Question

Okay, so I’m calling a function that draws three lines to display an ‘I’ on-screen.
Then, I call another function which calls this function, but adds 1 to the x variable to
make a the letter bold. Then I want to put x++ add in the ‘tick’ function, which executes
every frame. It isn’t working, but why? If you don’t understand what I mean, please check this page, it’s the tutorial I’m following: http://www.devmaster.net/articles/intro-to-c++-with-game-dev/part3.php

Declare global x and y and call function tick:

int x = 0;
int y = 0;
void Game::Tick( float a_DT )
{
    m_Screen->Clear( 80 );
    DrawI(0,0);
    x++;
}

Making functions:

void Game::DrawI(int x, int y)
{
    m_Screen->Line( 100 + x, 50 + y, 200 + x, 50 + y, 0xffffff );
    m_Screen->Line( 150 + x, 50 + y, 150 + x, 300 + y, 0xffffff );
    m_Screen->Line( 100 + x, 300 + y, 200 + x, 300 + y, 0xffffff );
}

void Game::DrawFatI()
{
    DrawI(1,0);
    DrawI(0,1);
    DrawI(0,0);
    DrawI(1,1);
}

Thanks for checking.

Answer 1

When you call DrawI it has x and y passed to it as parameters:

void Game::DrawI(int x, int y)

These mask the global x and y, such that within DrawI everywhere you say x and y it refers to the function parameters. (You can access the global ones using ::x and ::y though).

Since you always call DrawI with literals, not variables, e.g. DrawI(1,0); the values of x and y in DrawI never change, they are always just these fixed values and not the global variable, hence Line is always passed the same values, even if your global x has changed.

I would make two recommendations to avoid this:

1. Don’t have multiple variables with the same name where one masks the other
2. Don’t use globals, unless there’s a good reason to. (This doesn’t seem to be one as far as I can see)