VS2013编译器错误，函数调用中带有#define符号 [英] VS2013 compiler error with a #define symbol in a function call

问题描述

在以下代码中：

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);
}

以下行生成编译错误 coef_b_green 必须是可修改的左值

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);

如何避免它？



< b>我尝试了什么：



这没关系。 coef_b_red 不会产生问题：

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);
}

我也试过不同的braquet等级

I also tried different braquet levels

推荐答案

以下代码

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;
}

在我的Linux机器箱中编译并生成一些输出。

请注意， C ++ 为 #define 提供了更好的替代方案，例如，你可以写一下

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

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));

CPallini absolut正确，但我会更进一步：#define是为了避免WHERE EVER成为可能，因为它使代码和编译器（以及链接器和优化）的可读性变得复杂。



我的编译器使用上面的编译器码。我猜你有一些拼写错误（在你的原始代码中看起来像隐形字符）或者VS 2013有一些问题。



但是真的：避免#define因为它会杀死一些代码处理逻辑，它将帮助您编写更好的代码，以便更好的应用程序



想想代码流程从255到。 255.4999。喜欢：

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:

limit_uchar(255.3);

根据您期望输入的方式，该功能可能会略微平滑

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

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

将常见案例置于最顶层。

By bringing the common cases at the top.

我通过更改解决了问题：

I solved the problem by changing:

#define VMIN -210.0f

By：

By:

#define VMIN (-210.0f)

问题发生在VS编译器行：

The problem was at the VS compiler line:

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

编译器不懂双 -

The compiler do not understand double --

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

注意：我接受了解决方案-1但使用了const：

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));

只要未接受constexpr我的VS2013编译器。

结果它使用 #define 从400 megaoperationss /秒增加 assigncolor（）功能的速度到454 MOPS使用 const float





不幸的是，如果你使用C或openCL，你应该使用#define但设置负值在括号内

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

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