CS253 Constexpr

constexpr

constexpr, when applied to a variable, means that you can’t change it, like const.
It also means that the value was computed at compile-time.
The compiler is entitled to assume that the value never changes, so 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

constexpr auto pi = 3.14159;
cout << pi << '\n';

3.14159

constexpr double avo=6.022e23;
avo = 1.234;
cout << avo;

c.cc:2: error: assignment of read-only variable 'avo'

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