The difference between :: and : in c++

Double colon (::) is used for namespace scope resolution and class static member access, and single colon (:) is used for base class initialization and implicit type conversion.

The difference between :: and : in C

In the C programming language, double colon ( The ::) and single colon (:) operators have different usage and meaning.

Double colon(::)

Double colon(::) The operator is used in the following scenarios:

• Namespace scope resolution: Used when accessing elements in external namespaces in nested namespaces. For example:

namespace outer {
  int x = 10;
}

namespace inner {
  void printX() {
    std::cout << outer::x << std::endl;
  }
}

• Class static member access: Used when accessing static member functions or variables of a class. For example:

class MyClass {
public:
  static int numInstances = 0;
  
  static void printNumInstances() {
    std::cout << numInstances << std::endl;
  }
};

Single colon(:)

Single colon(:) The operator is used in the following scenarios:

• Base class initialization: Used when initializing base class members in the derived class constructor. For example:

class Base {
public:
  int x;
};

class Derived : public Base {
public:
  Derived(int x) : x(x) {}
};

• Implicit type conversion: Used when coercing a value of one type to another type. For example:

int x = 10;
double y = static_cast<double>(x);

Summary

Double colon (::) is used for namespace scope resolution and class static member access, while single colon (:) Used for base class initialization and implicit type conversion.

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