C++ - How can we call a class without calling its attributes? - c++

Question

I need to implement the class Multiplier for a school exercise, but I do not understand how the teacher was able to call prod() without calling its inputs.
The goal of the code is to read a sequence of integers until the product of their absolute values is greater than 200.
Can somebody help me understanding please?
Here is the code:
#include <iostream>
using namespace std;
int main()
{
Product mult(200);
cout << "Enter numbers: " << endl;
do{
cin >> mult;
} while(!mult.exceed_limit());
cout << "The absolute values product is " << mult() << " . " << endl;
return 0;
}

Answer 1

A class can implement the "call" operation by overloading the operator() member function.
For example
class MyType {
public:
void operator()(int param) const {
std::cout << "MyType(int) called with: " << param << "\n";
}
void operator()() const {
std::cout << "MyType() called\n";
}
};
int main() {
MyType instance;
instance(12);
instance();
return 0;
}

Answer 2

Multiplier prod(100); - Multiplier must have defined a constructor that takes an integer as input, eg:
class Multiplier
{
...
public:
Multiplier(int value);
...
};
cin >> prod - Multiplier must have overloaded operator>> for input, eg:
class Multiplier
{
...
};
istream& operator>>(istream&, Multiplier&);
prod.limit_exceeded() - Multiplier must have defined a member limit_exceeded() method, eg:
class Multiplier
{
...
public:
bool limit_exceeded() const;
...
};
cout << prod() - Multiplier must have overloaded operator() (and the return value is then streamed to cout via operator<<), eg:
class Multiplier
{
...
public:
int operator()() const;
...
};

Answer 3

Lets see what we need
int main()
{
Multiplier prod(3);
A constructor. The parameter is probably the number of factors to be multiplied.
std::cout << "Enter numbers: " << std::endl;
do{
std::cin >> prod;
A way to "input" the factors.
} while(!prod.limit_exceeded());
A method to see if the entered factors equals the number of desired factors.
std::cout << "The product of the absolute values is " << prod() << " . " << std::endl;
A call operator that returns the resulting product.
return 0;
}
So lets do that:
struct Multiplier {
Multiplier(size_t n) : max_factors(n),num_factors(0),product(1) {}
size_t max_factors;
size_t num_factors;
double product;
double operator()() const { return product;}
bool limit_exceeded() const { return max_factors <= num_factors;}
};
Constructor takes number of factors, product holds the result, operator() returns the result and limit_exceeded() checks if all factors have been entered. Finally, a an overload for operator>> to read the factors:
std::istream& operator>>(std::istream& in, Multiplier& m){
double x;
if (in >> x) {
m.product *= x;
++m.num_factors;
}
return in;
}
It is a bit uncommon for std::cin >> prod; to not read prod but instead to modify prod, but thats fine.
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