C++ Const

Const Memory

Changing the value of a const:

• Storage for a const value may be changed by using pointers.
• Compilers may choose to just replace occurances of the const with its value.
• See http://cpp.sh/86yky for an example.

const int year = 2020;
int * time_machine;
time_machine = (int *)&year;
*time_machine = 2019;
const int *year_stored = &year;
std::cout << *year_stored << std::endl; // output: 2019
std::cout << year << std::endl; // output: 2020

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Const pointers

Pointer to a const value (i.e. can’t change the value)

const int *ptr;
int const *ptr; // does the same as the line above

const pointer to a value (i.e. can’t change the pointer)

int * const ptr;

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Const with Classes

const in class methods:

• Indicates that class member variables won’t be modified in this method

class TimeMachine {
private:
	int year;
public:
	int get_year() const {
		return year;
	}
}

// When accessing a reference of a class, we can only use the const methods
int my_age(const TimeMachine& time_machine) {
	// This is an error if get_year is not const
	return time_machine.get_year() - 1988; 
}