C++ const references and non const references

A const reference is a reference to a const object.

 
const int i = 10; 
const int &ref = i; 

Ref can be read, but not modified. This makes sense, because I itself cannot be modified, and of course cannot be modified by ref. So it is illegal to assign a const variable to a non-const reference.

 
int &ref1 = i; // error: nonconst reference to a const object 

A non-const reference is a reference to a variable of type non-const.
Const references can be initialized to different types of objects or right values (such as literal constants), but not non-const references.

 
// legal for const references only 
int i = 10; 
const int & ref = 42; 
const int & ref1 = r + i; 
double d = 3.14; 
const int &ref2 = d; 

In the case of binding to different types of ref2, the compiler converts the code associated with ref2 as follows:

 
int temp = d; 
const int &ref2 = temp; // bind ref2 to temporary 

Ref2 is actually bound to a temporary variable, and if ref2 is not const, then it is reasonable to change the value of d by modifying ref2, but d does not change. So to avoid this problem, ref2 can only be const.

A non-const reference can only be bound to an object of the same type as the reference, whereas a const reference can be bound to an object of a different but related type or to an rvalue.