What is :: in c language

Double colon (::) in C is used for: 1. Global namespace access; 2. Namespace qualification; 3. Enum constant access; 4. Static method call; 5. Base class reference.

Double colon (::) in C

In C, double colon (::) operation The symbol has the following uses:

1. Global namespace access

• Used when accessing global variables or functions not declared in the current namespace.

• For example:

  <code class="cpp">::x = 10; // 访问全局变量 x</code>

2. Namespace qualification

• Specify the variable or function to which Namespaces.

• For example:

  <code class="cpp">namespace std {
  int a;
  }
  
  int main() {
  std::a = 10; // 访问 std 命名空间中的变量 a
  }</code>

3. Enumeration constant access

• Access in enumeration type used when using constants.

• For example:

  <code class="cpp">enum Color {
  Red,
  Green,
  Blue
  };
  
  int main() {
  Color color = ::Color::Red; // 访问枚举常量 Red
  }</code>

4. Static method call

• Used when calling static class methods .

• For example:

  <code class="cpp">class MyClass {
  public:
    static void print() {
      cout << "Hello!" << endl;
    }
  };
  
  int main() {
  MyClass::print(); // 调用静态方法 print
  }</code>

5. Base class reference

• Reference the base in the derived class Used when class.

• For example:

  <code class="cpp">class Base {
  public:
    void print() {
      cout << "Base class" << endl;
    }
  };
  
  class Derived : public Base {
  public:
    void print() {
      ::Base::print(); // 引用基类方法 print
    }
  };</code>

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