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4. Main Function and Main Loop

Learn about the structure of the Main function and the main loop.

4.1 Three Parts of the Main Function

  • The Main function can be divided into three main parts
Part Description What to do
Before main loop Section executed first after program startup Screen settings, texture creation, font initialization, etc.
Inside main loop Section inside the loop that repeats dozens of times per second Input processing and drawing
After main loop Section executed when the program ends (If necessary) Saving game data, etc. 
# include <Siv3D.hpp>

void Main()
{


	while (System::Update())
	{

	}
}

# include <Siv3D.hpp>

void Main()
{
	while (System::Update())
	{



	}
}

# include <Siv3D.hpp>

void Main()
{
	while (System::Update())
	{

	}


}
  • When you start developing in earnest, most of your work will be written inside the main loop
  • It's rare to write anything after the main loop

4.2 Main Loop Execution Frequency

  • The main loop repeats according to the refresh rate of the monitor 🖥️ running the program. Generally 60 or 120 times per second
  • If you write a loop like for(;;) in a regular C++ program, it would loop tens of thousands of times per second as long as performance allows, but while (System::Update()) is different
  • The System::Update() function internally performs waiting synchronized with the monitor's display timing, controlling it to be a loop frequency that matches the monitor's display timing (dozens of times per second)
    • It doesn't always match the monitor's refresh rate exactly. Depending on timing, it may slightly fall below the refresh rate
    • If very heavy processing is performed within the main loop, it may fall significantly below
  • Running the following program will display the FPS in the top-left of the screen for confirmation
    • FPS (Frames Per Second) is an indicator of how many times the screen is updated per second
Display how many times per second the main loop repeats (FPS) in the top-left of the screen
# include <Siv3D.hpp>

void Main()
{
	while (System::Update())
	{
		ClearPrint();
		Print << Profiler::FPS();
	}
}

4.3 What to Do Before the Main Loop

  • When you want to display images in your program, the "job of loading images" should be written before the main loop
  • By loading images only once before the main loop to create textures, then drawing them inside the main loop, you can achieve your goal with the least amount of work

OK

  • Before the main loop, load images only once to create textures
  • After that, just draw the already-created textures inside the main loop
# include <Siv3D.hpp>

void Main()
{
	// Load image data from image file and create texture
	const Texture texture{ U"example/windmill.png" };

	while (System::Update())
	{
		// Draw the texture
		texture.draw();
	}
}
  • If you write texture creation inside the main loop, the texture gets destroyed at the end of the loop, so wasteful processing of create-then-destroy is repeated every frame

Repeated create-then-destroy

  • Inside the main loop, images are loaded and textures are created and destroyed every loop
  • When Siv3D detects such problems, it outputs a warning message and ends the main loop
# include <Siv3D.hpp>

void Main()
{
	while (System::Update())
	{
		// Load image data from image file and create texture
		const Texture texture{ U"example/windmill.png" };

		// Draw the texture
		texture.draw();
	} // Texture is destroyed at end of loop
}
  • Don't worry if you accidentally write such code. Siv3D automatically detects "repeated create-then-destroy" and displays a message box to end the program

Review Checklist

  • Learned the three parts of the Main function
  • Learned that System::Update() controls the execution frequency of the main loop
  • Learned to write heavy jobs like "loading images" before the main loop
  • Learned that even if you mistakenly repeat "create-then-destroy" in the main loop, Siv3D automatically detects the problem and ends the program