What Is an 'Undeclared Identifier' Error and How to Fix It

How do i fix this specific undeclared identifier error?

You have not defined a type called Window with which you could define a variable like

Window *window;

You have defined a struct Window with which you define your window variable in main() like

struct Window *window;

in the same way you are already defining all your function prototypes' window parameters.

How do I fix this undeclared identifier? (C++)

This is what your SynthesizerTwo.cpp file looks like after the pre-processor has included the files, with most cut out:

class SynthesizerTwo : public IPlug
{
...
  double mFrequency;
  void SynthesizerTwo::CreatePresets() {
      
      MakeDefaultPreset((char*)"-", kNumPrograms); //This is what gives me the error
  }
...

const int kNumPrograms = 1; //Original Declaration

As you can see, kNumPrograms is used before it was declared. Declaring it afterwards doesn't help. Besides, if you include "SynthesizerTwo.h" into any other file, there would not be declaration at all.

Solution: Declare variables before their use.

But the thing is, I've tried declaring it in the .h and in the .cpp above the .h's #include. It gave me linker errors, as stated in the OP.

If you define a namespace scope variable in a header, then you should declare it inline.

---

#define __SYNTHESIZERTWO__

You've defined an identifier that is reserved to the language implementation. As a consequence, the behaviour of your program is undefined (would be if it compiled in the first place). You should use another header guard.

(char*)"-"

This seems dangerous. Avoid casting away constness.

Why am I getting error: use of undeclared identifier error?

C originally did not have native support for boolean values.
In order to get the things working, you need to import a header file name <stdbool.h>

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    int* ptri = NULL;
    char* ptrc = NULL;
    bool* ptrb = NULL;
    printf("%lu %lu %lu", sizeof(ptri), sizeof(ptrc), sizeof(ptrb));
}

C++ undeclared identifier error

This is, as I wrote in the comments, most likely caused by your circular includes. Connection.hpp includes LogicSimulator.hpp which itself includes Connection.hpp.

In your case, you don't even need the includes. For Pointers and References, a forward declaration is fine:

Connection.hpp:

#pragma once

class CircuitObject;
class LogicSimulator;

class Connection
{
public:
    Connection(CircuitObject& c1, CircuitObject& c2, LogicSimulator& 
        simulator);
private:
    int state;
    CircuitObject& c1;
    CircuitObject& c2;
    LogicSimulator& simulator;
};

LogicSimulator.hpp:

#pragma once

#include <SFML\Graphics.hpp>
#include <vector>

class CircuitObject;
class Connection;

class LogicSimulator
{
public:
    std::vector<CircuitObject*> circuitObjects;
    std::vector<CircuitObject*> selectedCircuitObjects;
    std::vector<Connection*> connections;
    sf::RenderWindow Window;

    void Init();
private:
    void start();
    void draw();
};

How do I resolve undeclared identifier error from the code?

Just modify the constructor of Box Class. You just missed , between parameters and name of b.

Box(double l = 2.0, double b = 2.0, double h = 2.0) {

Please see the full code as following:

#include <iostream>

using namespace std;

class Box {
public:
    static int objectCount;

    //Constructor definition
    Box(double l = 2.0, double b = 2.0, double h = 2.0) {
        cout << "Constructor Called." << endl;
        length = l;
        breadth = b;
        height = h;

        //Increase everytime the object is created
        objectCount++;
    }
    double Volume() {
        return length * breadth* height;
    }
private:
    double length;
    double breadth;
    double height;
};
// Initialize static member of class Box
int Box::objectCount = 0;

int main(void) {
    Box Box1(3.3, 1.2, 1.5);
    Box Box2(8.5, 6.0, 2.0);

    cout << "Total objects: " << Box::objectCount << endl;

    return 0;
}

Please get your result as following:

Constructor Called.
Constructor Called.
Total objects: 2

---

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