Cpp 8: Constant Expressions

1. What are Constants?

Two Types of Constants:

• Named:
  • has identifier, by placing attribute const adjacent to object type:
  • const double gravity1;: compilation error: const variables must be initialized
  • const double gravity2{9.8};: ok
• Literals:
  • no identifier
  • 5;
  • 1.2;
  • "Hello world!"

2. What are Constant Expressions?

The standard defines a constant expression:

• which is an expression that must be evaluatable at compile-time

Containing the following:

• Literals (e.g. 5, 1.2)
• Most operators with constant expression operands (e.g. 3 + 4, 2 * sizeof(int)).
• Const integral variables with a constant expression initializer (e.g. const int x { 5 };)
  • This is a historical exception, constexpr variables are preferred (in modern C++).
• Constexpr variables (future topic “Constexpr variables”).
• Constexpr function calls with constant expression arguments (future topic “Constexpr functions”).

3. Quiz Constants

For each statement, identify:

• Whether the initializer is a constant or non-constant expression.
• Whether the variable is a constant or non-constant expression.

Qn	C++ Statement	Initializer	Variable
a)	char a { 'q' };	q (const) - a literal	a (non-const) const keyword unused
b)	const int b { 0 };	0 (const) - a literal	b (const) - const int variable - with const express initializer 0
c)	const double c { 5.0 };	5.0 (const) - a literal	c (non-const) - non-integral variable Note: only const integral variables with a constant expression initializer are compile-time constants
d)	const int d { a * 2 }; - where char a { 'q' };	a*2 (const) - a is non-const as missing const keyword	d (non-const) - a is non-const exp initializer
e)	int e { c + 1.0 }; - where const double c { 5.0 };	c (non-const) - its a non-integral variable (double)	e (non-const) - missing const keyword - non-integral initializer
f)	const int f { d * 2 }; - where const int d { 0 };	d (const) - integral const expresssion - with const exp initializer 0	f (const) -const integral variable - with const expres init d
g)	const int g { getNumber() }; - where getNumber returns an int by value	getNumber() (non-const)	g (non-const)
h)	const int h{};	default initializer 0 (const)	h (const) - integral variable - with const expression init