Thread: passing operator as a parameter

Hi,

I want to have a function that evaluates 2 char vars (like a truth table)

for example:

since

T | F : T

then

Code:

myfunc('t', 'f', ||);  /*defined as: bool myfunc(char lv, char rv, ????)*/

should return true;

how can I pass the third parameter? (I know is possible to pass it as a char* but then I will have to have another table to compare operator string and then do the operation which is something I would like to avoid)

Is it possible to pass an operator like ^(XOR) or ||(OR) or &&(AND), etc in a function/method?

Thanks in advance

Originally Posted by tabstop

You could pass in a function (like operator||). If you do, though, keep in mind that operator||('f', 'f') is true.

Also keep in mind that operators cannot be overloaded for basic types - to overload an operator, a user defined type - usually a class - needs to be involved.

It is not, for example, possible to overload operator|| for two char's.

To answer the original question, yes it is necessary to implement some form of mapping between data passed (eg a string "or") and the operation performed.

Why do you want to compare characters? Probably it would be better to create a function that converts chars to booleans, and then it is easy to use any standard operators.

Code:

bool CharToBool(char c)
{
    if (c=='t' || c=='T') return true;
    return false;
}

int main()
{
    bool r;
    r=CharToBool('t')||CharToBool('f');
    r=CharToBool('T')&&CharToBool('F');
}

It sounds like you want to use function objects or function pointers. (On second thought, you never want to use function pointers in C++)

Here's an example using templates.

Code:

template <class T> bool myfunc(bool lv, bool rv, T oper)
{
	return oper(lv, rv);
}

struct Or
{
	bool operator()(bool lv, bool rv)
	{
		return lv || rv;
	}
};

int main()
{
	bool result = myfunc(true, false, Or());
	return 0;
}

Here's the same thing using inheritance

Code:

struct LogicalOperator
{
	virtual bool operator()(bool lv, bool rv) const = 0;
};

struct Or: public LogicalOperator
{
	bool operator()(bool lv, bool rv) const
	{
		return lv || rv;
	}
};

bool myfunc(bool lv, bool rv, const LogicalOperator& oper)
{
	return oper(lv, rv);
}

int main()
{
	bool result = myfunc(true, false, Or());
	return 0;
}

I have the feeling you won't understand all of this, so don't be afraid to ask questions.