Show Lecture.Const as a slide show.

CS253 Const

const

• const, when applied to a variable, means that you can’t change it.
• It’s like a final variable in Java.
• It does not mean that the memory is read-only; it just means that you can’t write it.
• A given piece of memory may be read-only to one part of a program, but read-write to another.
• const is also used to indicate an accessor (getter) method.

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”

• The string is read-write in main(), but read-only in show_name().
• The string memory itself is read-write. It’s just that show_name() promises not to change it.

It’s all relative

int president = 0;
int &prez = president;
const int &potus = president;
prez = 1;   // ok
potus = 2;  // bad

c.cc:5: error: assignment of read-only reference 'potus'

• Wait—is president read-only or read-write?
• It depends how you access it.
• prez is a read-write view (aspect, avatar) of president.
• potus is a read-only view of president.
• Same as how my lunch is read-write to me, but read-only to you.

constexpr

• constexpr, when applied to a variable, means that you can’t change it, it never changes, and was computed at compile-time.
• Since the value never changes, the compiler it can substitute the value wherever you use the variable.
• Use it for true constants:
  • π, Avogadro’s Number
  • number of days in a week
  • number of squares in a chessboard
• We used to do this in the preprocessor with #define TRIPLE_POINT 32.018, which still works, but this is better.
• It’s a shame to call it a “variable” if it never “varies”, but that’s how we talk.

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