无法从“ void”转换为“ int”

Question

Cannot convert from "void" to "int" in C++ - anyone know why? is there a function i have to use?

int calc_tot,go2;
go2=calculate_total(exam1,exam2,exam3);
calc_tot=read_file_in_array(exam);

Answer 1

go2=calculate_total(exam1,exam2,exam3);
calc_tot=read_file_in_array(exam);

My guess would be that one of those two functions returns a void, so you can't assign that value to an int. Since a "void" function doesn't return anything, you can't assign its return value to an int.

I would expect code like this to give you such an error:

void voidfunc () {
  // Do some things
}

int main () {
    int retval = voidfunc();
    return 0;
}

Though my compiler gives:

$ g++ main.c
main.c: In function ‘int main()’:
main.c:6: error: void value not ignored as it ought to be

Answer 2

void is the same as saying no type. There is no information in a void. You cannot convert no information into a number, hence the error.

Perhaps if you give us more information about the types of your functions, or where the exact error occurred, we can help you more.

Answer 3

Per your comments, calculate_total is declared wrong. If the function needs to return a value, it should be declared as:

int calculate_total(/*...*/);

Notice the int in front of the function name, instead of void .

And in the function body:

int calculate_total(/*...*/)
{
  int total = 0;
  /* ... calculate the total */
  return total;  // Return the total.
}

If you insist on having a function returning void , you can add another argument to the function:

void calculate_total(int * total, /*...*/);

The function then becomes:

void calculate_total(int * total, /*...*/)
{
  if (total) /* check for null pointers */
  {
    *total = 0;
    for (/*...*/)
    {
      *total += /*...*/
    }
  }
  return;
}