Hey everybody, 

A little over a month ago, I made a post asking for help with my Missile Command game that I was making with regards to getting the Cannon to angle properly, and I was very pleased with all of the support.  It really lit a fire under me (even moreso before I worked up the courage to ask the first time).

Well, the game is not finished yet, but I have made a lot of progress since then ( it's very motivating  ) and I've been able to work through the code and logic for shooting missiles from the cannon.

The missile logic is almost ready, but there is one small bug that I discovered last week that has been wracking my brain non-stop, so I'm posting a new topic here.

Here is an animated GIF of the missile behavior so far, in this shot, it's working as expected when you click the Left mouse button to fire one missile at a time:

https://i.imgur.com/WyM2oxi.gifv

But what I didn't expect to happen, was here, when you click the Left button to fire a new missile when the other missile is still in transit:

https://i.imgur.com/PFJTDV1.gifv

When the next missile is fired, the original missile changes course, re-angles itself and flies towards the next clicked spot, abandoning it's first clicked target, which is not what I wanted lol.

It's like the missiles turn into smart missiles all of a sudden.  The good news is that they're being destroyed when they hit the spot anyways, so that's something.

So, for anyone reading this post, how can I make the previous missiles stay on their original target, even if I click the mouse to summon a new missile?

Each missile's path should be independent of each other.

This will also play into the logic for the upcoming falling Asteroid class I'll be adding, so solving this one for me will also allow me to make independent asteroids and I'm almost done the game! lol

Here's the code:

Game.h

#ifndef GAME_H_
#define GAME_H_

// Preprocessors
#include "SDL.h"
#include "SDL_image.h"
#include "SDL_ttf.h"
#include "SDL_mixer.h"
#include <iostream>
#include <sstream>
#include <stdio.h>
#include <math.h>
#include <cmath>
#include <vector>

// Inclusions for game objects
#include "Cannon.h"
#include "Missile.h"
#include "Ground.h"
#include "Silo.h"

#define FPS_DELAY           500
#define SCREEN_WIDTH        800
#define SCREEN_HEIGHT       600
#define MAX_ROTATION        0
#define MIN_ROTATION        180
#define DEG_TO_RAD(deg)     degrees * ( 3.141592653589793238 / 180)
#define RAD_TO_DEG(rad)     radians * ( 180 / 3.141592653589793238 )
#define OVER_PI             ( 180 / 3.141592653589793238 )

class Game
{
    public:
        Game();
        ~Game();

        bool Init();
        void Run();


    private:
        SDL_Window* window;
        SDL_Renderer* renderer;

        SDL_Texture* texture;

        // Timing
        unsigned int lastTick, fpsTick, fps, frameCount;

        // Test
        float testX, testY;

        /*
        Fonts for testing
        */
        TTF_Font* fontMouseX;
        TTF_Font* fontMouseY;
        TTF_Font* fontCannonAngle;
        TTF_Font* fontFire;
        TTF_Font* fontAmmo;

        std::stringstream xValue;
        std::stringstream yValue;
        std::stringstream angleValue;
        std::stringstream fireValue;
        std::stringstream ammoValue;

        SDL_Texture* xTexture;
        SDL_Texture* yTexture;
        SDL_Texture* aTexture;
        SDL_Texture* fTexture;
        SDL_Texture* ammoTexture;

        int xWidth, xHeight, yWidth, yHeight, aWidth, aHeight;
        int fWidth, fHeight, ammoWidth, ammoHeight;

        /*
        END
        */


        // Game objects
        Cannon* cannon;                                 // The cannon, both the pipe and the base

        std::vector<Missile*> missileVec;                // A vector of missiles

        Silo* silo;
        Ground* ground;                                 // The ground

        SDL_Cursor* cursor;                             // The crosshair cursor

        // Flag to check if the cannon
        // is currently firing a missile
        bool isFiring;
        int clickedX;
        int clickedY;

        void Clean();                                   // Cleanup function
        void Update(float delta);                       // Update game elements
        void Render(float delta);                       // Render game elements

        void NewGame();                                 // Start a new game

        void SetCannonAngle( float vecX, float vecY);   // Set the angle of cannon pipe
        double SetMissileAngle( float vecX, float vecY);  // Set the angle of missile
        double ConvertDegrees(double radians);          // Convert radians to degrees
        double ConvertRadians(double degrees);          // Convert degrees to radians
        void ExplodeMissile();                          // Explode the missile once it
                                                        // has reached the target

        void CheckAsteroidCollisions();                 // See if missile hits asteroid
        void CheckSiloCollisions();                     // See if asteroid hits silo
        int GetSiloCount();                             // Check if there are silos remaining



};


#endif // GAME_H_

Game.cpp

#include "Game.h"

Game::Game()
{
    window = 0;
    renderer = 0;
}

Game::~Game()
{

}

bool Game::Init()
{
    // Initialize the SDL video and audio subsystems
    SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO);

    // Create window
    window = SDL_CreateWindow("Missile Command v1.0", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
                              800, 600, SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL);

    if(!window)
    {
        std::cout << "Error creating window: " << SDL_GetError() << std::endl;
        return false;
    }

    // Create renderer
    renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
    if(!renderer)
    {
        std::cout << "Error creating renderer: " << SDL_GetError() << std::endl;
        return false;
    }

    // Enable TTF loading
    TTF_Init();

    // Load the fonts
    fontMouseX = TTF_OpenFont("lato.ttf", 14);
    fontMouseY = TTF_OpenFont("lato.ttf", 14);
    fontCannonAngle = TTF_OpenFont("lato.ttf", 14);
    fontFire = TTF_OpenFont("lato.ttf", 14);
    fontAmmo = TTF_OpenFont("lato.ttf", 14);

    // Initialize resources
    SDL_Surface* surface = IMG_Load("test.png");
    texture = SDL_CreateTextureFromSurface(renderer,surface);
    SDL_FreeSurface(surface);

    // Set mouse cursor to crosshair
    cursor = SDL_CreateSystemCursor(SDL_SYSTEM_CURSOR_CROSSHAIR);
    SDL_SetCursor(cursor);

    // Initialize timing
    lastTick = SDL_GetTicks();
    fpsTick = lastTick;
    fps = 0;                // Set starting FPS value
    frameCount = 0;         // Set starting frame count

    testX = 0;
    testY = 0;

    return true;
}

void Game::Clean()
{
    // Clean resources
    SDL_DestroyTexture(texture);
    SDL_FreeCursor(cursor);
    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);

    // Clean test data
    SDL_DestroyTexture(xTexture);
    SDL_DestroyTexture(yTexture);
    SDL_DestroyTexture(aTexture);
    SDL_DestroyTexture(fTexture);
    SDL_DestroyTexture(ammoTexture);
}

void Game::Run()
{
    // Create game objects
    cannon = new Cannon(renderer);
    ground = new Ground(renderer);
    silo = new Silo(renderer);

    // Start a new game
    NewGame();

    // Main loop
    while(1)
    {
        // Event handler
        SDL_Event e;

        // If event is a QUIT event, stop the program
        if(SDL_PollEvent(&e))
        {
            if(e.type == SDL_QUIT)
            {
                break;
            }

            // If a mouse button was pressed...
            else if(e.type == SDL_MOUSEBUTTONDOWN)
            {
                // If the left mouse button was pressed,
                // set the "fire" flag to TRUE and
                // record x,y position of click
                if(e.button.button == SDL_BUTTON_LEFT)
                {
                    clickedX = e.button.x;
                    clickedY = e.button.y;
                    isFiring = true;
                }
            }
        }

        // Calculate delta and fps
        unsigned int curTick = SDL_GetTicks();
        float delta = (curTick - lastTick) / 1000.0f;

        // Cap FPS delay to specific amount
        if(curTick - fpsTick >= FPS_DELAY)
        {
            fps = frameCount * (1000.0f / (curTick - fpsTick));
            fpsTick = curTick;
            frameCount = 0;
            //std::cout << "FPS: " << fps << std::endl;
            char buf[100];
            snprintf(buf,100,"Missile Command v1.0 (fps: %u)", fps);
            SDL_SetWindowTitle(window,buf);
        }
        else
        {
            frameCount++;
        }

        lastTick = curTick;

        // Update and render the game
        Update(delta);
        Render(delta);
    }

    delete cannon;
    //missileVec.clear();
    delete ground;
    delete silo;

    Clean();

    // Close the fonts
    TTF_CloseFont(fontMouseX);
    TTF_CloseFont(fontMouseY);
    TTF_CloseFont(fontCannonAngle);
    TTF_CloseFont(fontFire);
    TTF_CloseFont(fontAmmo);

    TTF_Quit();

    SDL_Quit();
}

void Game::NewGame()
{

}

void Game::Update(float delta)
{
    // Game logic

    // Input, get mouse position to determine
    // the cannon pipe's angle for rendering
    int mX, mY;
    SDL_GetMouseState(&mX,&mY);

    // Assign the vector to variables
    float vectorX = mX - cannon->x;
    float vectorY = mY - cannon->y;

    // "Snap" the cannon angle to the angle between
    // cannon and mouse
    SetCannonAngle( vectorX, vectorY );

    // If cannon is firing, create a missile and add
    // it to the missile vector container, and set
    // "isFiring" to false again
    if( isFiring == true )
    {
        Missile* newMissile = new Missile(renderer, cannon->x, cannon->y);
        missileVec.push_back(newMissile);
        isFiring = false;
    }

    // If there are missiles in the vector container, set their direction
    // angle, fire them and update the missile positions
    for( int i = 0; i < missileVec.size(); i++ )
    {
        // Update the missile's trajectory upon travel
        missileVec[i]->endX = clickedX;
        missileVec[i]->endY = clickedY;

        // Set a vector between the clicked location
        // and the missile
        float targetX = clickedX - missileVec[i]->x;
        float targetY = clickedY - missileVec[i]->y;

        // Set facing angle for missile
        missileVec[i]->angle = SetMissileAngle(targetX,targetY);

        // Update the missile's position
        missileVec[i]->Update(delta);

        // If the missile is triggered to be destroyed,
        // destroy it
        if( missileVec[i]->destroyNow == true )
        {
            // Remove the destroyed missile
            // from the vector
            missileVec.pop_back();
        }
    }

    /*// If there are no more silos,
    // start a new game
    if(GetSiloCount() == 0)
    {
        NewGame();
    }*/



    // Check if the cannon has fired a missile
    //cannon->missile->Update(delta, mX - cannon->x, mY - cannon->y);

    /*
    Test Data
    */

    // Stream the X, Y, Angle and isFired values
    xValue.str("");     // Clear the stream before piping the xValue
    xValue << "X: " << mX;

    yValue.str("");     // Clear the stream before piping the yValue
    yValue << "Y: " << mY;

    angleValue.str(""); // Clear the stream before piping the angleValue
    angleValue << "Angle: " << cannon->angle;

    fireValue.str(""); // Clear the stream before piping the fireValue
    fireValue << "isFiring: " << std::boolalpha << isFiring;

    ammoValue.str(""); // Clear the stream before piping the ammoValue
    ammoValue << "Missiles in Vector: " << missileVec.size();

    // Set font color to WHITE
    SDL_Color textColor = {255,255,255};

    //***************************************
    // DEBUG - Prepare the fonts for testing
    //***************************************

    // Render the X-coordinate text
    SDL_Surface* temp = TTF_RenderText_Solid( fontMouseX, xValue.str().c_str(), textColor );
    xTexture = SDL_CreateTextureFromSurface( renderer, temp );
    xWidth = temp->w;
    xHeight = temp->h;
    SDL_FreeSurface(temp);

    // Render the Y-coordinate text
    temp = TTF_RenderText_Solid( fontMouseY, yValue.str().c_str(), textColor );
    yTexture = SDL_CreateTextureFromSurface( renderer, temp );
    yWidth = temp->w;
    yHeight = temp->h;
    SDL_FreeSurface(temp);

    // Render the angle text
    temp = TTF_RenderText_Solid( fontCannonAngle, angleValue.str().c_str(), textColor );
    aTexture = SDL_CreateTextureFromSurface( renderer, temp );
    aWidth = temp->w;
    aHeight = temp->h;
    SDL_FreeSurface(temp);

    // Render the isFiring text
    temp = TTF_RenderText_Solid( fontFire, fireValue.str().c_str(), textColor );
    fTexture = SDL_CreateTextureFromSurface( renderer, temp );
    fWidth = temp->w;
    fHeight = temp->h;
    SDL_FreeSurface(temp);

    // Render the missileVec.size() text
    temp = TTF_RenderText_Solid( fontAmmo, ammoValue.str().c_str(), textColor );
    ammoTexture = SDL_CreateTextureFromSurface( renderer, temp );
    ammoWidth = temp->w;
    ammoHeight = temp->h;
    SDL_FreeSurface(temp);
}

void Game::SetCannonAngle(float vecX, float vecY)
{
    // "Snap" cannon angle to angle where mouse
    // cursor is located in order to track it
    double theAngle = atan2(vecY, vecX);

    // Pass radian angle to convert to degrees
    theAngle = ConvertDegrees(theAngle);

    // Convert degrees format from -180 to 180
    // to 0 to 360
    if( theAngle < 0 )
    {
        theAngle = 360 - (-theAngle);
    }

    if(theAngle > MAX_ROTATION && theAngle < 90)
    {
        cannon->angle = MAX_ROTATION;
    }
    else if(theAngle < MIN_ROTATION && theAngle > 90)
    {
        cannon->angle = MIN_ROTATION;
    }
    else
    {
        cannon->angle = theAngle;
    }
}

double Game::SetMissileAngle(float vecX, float vecY)
{
    // "Snap" missile angle to angle where mouse
    // cursor is located in order to track it
    double theAngle = atan2(vecY,vecX);

    // Pass radian angle to convert to degrees
    theAngle = ConvertDegrees(theAngle);

    // Convert degrees format from -180 to 180
    // to 0 to 360
    if( theAngle < 0 )
    {
        theAngle = 360 - (-theAngle);
    }

    return theAngle;
}

double Game::ConvertDegrees(double radians)
{
    // Convert the angle from radians to
    // degrees and return the value
    return RAD_TO_DEG(rad);
}

double Game::ConvertRadians(double degrees)
{
    // Convert the angle from degrees to
    // radians and return the value
    return DEG_TO_RAD(deg);
}

int Game::GetSiloCount()
{
    int siloCount = 0;

    // If the current silo is still alive,
    // the game is not over
    if(silo->state)
    {
        siloCount = 1;
    }

    return siloCount;
}

void Game::Render(float delta)
{
    // Clear the renderer
    SDL_RenderClear(renderer);

    // Render the game objects
    cannon->Render(delta);
    ground->Render(delta);
    silo->Render(delta);

    // If there is a missile in the vector,
    // render it
    for( int i = 0; i < missileVec.size(); i++ )
    {
        missileVec[i]->Render(delta);
    }

    /*
        Setting source rectangles and then
        rendering the x, y and angle fonts
    */

    // For x
    SDL_Rect rect;
    rect.x = 20;
    rect.y = 20;
    rect.w = xWidth;
    rect.h = xHeight;
    SDL_RenderCopy(renderer, xTexture, 0, &rect);

    // For y
    rect.x = 20;
    rect.y = 40;
    rect.w = yWidth;
    rect.h = yHeight;
    SDL_RenderCopy(renderer, yTexture, 0, &rect);

    // For angle
    rect.x = 20;
    rect.y = 60;
    rect.w = aWidth;
    rect.h = aHeight;
    SDL_RenderCopy(renderer, aTexture, 0, &rect);

    // For isFired
    rect.x = 20;
    rect.y = 80;
    rect.w = fWidth;
    rect.h = fHeight;
    SDL_RenderCopy(renderer, fTexture, 0, &rect);

    // For isFired
    rect.x = 20;
    rect.y = 100;
    rect.w = ammoWidth;
    rect.h = ammoHeight;
    SDL_RenderCopy(renderer, ammoTexture, 0, &rect);

    // Present the renderer to display
    SDL_RenderPresent(renderer);
}

Missile.h

#ifndef MISSILE_H_INCLUDED
#define MISSILE_H_INCLUDED

#include "Entity.h"

#include <math.h>

class Missile : public Entity
{
    public:
        Missile(SDL_Renderer* renderer, float cX, float cY);
        ~Missile();

        void Update(float delta);
        void Render(float delta);

        void Fire();

        void Move();

        // Starting and ending positions for missile
        float startX, startY, endX, endY;
        float directionX, directionY;
        float distance, speed;

        // The missile's angle
        double angle;

        // Flag to have missile destroyed
        // when certain conditions are met
        bool destroyNow;

    private:
        SDL_Texture* texture;           // Texture for missile
        SDL_Point center;
};

#endif // MISSILE_H_INCLUDED

Missile.cpp

#include "Missile.h"

Missile::Missile(SDL_Renderer* renderer, float cX, float cY) : Entity(renderer)
{
    // Create missile texture
    SDL_Surface* surface = IMG_Load("images/missile.png");
    texture = SDL_CreateTextureFromSurface(renderer,surface);
    SDL_FreeSurface(surface);

    // Set angle variable default,
    // width, height
    angle = 0.0f;
    width = 34;
    height = 13;

    // Set missile offset for positioning relative to cannon
    float xOffset = 8;

    // Set starting x,y position
    x = cX + xOffset;
    y = cY;

    // Set the missile's starting x and y position
    // and speed
    startX = x;
    startY = y;
    speed = 150;

    // Set the missile's center for rotation
    center.x = ( width / 2 );
    center.y = ( height / 2 );

    // Set destruction flag to false
    destroyNow = false;
}

Missile::~Missile()
{
    // Clean resources
    SDL_DestroyTexture(texture);
}

void Missile::Move()
{
    // Determine the distance from the starting
    // position to the ending position
    distance = sqrt( pow(endX - startX, 2) + pow(endY - startY, 2));

    // Determine direction of movement
    directionX = (endX - startX) / distance;
    directionY = (endY - startY) / distance;
}

void Missile::Update(float delta)
{
    // Call move function
    Move();

    // If the missile is moving,
    // update its position
    this->x += directionX * speed * delta;
    this->y += directionY * speed * delta;

    // If the missile goes BEYOND the
    // destination x and y (endX and endY),
    // set the missile's position to endX
    // and endY and stop movement
    if( sqrt( pow( this->x - startX, 2) + pow( this->y - startY, 2) ) >= distance )
    {
        this->x = endX;
        this->y = endY;
        this->destroyNow = true;
    }
}

void Missile::Render(float delta)
{
    // Render the missile
    SDL_Rect rect;
    rect.x = (int)(x + 0.5f);
    rect.y = (int)(y + 0.5f);
    rect.w = width;
    rect.h = height;

    // Render the missile at given angle
    SDL_RenderCopyEx(renderer,texture,0,&rect,angle,&center,SDL_FLIP_NONE);
}

I appreciate any feedback, guys.  Thanks in advance.

Also, let me know if you need more information about this post and I can provide it when I can.

Hi Glydion, one small suggestion, you should edit the post above, double click on the code and choose the C++ display option, it makes it easier to read

In general, try to post only the relevant bits of code if you can, instead of all code.

The bug lies in Game::Update. When you loop over all missiles, you set all missiles to head towards clickedX and clickedY (you set the end variables for all missiles).

Instead, only set the end variable when you create a new missile. Maybe add it to the constructor and pass it in there.

Doing this will uncover a new bug. All missiles should now fly to where they are intended to, but they will still keep on facing where you clicked. This is a similar bug to the one above -- you are using clickedX and clickedY instead of endX and endY to calculate targetX and targetY.

As a last thing, I would suggest changing the way you comment stuff. Currently, you are commenting what the code does, in a lot of detail. Instead, try commenting only the cases where it isn't obvious why the code is doing what it does.

Stuff like "Ground *ground; //The ground" is unnecessary, and it forces the reader (in most cases, the reader is you) to have to read a lot more than they really need.

I'd rather you called this an "unexpected feature" than a bug. Homing missiles are cool.

Maybe after you fix it, you can still keep that behavior so that it activates with a powerup or some special code.

15 hours ago, Alpha_ProgDes said:

I'd rather you called this an "unexpected feature" than a bug. Homing missiles are cool.

Maybe after you fix it, you can still keep that behavior so that it activates with a powerup or some special code.

Hey Alpha!  Thank you for writing that article about the journey for beginner programmers and the 10 "must make" games to break into the industry.  It has been a big inspiration for me in my journey to become a 2D game programmer so far and has helped me not get burnt out when learning this stuff.

It's been a great way to learn how to program 2D games incrementally and I've found it is easy to stay motivated with this model of progress.

My dream is to start my own indie company, Glydion, and make my own 2D games that remind players of the 16-bit era (Super Nintendo, Sega, Neo-Geo, etc.) which is where my passion is.

Yes lol, I'll make this "homing" feature a powerup for when I go through these 10 games again and make (version 2.0) versions of these games in the future.

22 hours ago, Lactose said:

In general, try to post only the relevant bits of code if you can, instead of all code.

 

The bug lies in Game::Update. When you loop over all missiles, you set all missiles to head towards clickedX and clickedY (you set the end variables for all missiles).

Instead, only set the end variable when you create a new missile. Maybe add it to the constructor and pass it in there.

Doing this will uncover a new bug. All missiles should now fly to where they are intended to, but they will still keep on facing where you clicked. This is a similar bug to the one above -- you are using clickedX and clickedY instead of endX and endY to calculate targetX and targetY.

 

As a last thing, I would suggest changing the way you comment stuff. Currently, you are commenting what the code does, in a lot of detail. Instead, try commenting only the cases where it isn't obvious why the code is doing what it does.

Stuff like "Ground *ground; //The ground" is unnecessary, and it forces the reader (in most cases, the reader is you) to have to read a lot more than they really need.

Hey Lactose, thank you for your feedback.  Yeah I think you're right, I do seem to be commenting on lines that are pretty self-explanatory, which must make it hard for others to sift through it.

I think I took that "comment everywhere" feedback from my early programming courses too seriously lol  .

Hmm, I've never thought of passing the clickedX and clickedY in the missile constructor.

I'll change it up with your suggestions and run some tests.  Thank you.

22 hours ago, MarcusAseth said:

Hi Glydion, one small suggestion, you should edit the post above, double click on the code and choose the C++ display option, it makes it easier to read

Hey Marcus!  Hope you've been doing well, friend.

I didn't even realize that the code I inserted was in HTML hehe.  I've changed it to C++ now for enhanced readability.  My bad.

Hey guys, just wanted to update on this topic.

Lactose hit the nail right on the head, as soon as I passed the clickedX and clickedY into the constructor, the missiles no longer "re-aligned" themselves to the next missile but continued on their original path niiiice.

Here's an animated GIF for a visual:

https://i.imgur.com/2ZsL5in.gifv

Now I have a new bug hehe, all of the missiles, regardless of how many have been fired, will all be destroyed (disappear) at the exact same time lol.

Let me try to fix that and I'll post an update about it later.  Thanks again to all who responded.  Have a good one.

Hi everyone, an update to the new bug in my program.

I can't seem to figure out why all the missiles are popping out of the vector at the exact same time.

Here's another GIF to illustrate and the pertinent code below.

https://i.imgur.com/4dZjEJv.gifv

The only case where the missiles don't all disappear concurrently is if the first missile path is very far away from the cannon, as shown in the 1st launch test in the GIF above.  

In this example, I'm able to get 4 quick shots off before the original 1st missile reaches the top of the screen.

Here's what I got for code:

Game.cpp

void Game::Update(float delta)
{
    // Game logic
    int mX, mY;
    SDL_GetMouseState(&mX,&mY);

    float vectorX = mX - cannon->x;
    float vectorY = mY - cannon->y;

    SetCannonAngle( vectorX, vectorY );

    if( isFiring == true )
    {
        Missile* newMissile = new Missile(renderer, cannon->x, cannon->y, clickedX, clickedY);
        missileVec.push_back(newMissile);
        isFiring = false;
    }

    // If there are missiles in the vector container, set their direction
    // angle, fire them and update the missile positions
    for( int i = 0; i < missileVec.size(); i++ )
    {
        float targetX = missileVec[i]->endX - missileVec[i]->x;
        float targetY = missileVec[i]->endY - missileVec[i]->y;

        missileVec[i]->angle = SetMissileAngle(targetX,targetY);

        missileVec[i]->Update(delta);

        if( missileVec[i]->destroyNow == true )
        {
            missileVec.pop_back();
        }
    }
}

Missile.cpp

void Missile::Update(float delta)
{
    // Call move function
    Move();

    // If the missile is moving,
    // update its position
    this->x += directionX * speed * delta;
    this->y += directionY * speed * delta;

    // If the missile goes BEYOND the
    // destination x and y (endX and endY),
    // set the missile's position to endX
    // and endY and stop movement
    if( sqrt( pow( this->x - startX, 2) + pow( this->y - startY, 2) ) >= distance )
    {
        this->x = endX;
        this->y = endY;
        this->destroyNow = true;
    }
}

As per the above code, each missile has a Boolean destroy flag (destroyNow) that signals when to pop the missile out of the vector, which I have set to when the missile reaches its destination on the screen.

So this works when the first missile has to travel a longer distance than the subsequent missiles that succeed it, but when the first missile has a short path and then you click the Left mouse button to set a longer path for the next missile, both of them are popped out of the vector and disappear at the same time, when their destruction should be independent of each other.

I've been racking my brain trying to figure it out for 2 days' straight so my brain is now mush lol.

How would you all go about removing missiles independently of each other given my code above?

I appreciate all feedback, thank you in advance.

if( missileVec[i]->destroyNow == true )
{
    missileVec.pop_back();
}

This seems fishy to me... if your first missile is set to be destroyed you are doing pop_back() effectively destroying the last missile which has nothing to do with the first

Also if I am not mistaken, by doing pop_back() on a vector of raw pointers you now have leaked a missile because delete is never called on that missile thus its destructor is not fired and you thrown away the only pointer to it

I would try using a different data structure to store the Missiles, one that allows you to remove from the middle of it as well like a DoubyLinkedList (called list in C++ I think) or maybe a map where the key is the missile ID which you assign to every missile at spawn and have a static variable distributing IDs, or maybe I would try to keep it a vector but when you find a missile that need to be destroyed, you swap it with the last, carefully destroy the last element and decrese your counter loop by one so you process again the current ID which is the previous(before the swap) last element.

Missile* newMissile = new Missile(renderer, cannon->x, cannon->y, clickedX, clickedY);
missileVec.push_back(newMissile);

If it isn't clear what you are doing here, you are essentially creating a pointer named newMissile, which then has its value set to point to the memory that is allocated by using new, so now only newMissile refers to that memory.

Then you use push_back on it, which copies the value of the pointer into the vector, so now we have two things pointing at that memory.

for( int i = 0; i < missileVec.size(); i++ )
{
  float targetX = missileVec[i]->endX - missileVec[i]->x;
  float targetY = missileVec[i]->endY - missileVec[i]->y;

  missileVec[i]->angle = SetMissileAngle(targetX,targetY);

  missileVec[i]->Update(delta);

  if( missileVec[i]->destroyNow == true )
  {
    missileVec.pop_back();
  }
}

Now this part is an issue, basically you're iterating over each pointer to a missile. The problem is that presumably the first missile in the container will be the earliest fired one, and likely to explode the first. What happens when it explodes? it sets destroynow to true, and then the next time the loop runs the first missile it sees is one that is destroyed.

Problem is push_back and pop_back do exactly what they say, they push or pop the element on the back. So now when your first missile explodes, whether you have 1 missile or 10, it removes the last one. Then it goes to the next ones, they may not be destroyed yet, so it exits the loop. But what happens the next update loop? It checks the first missile, sees it is destroyed, and pops the back again. Basically you're going to infinitely pop off all of your missiles.

In addition, you are popping the missile pointers off without ever calling delete on them to free the memory they point to.

There's a few methods you could use to remove elements in a vector like this. If you use an iterator(missileVec::iterator) then you can use erase when you get to a missile you want gone. Erase will remove that element and shift the other elements back to fill in the space. Another method is swap and pop, essentially if there are more than two elements in the vector you take the last element and swap it with the one you want to erase, then pop it off the back.

Just make sure to delete these missiles, or use a smart pointer like unique_ptr, which is vector safe.

EDIT: Managing memory and ownership like this is a pretty big part of any program, especially stuff like this for games. It might be worthwhile for you to research your options a bit and understand some of the various ways you can store things and pass ownership or references around.

I looked at your code real fast :

Why does the missile need to delete a texture ?

You should have only 1 texture loaded for all missiles.