VS2013 compiler error with a #define symbol in a function call

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VS2013

In following code:

#define VMIN -210.0f
#define VMAX 220.0f
#define coef_a (0.5f*(VMIN+VMAX))
#define coef_b_green (510.0f/(VMAX-VMIN))
#define coef_b_red (510.0f/(VMIN-VMAX))

//#define LIMITUCHAR(a) ((a) < (255) ? (a) : 255)
uchar limit_uchar(float x)
{
	if (x<255.5f) 
	{
		if (x<0) return (uchar) 0;
		else return (uchar) x;
	}
	else return (uchar) 255;
}

/*
Generates a color as a function of v: if v<vmed, assign red, if v>vmed assign green
At the middle color=black
*/
void assigncolor(float v,uchar color[3])
{
	color[0]=0;
	color[1]=limit_uchar((v-coef_a)*coef_b_green);
	color[2]=limit_uchar((v-coef_a)*coef_b_red);
}

The following line generates compilation error due coef_b_green must be a modifiable lvalue

color[1]=limit_uchar((v-coef_a)*coef_b_green);

How can I avoid it?

What I have tried:

This is OK. coef_b_red does not create problem:

void assigncolor(float v,uchar color[3])
{
	color[0]=0;
	color[1]=limit_uchar((v-coef_a)*510.0f/(VMAX-VMIN));
	color[2]=limit_uchar((v-coef_a)*coef_b_red);
}

I also tried different braquet levels

Posted 13-Mar-18 2:16am

Javier Luis Lopez

Updated 13-Mar-18 3:44am

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3 solutions

Solution 2

CPallini is absolut correct, but I would go a big step further: #define are to avoid WHERE EVER possible because it complicates the readability of code AND the compiler (and so linker and optimization).

My compiler did compile with the above code. I guess that you have some typo (in your original code like invisible chars) or VS 2013 has some issues.

But really: avoid #define because that kills some code processing logic which will help you to write better code and so a better app

Think about the code flow in the range from 255. to 255.4999. Like:

C++

limit_uchar(255.3);

Depending how you "expect" your input the function may run a slight smoother

C++

uchar limit_uchar(float x)
{
  if (x>255.) {
     return (uchar) 255
  }
	
  if (x<0) {
    return (uchar) 0;
  }
  return (uchar) x;
}

By bringing the common cases at the top.

Posted 13-Mar-18 3:08am

KarstenK

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CPallini · Accepted Answer · 2018-03-13T02:46:00

The following code

#include <iostream>
using namespace std;

typedef unsigned char uchar;

#define VMIN -210.0f
#define VMAX 220.0f
#define coef_a (0.5f*(VMIN+VMAX))
#define coef_b_green (510.0f/(VMAX-VMIN))
#define coef_b_red (510.0f/(VMIN-VMAX))

//#define LIMITUCHAR(a) ((a) < (255) ? (a) : 255)
uchar limit_uchar(float x)
{
  if (x<255.5f)
  {
    if (x<0) return (uchar) 0;
    else return (uchar) x;
  }
  else return (uchar) 255;
}

/*
Generates a color as a function of v: if v<vmed, assign red, if v>vmed assign green
At the middle color=black
*/
void assigncolor(float v,uchar color[3])
{
  color[0]=0;
  color[1]=limit_uchar((v-coef_a)*coef_b_green);
  color[2]=limit_uchar((v-coef_a)*coef_b_red);
}


int main()
{
  float v = 0.5;
  uchar c[3];
  assigncolor(v, c);

  for (const auto & x : c)
    cout << static_cast<unsigned int>(x) << endl;
}

Compiles and produces some output in my Linux box.
Please note, C++ offers better alternatives to #define, for instance, you might write

C++

constexpr float VMIN = -210.0f;
constexpr float VMAX = 220.0f;
constexpr float coef_a = (0.5f*(VMIN+VMAX));
constexpr float coef_b_green = (510.0f/(VMAX-VMIN));
constexpr float coef_b_red = (510.0f/(VMIN-VMAX));

Javier Luis Lopez · Accepted Answer · 2018-03-13T03:45:00

I solved the problem by changing:

#define VMIN -210.0f

By:

#define VMIN (-210.0f)

The problem was at the VS compiler line:

#define coef_b_green (510.0f/(VMAX-VMIN))

The compiler do not understand double --

#define coef_b_green (510.0f/(VMAX--210.0f))

NOTE: I accepted the solution-1 but using const:

const float VMIN =-210.0f;
const float VMAX =(220.0f);
const float coef_a= (0.5f*(VMIN+VMAX));
const float coef_b= (510.0f/(VMAX-VMIN));

As long as constexpr was not accepted by my VS2013 compiler.
As result it was increased the speed of assigncolor() function from 400 megaoperationss/second using #define to 454 MOPS using const float

unfortunalely if you arte using C or openCL you should use #define but setting negative values inside parentheses

VS2013 compiler error with a #define symbol in a function call

3 solutions

Solution 1

Solution 2

Solution 3

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