5.6. References to const

5.6. References to `const`
	Chapter 5. Functions

5.6. References to `const`

[ fromfile: functions.xml id: constreference ]

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

#include <QtCore>

class Person {
public:
    void setNameV( QString newName) {
        newName += " Smith";                
        m_Name = newName;
    }
    void setNameCR( const QString& newName) {
//      newName += " Python";               
        m_Name = newName;
    }
    void setNameR( QString& newName) {
        newName += " Dobbs";                
        m_Name = newName;
    }
private:
    QString m_Name;
};

int main() {
    Person p;
    QString name("Bob");
    p.setNameCR(name);                       
//  p.setNameR("Monty");                    
    p.setNameCR("Monty");                   
    p.setNameV("Connie");                   
    p.setNameR(name);                       
    qDebug() << name;         
}

	Changes a temporary that's about to be destroyed.
	Error: Can't change const&.
	Changes the original `QString`.
	No temporaries are created.
	Error: Cannot convert to a `QString`&.
	char* converts to temporary and gets passed by const reference.
	Temporary `QString` #1 is created to convert char* to `QString`. Temporary #2 is created when it is passed by value.
	No temporaries are created.


5.5. Parameter Passing by Reference		5.7. Function Return Values