如何将 const 引用分配给非 const 变量？

Question

Consider following code, onData callback invoked and then I assign the const reference to a variable. I want to understand how it works. I was told you can't modify const references but here it looks like you can by assigning to a non-const variable.

void onData(const evpp::TCPConnPtr &socket) {
   evpp::TCPConnPtr connection = socket;
}

In real world application, I assign this to Object Instance's member variable.

Answer 1

In simple terms, a reference is just an alias to the referenced object. When a reference is const it means you may not modify the object via that reference (the object can be modified via other means, this distinction is important, especially with multithreading). Copying a const reference is like copying the referenced object:

 std::string x = "foo";
 const std::string& x_const_ref = x;  // constant reference to x
                                      // x can be modifed but not via x_const_ref
 //x_const_ref = "moo";  // ERROR !
 x = "bar";              // OK !
 std::string y = x;                   // y is initialized with a copy of x
 std::string z = x_const_ref;         // z is initialized with a copy of x 

y and z will both hold the string "bar" , though x is not modified in the last two lines. And if TCPConnPtr has no weird copy constructor then thats basically also what happens in your code (if it does try to modify socket you would get a compiler error).