Warning: Function will always return True [closed] - c++

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I have an isPrime function that is always returning true, regardless of the inputted number. This is the same for a couple other bool functions in my program.
My simple isPrime function:
bool isPrime(mpz_class num)
{
bool prime = true;
for (int i=2; i<num; i++)
if (num % i == 0)
prime = false;
return prime ;
}
Calling it (I suspect this is where the problem is but I don't know what the problem is):
do
{
do
{
cout << "Enter Prime 1: ";
getline(cin, sa);
isNum(sa);
firstPrime = sa;
}
while(!isNum);
isPrime(firstPrime);
}
while(!isPrime);
The isNum function is also returning "true" every time.
Runtime error:
warning: the address of 'void isNum(std::string)' will always evaluate as 'true' [-Waddress]|
Does anyone see the problem?

Answer 1

You need to store the return value from the call to isNum() before you can check it in the while look. What you're doing now with
while(!isNum)
is that you're checking the address of the function and not it's return type. What you probably mean to do is something like this:
bool isPrimeRetVal;
do
{
bool isNumRetVal;
do
{
cout << "Enter Prime 1: ";
getline(cin, sa);
isNumRetVal = isNum(sa);
firstPrime = sa;
}
while(!isNumRetVal);
isPrimeRetVal = isPrime(firstPrime);
}
while(!isPrimeRetVal);
Note the same issue with while(!isPrime).

Answer 2

You ignore the return value of isPrime and instead check the value of isPrime (which doesn't change, it's always a valid function).

Answer 3

You are not calling the function by merely mentioning its name, e.g. with while (!isNum).
The function name in that context evaluates to a pointer to that function. Since all non-null pointers evaluate to boolean true and a pointer to any function cannot be equal to the null pointer, isNum always evaluates to true and !isNum is always false.
Instead of this:
do { ... isPrime(x); } while (!isPrime);
you need to write something more like this:
do { ... } while (!isPrime(x));

Answer 4

These two lines:
isPrime(firstPrime);
while(!isPrime);
combine to suggest you're not clear on how functions are called in C++. The first line calls the function and then throws the result away. The second line doesn't even call the function, it just looks at its address (which will never be zero).
You probably want:
while(!isPrime(firstPrime));
This passes firstPrime to the function, and then looks at the return value of the function.

Answer 5

You are not using return value of isPrime function. IsPrime is a function and you should store its value in some variable and use that variable in while loop.
The same goes with the isNum variable.