What does :: mean in c++

The double colon (::) in C is mainly used for the following purposes: accessing elements in the global namespace. Access static members of a class. Specify the inheritance order in multiple inheritance. Casting.

Double colon (::) in C

In the C programming language, double colon (:: ) is a range resolution operator with the following uses:

• #Accessing elements in the global namespace:
  If an element does not explicitly belong to namespace, you can use the :: operator to access declarations in its global namespace. For example:

  <code class="cpp">::std::cout << "Hello, world!"; // 输出 "Hello, world!" 到控制台</code>

• Accessing static members of a class:
  You can use the :: operator to access static members of a class (for example, static methods or static variables ) without creating an instance of the class. For example:

  <code class="cpp">class MyClass {
  public:
      static int myStaticVariable;
      static void myStaticMethod() {}
  };
  
  int main() {
      ::MyClass::myStaticVariable = 10; // 访问静态变量
      ::MyClass::myStaticMethod(); // 调用静态方法
  }</code>

• Specify the inheritance order in multiple inheritance:
  In multiple inheritance, you can use the :: operator to specify the inheritance order, especially It is when a subclass overrides a member of the base class with the same name. For example:

  <code class="cpp">class Base1 {
  public:
      void foo() { std::cout << "Base1::foo()\n"; }
  };
  
  class Base2 {
  public:
      void foo() { std::cout << "Base2::foo()\n"; }
  };
  
  class Derived : public Base1, public Base2 {
  public:
      void foo() { Base2::foo(); } // 指定从 Base2 继承 foo() 方法
  };</code>

• Forced type conversion:
  You can use the :: operator to force one type to another type, for example:

  <code class="cpp">int x = 10;
  double y = ::static_cast<double>(x); // 将 int x 转换为 double y</code>

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