C ++中的void function（）和void ClassName :: function（）有什么区别？

Question

What is the difference between them? Are they both one-per object? (beginner question)

Answer 1

The void function can be called without an instance of any specific class... ie just call...

function()

For ClassName::function() you need to call it on a specific object of type ClassName :

ClassName my_class;
my_class.function();

When called, ClassName::function() will have a hidden implicit function argument this , which can be used inside the function body. It is a pointer to the ClassName object on which ClassName::function has been invoked. You can also access other class members.

Another related difference is that you can take a pointer to function() and call it at any time, whereas when you take a pointer-to-member-function to ClassName::function() you'll also need a specific ClassName object at the time you want to run the function.

Answer 2

A namespace scope function can be called from anywhere without a object:

int foo();

void bar() {
  int x = foo();
}

A member object needs an instance to be called, which is implied within the context of another member function:

struct B {
  int foo();

  int fred() {
    if (true) { 
      return foo(); // OK, local object implied
    }
    return n; // member function can also access member variables
  }

  static int george(); // static members can be called without instances!

  int n;
};

void baz() {
  B b;
  int x = B::george(); // static class members don't need instances.
  int y = b.foo();     // member function on object OK
//int z = foo();       // XXX: this wouldn't compile
}

Answer 3

void ClassName::function()

is call to a function inside your class. member function . and should be called by using object of your class. while,

void function() 

is just a function and can be called without object.