pong1.txt

#include <iostream>
#include <SDL.h>
#include <SDL_mixer.h>
#include <SDL_image.h>

using namespace std;

int main (int argc, char* args[])
{
const int screen_width = 640;
const int screen_height =480;
const int FPS = 200;
bool b[4] = {0,0,0,0,};
bool running = true;
Uint32 start;
SDL_Event* ball;

    SDL_Init(SDL_INIT_EVERYTHING);

    SDL_Window* window = NULL;
    window = SDL_CreateWindow("Pong", 100, 100, screen_width, screen_height, SDL_WINDOW_SHOWN |SDL_WINDOW_RESIZABLE );

    if (window == NULL)
    {
        cout << "Window load Error!" << endl;
        running = false;
    }

    SDL_Renderer* render = NULL;
    render = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC | SDL_RENDERER_TARGETTEXTURE);

    if(render == NULL)
    {
        cout << "Rendering Error!" << endl;
        running = false;
    }

    SDL_Texture* stage1_image = NULL;
    stage1_image = IMG_LoadTexture(render,"stage1.png");

    SDL_Rect stage1_rect;
    stage1_rect.x = 10;
    stage1_rect.y = 10;
    stage1_rect.w = screen_width;
    stage1_rect.h = screen_height;

    SDL_Texture* red_bar_image = NULL;
    red_bar_image = IMG_LoadTexture(render, "redbar.png");

    SDL_Rect red_bar_rect;
    red_bar_rect.x = 10;
    red_bar_rect.y = 240;
    red_bar_rect.w = 50;
    red_bar_rect.h = 50;

    SDL_Texture* blue_bar_image = NULL;
    blue_bar_image = IMG_LoadTexture(render, "bluebar.png");

    SDL_Rect blue_bar_rect;
    blue_bar_rect.x = 580;
    blue_bar_rect.y = 240;
    blue_bar_rect.w = 50;
    blue_bar_rect.h = 50;

    SDL_Texture* ball_image = NULL;
    ball_image = IMG_LoadTexture(render, "ball.png");

    SDL_Rect ball_rect;
    ball_rect.x = 320;
    ball_rect.y = 240;
    ball_rect.w = 75;
    ball_rect.h = 50;

    SDL_Event* mainevent = new SDL_Event();

                     //MainEvent is to poll event in wile loop
    while(running)
    {

        start = SDL_GetTicks();

    while (SDL_PollEvent(mainevent))
    {
        switch (mainevent->type)
        {
            case SDL_QUIT:
            running = false;
            break;
            case SDL_KEYDOWN:
                ball_rect.x --;
                ball_rect.x --;
                ball_rect.x --;
                ball_rect.x --;
                ball_rect.x --;
                ball_rect.x --;
                ball_rect.x --;
                ball_rect.x --;
                ball_rect.x --;
                ball_rect.x --;
                ball_rect.x --;
        switch(mainevent->key.keysym.sym)
        {
            case SDLK_w:
            b[0] = 1;
            break;
            case SDLK_s:
            b[1] = 1;
            break;
            case SDLK_o:
            b[2] = 1;
            break;
            case SDLK_l:
            b[3] = 1;
            break;
            case SDLK_ESCAPE:
            running =false;
            break;
        }
            break;
            case SDL_KEYUP:
        switch(mainevent->key.keysym.sym)
        {
            case SDLK_w:
            b[0] = 0;
            break;
            case SDLK_s:
            b[1] = 0;
            break;
            case SDLK_o:
            b[2] = 0;
            break;
            case SDLK_l:
            b[3] = 0;
        }
            break;

        }
            if(b[0])
            {
            red_bar_rect.y--;
            red_bar_rect.y--;
            red_bar_rect.y--;
            }
            if (b[1])
            {
            red_bar_rect.y++;
            red_bar_rect.y++;
            red_bar_rect.y++;
            }
            if(b[2])
            {
            blue_bar_rect.y--;
            blue_bar_rect.y--;
            blue_bar_rect.y--;
            }
            if(b[3])
            {
            blue_bar_rect.y++;
            blue_bar_rect.y++;
            blue_bar_rect.y++;
            }

        SDL_RenderClear(render);
        SDL_RenderCopy(render, stage1_image, NULL, &stage1_rect);
        SDL_RenderCopy(render, red_bar_image, NULL, &red_bar_rect);
        SDL_RenderCopy(render, blue_bar_image, NULL, &blue_bar_rect);
        SDL_RenderCopy(render, ball_image, NULL, &ball_rect);
        SDL_RenderPresent(render);
    }
        ball_rect.x--;
        ball_rect.x--;
        ball_rect.x--;
        ball_rect.x--;
        ball_rect.x--;

        if (ball_rect.x <= red_bar_rect.x)
        {
        ball_rect.x = 320;
        }
     /*if(1000/FPS > SDL_GetTicks() - start)
      {
       SDL_Delay(1000/FPS - (SDL_GetTicks() - start));
      }*/
        SDL_RenderClear(render);
        SDL_RenderCopy(render, stage1_image, NULL, &stage1_rect);
        SDL_RenderCopy(render, red_bar_image, NULL, &red_bar_rect);
        SDL_RenderCopy(render, blue_bar_image, NULL, &blue_bar_rect);
        SDL_RenderCopy(render, ball_image, NULL, &ball_rect);
        SDL_RenderPresent(render);

}

    SDL_DestroyWindow(window);
    SDL_DestroyRenderer(render);
    SDL_DestroyTexture(stage1_image);
    SDL_DestroyTexture(red_bar_image);
    SDL_DestroyTexture(blue_bar_image);
    SDL_DestroyTexture(ball_image);



}

That looks correct to me. It's probably related to where you tried to define it. Also, I've updated my fork with some more improvements and long explanations about why I did them that way.