Scope Resolution Operator Use Cases

Summary of some common use cases of scope resolution operator.

1. Access namespace mmebers(variables, functions, classes)

namespace A {
    int a = 10;
    void printA() {
        std::cout << a << '\n';
    }
}
 
int main() {
    std::cout << A::x << '\n'; // Outputs 10
    myNamespace::printA(); // Outputs 10
    return 0;
}

2. Access global variables, function, …

int x = 10; // Global x
 
int main() {
    int x = 20;
    std::cout << ::x << '\n'; // Print 10
}

3. Enum class members

enum class Color { RED, GREEN, BLUE };
 
Color myColor = Color::RED;

4. Access static class members

class A {
public:
    static int x;
    static void printX() {
        std::cout << x << std::endl;
    }
};
 
int A::x = 10; // Initialize static member variable
 
int main() {
    std::cout << A::x << '\n';
    A::printX();
    return 0;
}

5. Call class member functions with a pointer

class A {
public:
    void printVal(int x) {
        std::cout << x << std::endl;
    }
};
 
int main() {
    A* p = new A;
    p->A::printVal(10);
    return 0;
}

6. Access base class members

class A {
public:
    int x;
    void foo() {
        std::cout << "A foo(), x = " << x << '\n';
    }
};
 
class B : public A {
public:
    void setValue(int value) {
        A::x = value + 1; // Explicit access to x from the Base class
        A::foo();
    }
    void foo() {
        std::cout << "B foo(), x = " << x << '\n';
    }
};

7. Defining and Using Type Aliases with Namespaces

namespace A {
    // Type alias for an integer vector
    using IntVector = std::vector<int>;
 
    // Another type alias for a pointer to a function returning int and taking two int parameters
    using FuncPtr = int(*)(int, int);
}
 
int main() {
    A::IntVector numbers = {1, 2, 3, 4, 5};
 
    // Using the type alias for a function pointer defined in MyNamespace
    auto lm = [](int a, int b){
        return a + b;
    };
    A::FuncPtr fptr = lm;
    int result = fptr(10, 20);
 
    return 0;
}

8. Resolving ambiguity in multiple inheritance

class A {
public:
    int x = 5;
};
 
class B : public A { // inherits from class A
public:
    int i = 6;
};
 
class C : public A { // inherits from class A
public:
    int i = 7;
};
 
class D : public B, public C { // inherits from both classes B and C
};
 
int main() {
    D obj;
    std::cout << obj.B::x << std::endl; // Accessing x from B's A
    std::cout << obj.C::x << std::endl; // Accessing x from C's A
    return 0;
}