Thread: Want a function that accepts either a bool value or a function ptr that returns bool

Basically, I have a function called "monitor" and basically, this function works by waiting until some expression is false, and then it returns.

Right now, for this specific instance, it compares two uint16_t's. If they are equal, it will "sleep" with a condition variable. When one of the uint16_t's is changed, the changing thread will signal on the condition variable, and the function will wake up, do the comparison, and finally return.

How can I generalize this "monitor function" so that it could accept a generic function that returns a bool value?

For example, right now, I have

Code:

void Monitor(uint16_t start_val, uint16_t &tail)
{
     mutex.lock()
     while (start_val == tail) {
          condition.wait(mutex);
     }
}

but I want to generalize the input parameters to accept a boolean function. The programmer can specify any number of parameters to this boolean function, so basically, the loop would be

Code:

void Monitor(bool function)
{
     mutex.lock()
     while (function(param p1, param p2, ..., param pn) {
          condition.wait(mutex);
     }
}

Is there any way to do this?

Considering that you want to pass "any number of parameters", you would probably need a variadic template:

Code:

template<typename Function, typename... Arguments>
void Monitor(Function function, Arguments... arguments)
{
    mutex.lock();
    if (function(arguments...))
    {
        condition.wait(mutex);
    }
}

Originally Posted by joshuastuden

but how do I invoke Monitor, which is a member function of a class?

obj->Monitor(compare(val1, val2, val3));

?

Notice that it invokes the function for you with the supplied arguments, so of course you cannot just call the function: you would end up with what the function returns, i.e., a bool, which is of course not what the function template expects. Hence:

Code:

obj->Monitor(compare, val1, val2, val3);

Originally Posted by joshuastuden

How do I ensure that the function returns a boolean?

On one hand, that's complicated. The solutions typically lie in something called concepts, but I'm afraid I have never used them, and as it stands it is on the path to standardisation but was rejected from the most recent version of the C++ standard (C++17), though a version of it may eventually be published in C++20. So generally, I'd say you're out of luck unless you want to go with a non-standard implementation, e.g., Boost Concept Check Library.

On the other hand, that's simple: you don't. If the function doesn't return something that can be implicitly converted to bool, a compile error will result.

By the way, this function is probably wrong:

Code:

bool compare(int a, int b, int c)
{
     return a == b == c;
}

If your intention was to compare to see if a and b and c are all equal to each other, then you should have written:

Code:

return a == b && b == c;

Otherwise, suppose a = 0, b = 0, and c = 0. Then a == b evaluates to true, and true == c evaluates to false since 0 is converted to false, and true == false evaluates to false.