I just learned about What is the meaning of "operator bool() const" , now I’m wondering what is the meaning of the second part of this definition?

struct always_false {
  operator bool() const { return false; }
  bool operator = (bool b) const { return b; }
  };

Specifically, what exactly is bool operator = (bool b) const { return b; } doing?

>Solution :

operator=() in a class foo allows you to define what it means to assign something to a foo object, ie:

 struct foo{
    int m_val;
    int operator=(int v){
      m_val = v;
   }
 }
 
 foo ff;
 ff = 42; // actually calls ff.operator=(42)

In your case, foo is in fact always_false, so you can do:

 always_false f;
 f = true; // ignored, and returns true
 f = false; // ignored and returns false

because there is an operator=(bool) defined for always_false.

However, it’s very odd and misleading to assign true since it’s supposed to always be false.