CS253: Software Development with C++

Fall 2021

Const

Show Lecture.Const as a slide show.

CS253 Const

const

Example

const int BOARD_SIZE = 8;
char chessboard[BOARD_SIZE][BOARD_SIZE];
for (int i=0; i<BOARD_SIZE; i++)
    for (int j=0; j<BOARD_SIZE; j++)
        chessboard[i][j] = ' ';

if (chessboard[3][2] == ' ')    // Empty position?
    chessboard[3][2] = 'R';     // put a rook there

cout << chessboard[3][2] << '\n';
R

Reference example

void show_name(const string &name) {
    cout << "The name is: “" << name << "”\n";
}

int main(int, char *argv[]) {
    string s = argv[0];
    show_name(s);
}
The name is: “./a.out”

It’s all relative

int president = 46;
int &prez = president;
const int &potus = president;
prez = 1;   // ok
potus = 2;  // bad
c.cc:5: error: assignment of read-only reference 'potus'

constexpr

Examples

constexpr auto pi = 3.14159;
cout << pi << '\n';
3.14159
constexpr double avo=6.022e23;
avo = 1.234;   // badness
cout << avo;
c.cc:2: error: assignment of read-only variable 'avo'
// Oops: process id not known at compile time:
constexpr double pid = getpid();
cout << pid;
c.cc:2: error: call to non-'constexpr' function '__pid_t getpid()'

Trying to cheat

Let’s try to fool constexpr:

constexpr int answer = 42;
int *p = &answer;
*p = 8675309;
cout << answer << '\n';
c.cc:2: error: invalid conversion from 'const int*' to 'int*'
constexpr int answer = 42;
int *p = const_cast<int *>(&answer);
*p = 8675309;
cout << *p << ' ' << answer << '\n';
8675309 42
Why two different values?

answer is constexpr, so our compiler substituted 42 for answer in the machine code. No fetching, no memory locations—just an inline Platonic 42, unchanged by the cheating code.