枚举指针可以指向常量值吗？

Question

I am working on some code where I have an 'enum' defined to describe the type of error. Whenever a certain function is called, it takes an 'enum pointer' as a parameter. The function will then set the passed in error variable with a 'constant enum' value. When I run the program I get a run-time error. I have provided some of the code to describe my issue:

enum error
{
    No_Exist=0, 
    No_Error,
    Unknown,
};

bool process_something(..., error *err)
{
    ....
    ....
    ....
    *err = No_Error;
    return 1;
}

int main()
{
    error *err_val; 
    if(process_something(...,err_val))
    {
    .....
    .....
    }
    .....
    return (0);
}

I want to know whether I can assign the pointer enum variable with the value 'No_Error' or not?

Answer 1

When ever you see a function like:

void foo(int*);

It doesn't mean you need to pass an explicitly declared int* variable. Following is wrong usage:

int* p;
foo(p);

Following is correct, but not proper:

int* p;
int a;
p=&a;
foo(p); // p points to a

Following is also correct (but again not proper, IMO):

 int* p;
 p = new int;
 foo(p); // p points to allocated memory

The most correct usage (as per the problem in hand) would be:

int a;
foo(&a);

Remember that int* (or any T* ) says that pass me an address , and doesn't say pass me a pointer. Another example would be with reference. If a function is like:

void bar(int&);

Doesn't mean you must declare a int-reference, and pass it:

int a;
int &r = a;
bar(r);

But instead pass a itself. Function needs a reference (L-value).

Answer 2

This:

int main()
{
    error *err_val; 
    if(process_something(...,err_val))

should be:

int main()
{
    error err_val; 
    if(process_something(..., &err_val))

You need to pass the address of a valid error to create a pointer to somewhere process_something() can actually write. Your code triggers undefined behavior.

Answer 3

Now if you must use what you are using you should do it by reference.

For example,

int readnumber(string userinput, bool* ok);// or your enum instead of a boolean
//
bool okay;
auto val = readstring(someinput,&okay);
if (!okay)
{
    cout << "Something weird but we don't want that to crash our program";
}