD' button on the remote."); }
Serial.println();
}
void loop() {
//Pre-loop checks.
//empty
IRLoop(true);
}
void IRLoop(bool toldToContinueLooking) {
if (receiver.decode(&results)) {
switch(results.value) {
case IRCodes["Power Button"]:
Serial.println("Power Toggle");
break;
default:
if (DEBUG_MODE) {
Serial.print("Extraneous code: ");
Serial.print(results.value);
}
break;
}
}
if (toldToContinueLooking) { receiver.resume(); }
}
void IR_generateMap() {
//General Keys
IRCodes["Power Button"] = 16753245;
IRCodes["Mode Button"] = 16736925;
IRCodes["Back Button"] = 16750695;
IRCodes["EQ Button"] = 16769055;
IRCodes["USD Button"] = 16756815;
//Media Keys
IRCodes["PlayPause Button"] = 16720605;
IRCodes["Rewind Button"] = 16712445;
IRCodes["Fast Forward Button"] = 16761405;
//Volume Keys
IRCodes["Mute Button"] = 16769565;
IRCodes["Minus Button"] = 16754775;
IRCodes["Plus Button"] = 16748655;
//Numpad Keys
IRCodes["Numpad 0"] = 16738455;
IRCodes["Numpad 1"] = 16724175;
IRCodes["Numpad 2"] = 16718055;
IRCodes["Numpad 3"] = 16743045;
IRCodes["Numpad 4"] = 16716015;
IRCodes["Numpad 5"] = 16726215;
IRCodes["Numpad 6"] = 16734885;
IRCodes["Numpad 7"] = 16728765;
IRCodes["Numpad 8"] = 16730805;
IRCodes["Numpad 9"] = 16732845;
//Non-Key Codes
IRCodes["Excess Communication"] = 4294967295;
}
Can someone explain what I'm doing wrong to me in terms that someone of my early C++ knowledge-base would understand?

Answer 1

The expression in the 'case' must be compile time constant. Just use an if/else construction instead and it should work.
Thomas

Answer 2

I'd go in opposite way by using map to decode IR Code to handler function or by using enum. Something like this:
#include <StandardCplusplus.h>
#include <map>
#include <functional>
void power_key();
void num_key0();
void num_key1();
void num_key2();
void num_key3();
void num_key4();
void num_key5();
void num_key6();
void num_key7();
void num_key8();
void num_key9();
void num_key(int8_t);
typedef void(*handler_t)();
std::map<uint32_t, handler_t> keys;
void setup() {
keys[16753245UL] = power_key;
keys[16738455UL] = num_key0;
keys[16724175UL] = num_key1;
keys[16718055UL] = num_key2;
keys[16743045UL] = num_key3;
keys[16716015UL] = num_key4;
keys[16726215UL] = num_key5;
keys[16734885UL] = num_key6;
keys[16728765UL] = num_key7;
keys[16730805UL] = num_key8;
keys[16732845UL] = num_key9;
Serial.begin(57600);
}
void loop() {
uint32_t recvd_code = 16753245UL; // literal value, for testing purpose without IR receiver and remote
auto iter = keys.find(recvd_code); // find received code and execute it
if (iter != keys.end()) {
iter->second();
} else {
Serial.print(F("IR Code "));
Serial.print(recvd_code);
Serial.println(F("not found"));
}
delay(1000);
}
void power_key() {
Serial.println(F("Power key pressed"));
}
void num_key0() { num_key(0); }
void num_key1() { num_key(1); }
void num_key2() { num_key(2); }
void num_key3() { num_key(3); }
void num_key4() { num_key(4); }
void num_key5() { num_key(5); }
void num_key6() { num_key(6); }
void num_key7() { num_key(7); }
void num_key8() { num_key(8); }
void num_key9() { num_key(9); }
void num_key(int8_t num) {
Serial.print(F("Numeric key pressed: "));
Serial.println(num);
}
Initializer list somehow didin't work, so initialization must be done in setup and map can't be const.