References to const

Declaring a reference parameter to be const tells the compiler to make sure that the function does not attempt to change that object. For objects larger than a pointer, a reference to const is an efficient alternative to a value parameter because no data is copied. Example 5.14 contains three functions, each accepting a parameter in a different way.

Example 5.14. src/const/reference/constref.cpp

class Person {

 public:
 void setNameV( QString newName) {
 newName += " Smith"; <-- 1
 m_Name = newName;
 }

 void setNameCR( const QString& newName) {
// newName += " Python"; <-- 2
 m_Name = newName;
 }
 void setNameR( QString& newName) {
 m_Name += " Dobbs"; <-- 3
 m_Name = newName;
 }
 private:
 QString m_Name;
};

int main() {
 Person p;
 QString name("Bob");
 p.setNameCR(name); <-- 4
// p.setNameR("Monty"); <-- 5
 p.setNameCR("Monty"); <-- 6
 p.setNameV("Connie"); <-- 7
 p.setNameR(name); <-- 8
 cout << name;
}

 

(1)Changes a temporary that's about to be destroyed.

(2)Error: Can't change const&.

(3)Changes the original Qstring.

(4)No temporaries are created.

(5)Error: Cannot convert to a QString&.

(6)char* converts to temporary and gets passed by const reference.

(7)Temporary QString #1 is created to convert char* to QString. Temporary #2 is created when it is passed by value.

(8)No temporaries are created.