gcc convert int * to enum * error

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C++

#include <stdio.h>
typedef enum{
red=1,blue,green
}color;

void set_color(color *c)
{
  *c = blue;
}

int main()
{
  //int a = 3;
  //set_color(&a);//why this error?
  //set_color((color *)(&a));//It is right
  color c = red;
  set_color(&c); // this is right;
  return 0;
}

i use gcc -S to view the assemble language with set_color((color *)(&a)) and it is the same with set_color(&c),but why gcc does not do the convertion automatically?

Posted 27-Mar-13 1:59am

Keanu L

Updated 27-Mar-13 2:00am

v2

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3 solutions

Solution 1

Because you defined a type. Thus implicit type casting won't work. See: http://en.cppreference.com/w/cpp/language/implicit_cast[^]

Posted 27-Mar-13 2:06am

Zoltán Zörgő

Solution 3

In C (and C++), the datatypes 'int' and 'enum' are partly typesafe. For example:

C

typedef enum {E0 = 0, E1 = 1} ENUMtype_t;

ENUMtype_t e = E0;  // legal
int i = 0;          // legal
i = E0;             // legal
e = 0;              // legal(!)
e = 1;              // error(!)
e = i;              // error
e = (ENUMtype_t) i; // legal

Posted 27-Mar-13 5:08am

H.Brydon

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OriginalGriff · Accepted Answer · 2013-03-27T02:09:00

Solution 2

Because it is trying to prevent you making a mistake, and you not realizing it.
When you write

C++

set_color((color *)(&a));

You are explicitly converting it - telling the compiler "this is the value I wanted to send, I know what I am doing" so it lets it through.

When you write

C++

set_color(&a);

It could be a mistake - you might not have realised that "a" was not a color at all. So it raises an error so you can fix the problem before it becomes a run time problem and causes an odd bug.

Posted 27-Mar-13 2:09am

OriginalGriff

Comments

Keanu L 27-Mar-13 8:16am

In gcc, the int and the emun are allocated the same memory size 4 bytes, so it should be safe.

OriginalGriff 27-Mar-13 8:30am

Doesn't matter - it will be safe because all enums (unless sepcifically stated otherwise) are based on an integer. The compiler is telling you that this could be an unsafe operation from a program logic point of view, rather than from a execution run-time-error point of view.

Keanu L 27-Mar-13 8:52am

Thank you for your explanation. I have underundered the problem.You are right , from the view of compiler i should consider the program logic.