Rule

• Primitive semantics of logical operators

  • Operators have only two values (true and false)
  • The logic expression can determine the final value without complete calculation (short circuit rule)
  • The final result can only be true or false

Programming experiments: logical expressions

#include <iostream>
#include <string>

using namespace std;

int func(int i)
{
    cout << "int func(int i) : = " << i << endl;
    
    return i;
}

int main()
{
    if( func(0) && func(1) )
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }
    
    cout << endl;

    if( func(1) || func(0) )
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }

    return 0;
}

Output: [short circuit rule]
int func(int i) : = 0
Result is false!

int func(int i) : = 1
Result is true!

Overloading logical operators

Can logical operators be overloaded? What's the point of overloading logical operators?

Programming experiment: overloading logical operators

#include <iostream>
#include <string>

using namespace std;

class Test
{
private:
    int mValue;
public:
    Test(int v)
    {
        mValue = v;
    }
    int value() const
    {
        return mValue;
    }
};

bool operator && (const Test& l, const Test& r)
{
    return (l.value() && r.value());
}

bool operator || (const Test& l, const Test& r)
{
    return (l.value() || r.value());
}

Test func(Test i)
{
    cout << "Test func(Test i) : i.value() = " << i.value() << endl;
    
    return i;
}

int main()
{
    Test t0(0);
    Test t1(1);

    if( func(t0) && func(t1) )                     // Pay attention here!
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }
    
    cout << endl;

    if( func(1) || func(0) )                      // Pay attention here!
    {
        cout << "Result is true!" << endl;
    }
    else
    {
        cout << "Result is false!" << endl;
    }

    return 0;
}

Output:
Test func(Test i) : i.value() = 1
Test func(Test i) : i.value() = 0
Result is false!

Test func(Test i) : i.value() = 0
Test func(Test i) : i.value() = 1
Result is true!

• The essence of the problem

  • The function of C + + extending operator by function call
  • All parameters must be calculated before entering the function body
  • The calculation order of function parameters is uncertain
  • Complete failure of short circuit rule

if( func(t0) && func(t1) ) {}
if( func(t0) || func(t1) ) {}

<==>

if( operator && (func(t0), func(t1)) ) {}
if( operator || (func(t0), func(t1)) ) {}

After logical operators are overloaded, they can't fully implement the native semantics!!

• Some useful suggestions

  • Avoid overloading logical operators in actual engineering development
  • Replace logical operator overloading with overloaded comparison operator
  • Using member functions instead of logical operator overloading
  • Use global functions to overload logical operators [recommended]

Summary

• C + + syntax supports logical operator overloading
• The logic operator after overload does not satisfy the short circuit rule
• Do not overload logical operators in engineering development
• Replace logical operator overloads with comparison operators
• Replacing logical operator overloading with special member functions

The above contents refer to the series courses of Ditai Software Institute, please protect the original!