C / C++ / MFC

18-Apr-20 8:38

Since today is my day to ask stupid questions, here is another one.
I have some understanding how to pass a function as a parameter.
I can even add and use ONE parameter to such function.
I do not understand the

- syntax.

how to add more parameters.

What I am also missing is syntax on how to read / access , verify passed parameters.
I need to make a decision based on the parameter.

C++

void OpenGL_Stencil(void (*f)(int parameter));
void OpenGL_Stencil(void (*Stencil)(int parameter)) {
	Stencil(0);	// pass parameter ??

PS Take your time , my interned connection is like a yo-yo today.

modified 20-Apr-20 9:24am.

Greg Utas18-Apr-20 9:07

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18-Apr-20 9:07

To add more parameters, just extend the argument list: (int parameter1, int parameter2) and so on.

All you have at this point is a declaration and definition of the function OpenGL_Stencil. Its parameter is a function that returns void and takes one int parameter. The next step is to implement a function with that signature and pass it as an argument to OpenGL_Stencil.

void Something(int value) { /* code */ }

After which invoking OpenGL_Stencil(Something) should cause OpenGL_Stencil to invoke Something.

Vaclav_18-Apr-20 9:22

18-Apr-20 9:22

Yes, I got that far.
I need to read the passed parameter.

C++

<pre>void OpenGL_Stencil(void (*Stencil)(int parameter)) {
	
// clear all three buffers - not neccessary
	glClear(GL_STENCIL_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//enable estecil test
	glEnable(GL_STENCIL_TEST);
// turn off writing to color buffer
	glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
// enable only to verify the stencil
//glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
// setup stencil function
	glStencilFunc(GL_ALWAYS, 1, 1);							// ??
// setup stencil opertion
	glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);					// ??
// ??
	glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
// get stencil
// TDOD need parameter here
//OpenGL_Unit_Circle(0);
	Stencil(0);	// works with OpenGL_Unit_Circle(0) parameter
	glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
//
this is where I am missing how to test the parameter 
	// switch keep area
	//Stencil->paramater == 0;
//	if (Stencil->paramater == 0)
		glStencilFunc(GL_EQUAL, 1, 1);		///keep inside circle
//	else
//		glStencilFunc(GL_NOTEQUAL, 1, 1);	 // keep all outside of stencil
//
	glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);

C++

Greg Utas18-Apr-20 10:24

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18-Apr-20 10:24

I'm not certain what you're asking here.

Inside OpenGL_Stencil, the name of the parameter that was passed in (a function) is Stencil. If you're talking about the int parameter for Stencil, OpenGL_Stencil has to provide that itself. It isn't the parameter associated with Stencil, because that only describes Stencil's function signature and could actually be left out, the same as in a function declaration:

void OpenGL_Stencil(void (*Stencil)(int))

If OpenGL_Stencil doesn't know what value to pass to Stencil, you could provide it like this:

void OpenGL_Stencil(void (*Stencil)(int), int value)

after which you can invoke

Stencil(value)

Vaclav_18-Apr-20 10:52

18-Apr-20 10:52

May try it this way
the parameter is a function - Stencil
such function is passed with its own parameter - such in Stencil(parameter)

how do I access that parameter "value" in OpenGL_Stencil function?
I need to do

if(Stencil(parameter == x )

Greg Utas18-Apr-20 11:04

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18-Apr-20 11:04

You have to pass it as a parameter to OpenGL_Stencil, the way I described in the second half of my post.

Vaclav_19-Apr-20 5:47

19-Apr-20 5:47

OK, I think I have the concept - have passed function return value insed of being void.
Now I am still not sure about the proper syntax.
Here is my code

C++

OpenGL_Stencil_Circle_Build(parameter);         returns parameter OK

OpenGL_Stencil(OpenGL_Stencil_Circle_Build(6)); wrong syntax

And here is the error

Description	Resource	Path	Location	Type
Invalid arguments '
Candidates are:
int OpenGL_Stencil(void (*)(int))
'	A_STENCIL.cpp	/A_STENCIL/src	line 766	Semantic Error
invalid conversion from ‘int’ to ‘void (*)(int)’ [-fpermissive]	A_STENCIL.cpp	/A_STENCIL/src	line 766	C/C++ Problem

The declaration may be the issue ?
int OpenGL_Stencil(void (*f)(int parameter));

Could somebody please reply with correct syntax or help me with declaration ?

Greg Utas19-Apr-20 6:05

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19-Apr-20 6:05

I believe that this

invalid conversion from ‘int’ to ‘void (*)(int)

means that you are passing an int argument when a function argument is expected, specifically a function that takes one int parameter and returns void.

The problem appears to be

C++

OpenGL_Stencil(OpenGL_Stencil_Circle_Build(6))

where, based on previous posts in this thread, I assume you want OpenGL_Stencil to call OpenGL_Stencil_Circle_Build with a value of 6. If that's the case, you did not read my previous post very carefully, because you need to do it as follows:

OpenGL_Stencil(OpenGL_Stencil_Circle_Build, 6)

where OpenGL_Stencil is defined as

C++

void OpenGL_Stencil(void (*func) (int), int arg)
{
   //...
   func(arg);  // this is the line that will call OpenGL_Stencil_Circle_Build with a value of 6
}

SOLVED Re: Passing function with parameters as parameter

Vaclav_19-Apr-20 9:03

19-Apr-20 9:03

It is now working, however, the mechanics are NOT what you have presented.
The initial problem was to pass a parameter to the passed function such as
Stencil(parameter); // parameter);
Now in my poor English interpretation I have
A function OpenGL_Stencil with first argument int (*Stencil)(int) and second argument int parameter
The OpenGL_Stencil returns int which is not currently used.
The first argument - function takes the second argument as a -parameter- and returns it.
The returned value is used to switch the code which follows.
That was the original task , however, I could just use the passed second argument to do the switching, without the use of the return value.
But that is just the way I like to make sure the code is actually processing the first argument - the function.

I believe my original misunderstanding was trying to pass the second argument - parameter _ as a argument to the first argument - the function.

Next task - pass multiple arguments or a pointer....

Many thanks for your help, really appreciate it.
Cheers.

the c programming language 3rd edition - Google Search[^]

Richard MacCutchan18-Apr-20 21:21

Richard MacCutchan

18-Apr-20 21:21

Math problem with C code

Vaclav_18-Apr-20 6:47

Re: Math problem with C code

18-Apr-20 6:47

Just FYI, this is NOT a repost.
As usual I have posted the original in WRONG pew.
However, I found another even weirder failure of simple math case and I really do not have a clue what I am doing wrong.
Are my local variable assignments incorrect ?
Is "printf" lying to me ?

Here is the entire test code

C++

#define MAX_CIRCLES 5

struct TAG_Circles {
	float radius;
	float center;
	float center_x;
	float center_y;

} REAL_Circle[MAX_CIRCLES], IMG_Circle[MAX_CIRCLES];

void OpenGL_Stencil_Circle_Build(void);
// copy last circle to stencil
void OpenGL_Stencil_Circle_Build(void) {
	//
	int index = 5;
	{
		// last circle
		float radius = IMG_circle[index].radius;
		float center_x = IMG_circle[index].center_x;
		float center_y = IMG_circle[index].center_y;
		printf("\n Last circle #%i  radius struct %f radius  %f", index,
				REAL_Circle[index].radius, radius);
		printf("\n Last circle #%i center_x  struct %f center_x  %f", index,
				REAL_Circle[index].center_x, center_x);
		printf("\n Last circle #%i center_y  struct %f center_y  %f", index,
				REAL_Circle[index].center_y, center_y);

	}
}

Here is the output

Last circle #5  radius struct 0.065789 radius  0.131579
Last circle #5 center_x  struct 0.000000 center_x  0.118421
Last circle #5 center_y  struct 0.000000 center_y  0.000000

The questions are NOT what I am trying to accomplish with the code ,
the questions are:

1. why values printed FROM struct DO NOT match assigned values ?
2. why is "radius" TWICE of the value assigned from struct?

PS
Only

radius struct 0.065789

is of correct value.

Cheers

k505418-Apr-20 7:00

k5054

18-Apr-20 7:00

Could it be because you're assigning radius from IMG_circle but printing the value of REAL_circle?

Keep Calm and Carry On

SOLVED Re: Math problem with C code

Vaclav_18-Apr-20 8:16

Re: Math problem with C code

18-Apr-20 8:16

I am not so sure how to answer this (honestly).
Maybe I should get a different hobby - like basket weaving...
For what it is worth
I started with IMG and then realized it was wrong struct, but changed only part of the code !

Richard MacCutchan18-Apr-20 7:02

Richard MacCutchan

18-Apr-20 7:02

Vaclav_ wrote:
Here is the entire test code

As usual it is not the entire code, since none of your struct entities have been initialised with values. Please provide the values of all the variables that you are using, explain what you are trying to calculate, and why you consider the results are wrong.

Pass a struct as a function parameter or just use it as a global variable?

Vaclav_18-Apr-20 5:23

Re: Pass a struct as a function parameter or just use it as a global variable?

18-Apr-20 5:23

Yet another one of my favourites - academic questions.
Hopefully a decent discussion will follow, but please no more RTFM.
Cheers

k505418-Apr-20 6:17

k5054

18-Apr-20 6:17

Unless you have a good, compelling argument otherwise (and there are a few), you should prefer to pass structs as parameters. If its small (e.g. something like a struct timeval), and you don't need to modify the members, its OK to pass by value. If its large, then pass by reference (pointer). If you're not going to modify the struct in the function then you should mark the reference as const.

C++

struct small_obj {
   char description[16];
   int count;
};  // size = 20 bytes

struct large_obj {
    char text[1024][1024];
    double factor[1024][1024];
}; // size = 9 MB

void f1(small_obj arg);  // OK, puts 20 bytes on the stack, f1 gets a copy
                                // of the struct. Any modifications to the struct are
                                // local ot f1

void f2(small_obj* arg);  // stack is 4 bytes only. changes to f2 members will not
                                 // be seen by caller

void f3(const small_obj *arg); // stack is 4 bytes only. attempts within f3 to modify members
                               // should produce compiler warnings

void f4(small_obj& arg);   // similar to f2(), but uses C++ reference type

void f5(const small_obj& arg);   // similar to f3, but uses C++ reference type

void f6(large_obj arg);   // questionable.  This copys 9 MB onto the stack, which might
                          // be more than the OS can handle, and might incur performance
                          // penalties you can avoid by passing as a pointer or reference

The problem with global values, especially global values that are not local to a single file (e.g. declared as static), is it becomes increasingly more difficult to reason about where the variable gets modified. Given a function say print_item(void) you might not expect your global Item to get modified, but print_item() calls foo(void), which calls bar(void) which calls frobnicate(void), which modifies Item. If you are trying to trace down a bug, trying to see where Item gets modified can be difficult. On the other hand, calling print_item(const Item&), you can be (more or less) sure that Item doesn't get modified during the lifetime of print_item() (more or less because print_item() could be nasty and cast away constness, but that's an evil for another tale). Alternatively, you might have print_item(Item foo), which gets a local copy of Item, so the caller knows that whatever changes print_item might make to its copy, the callers Item remains unchanged.

Keep Calm and Carry On

Re: Pass a struct as a function parameter or just use it as a global variable?

Vaclav_18-Apr-20 6:30

computational cost of math functions

18-Apr-20 6:30

Thanks, you may have answered my other problem.
( I'll posting it soon.)

At this point I use struct sort-off attached to a class.
Not static.
I am saying sort-off because the main code is OpenGL and it does not work well in C++ class.
I do not foresee any other code to modify it.
But for learning experience I'll go with passing the struct via pointer to the function which is also "passed" as parameter to another function.
Pretty convoluted , mostly because OpenGL "pipe" architecture.
But so far it id working as expected.

Calin Negru17-Apr-20 5:06

Calin Negru

17-Apr-20 5:06

Is there a listing for the cost of math functions? (how much cpu time math functions take). i.e sin(), asin(), sqrt() ?

OriginalGriff17-Apr-20 5:10

OriginalGriff

17-Apr-20 5:10

No - it depends on the compiler, the library, the OS, the processor, the system loading, the ... you get the idea.
You want to find out? Write code to execute the same function for a huge range of values and run it many, many times. Time that, and average out the run time by the number of sin operations. Ruin this a dozen times, and average out the values.
Then take out the "payload sin call" and repeat the tests. The difference should be the execution time of the sin operation.

"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
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Calin Negru17-Apr-20 6:58

Calin Negru

17-Apr-20 6:58

Thanks, if we talk about functions ( and other math operations, additions, substractions, multiplication, division) with a wide margin cost difference can`t you make a ranking of them.
I want to make a raytracer and I`m just bracing myself against making foolish things/mistakes.

leon de boer17-Apr-20 7:04

leon de boer

17-Apr-20 7:04

That is more generally the compiler/optimizer responsibility. You are better off leaving that sort of thing to the end.

I can highly recommend learning the whole process on a simple software renderer
You will be hard pressed to find a better start point than this link because it's really a couple of simple files.
GitHub - ssloy/tinyrenderer: A brief computer graphics / rendering course[^]

He has a nice easy to follow sequence from raycaster to the renderer being the most complex but on 500 lines of code.
Home · ssloy/tinyrenderer Wiki · GitHub[^]

In vino veritas

modified 17-Apr-20 13:10pm.

Calin Negru17-Apr-20 8:14

Calin Negru

17-Apr-20 8:14

hey leon de boer

Quote:
I can highly recommend learning the whole process on a simple software renderer

I`m familiar to DirectX, I find raytracing intriguing, I want to give it a try.