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constexpr variables

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A variable declared constexpr is implicitly const and its value may be used as a constant expression.

Comparison with #define

A constexpr is type-safe replacement for #define based compile-time expressions. With constexpr the compile-time evaluated expression is replaced with the result. For example: 

int main()
{
   constexpr int N = 10 + 2;
   cout << N;
}

will produce the following code: 

cout << 12;

A pre-processor based compile-time macro would be different. Consider: 

#define N 10 + 2

int main()
{
    cout << N;
}

will produce: 

cout << 10 + 2;

which will obviously be converted to cout << 10 + 2;. However, the compiler would have to do more work. Also, it creates a problem if not used correctly.

For example (with #define): 

cout << N * 2;

forms: 

cout << 10 + 2 * 2; // 14

But a pre-evaluated constexpr would correctly give 24.

Comparison with const

A const variable is a variable which needs memory for its storage. A constexpr does not. A constexpr produces compile time constant, which cannot be changed. You may argue that const may also not be changed. But consider: 

int main()
{
   const int size1 = 10;
   const int size2 = abs(10);

   int arr_one[size1]; 
   int arr_two[size2]; 
}

With most compilers the second statement will fail (may work with GCC, for example). The size of any array, as you might know, has to be a constant expression (i.e. results in compile-time value). The second variable size2 is assigned some value that is decided at runtime (even though you know it is 10, for the compiler it is not compile-time).

This means that a const may or may not be a true compile-time constant. You cannot guarantee or enforce that a particular const value is absolutely compile-time. You may use #define but it has its own pitfalls.

Therefore simply use: 

int main()
{
    constexpr int size = 10;

    int arr[size];
}

A constexpr expression must evaluate to a compile-time value. Thus, you cannot use: 

constexpr int size = abs(10);

Unless the function (abs) is itself returning a constexpr.

All basic types can be initialized with constexpr. 

constexpr bool FailFatal = true;
constexpr float PI = 3.14f;
constexpr char* site= "StackOverflow";

Interestingly, and conveniently, you may also use auto: 

constexpr auto domain = ".COM";  // const char * const domain = ".COM"
constexpr auto PI = 3.14;        // constexpr double
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constexpr:
* constexpr
* constexpr variables
* Static if statement
* constexpr functions
Table Of Contents
0 Getting started
1 Literals
2 Basic type keywords
3 Operator precedence
4 Floating point arithmetic
5 Bit operators
6 Bit manipulation
7 Bit fields
8 Arrays
9 Flow control
10 const keyword
11 Loops
12 mutable keyword
13 friend keyword
14 keywords
15 Variable declaration keywords
16 auto keyword
17 Pointers
18 Type keywords (class, enum, struct, union)
19 Classes / structs
20 std::string
21 Enumeration
22 std::atomic
23 std::vector
24 std::array
25 std::pair
26 std::map
27 std::unordered_map
28 std::set and std::multiset
29 std::any
30 std::variant
31 std::optional
32 std::integer_sequence
33 std::function
34 std::forward_list
35 std::iomanip
36 Iterators
37 Basic I/O
38 File I/O
39 Streams
40 Stream manipulators
41 Metaprogramming
42 Returning multiple values from a function
43 References
44 Polymorphism
45 Value and reference semantics
46 Function call by value vs. by reference
47 Copying vs assignment
48 Pointers to class / struct members
49 The this pointer
50 Smart pointers
51 Unions
52 Templates
53 Namespaces
54 Function overloading
55 Operator overloading
56 Lambdas
57 Threading
58 Value categories
59 Preprocessor
60 SFINAE
61 Rule of three, five and zero
62 RAII
63 Exceptions
64 Implementation-defined behavior
65 Special member functions
66 Random numbers
67 Sorting
68 Regular expressions
69 Perfect forwarding
70 Virtual member functions
71 Undefined behavior
72 Overload resolution
73 Move semantics
74 Pimpl idiom
75 Copy elision
76 Fold expressions
77 Unnamed types
78 Singleton
79 ISO C++ Standard
80 User-defined literals
81 Type erasure
82 Memory management
83 Explicit type conversions
84 RTTI
85 Standard library algorithms
86 Expression templates
87 Scopes
88 Atomic types
89 static assert
90 constexpr
91 Date and time with std::chrono
92 Trailing return type
93 Function template overloading
94 Common compile linker errors
95 Design patterns
96 Optimizations
97 Compiling and building
98 Type traits
99 One definition rule
100 Unspecified behavio
101 Argument dependent name lookup
102 Attributes
103 Internationalization
104 Profiling
105 Return type covariance
106 Non-static member functions
107 Recursion
108 Callable objects
109 Constant class member functions
110 C++ vs. C++ 11 vs C++ 17
111 Inline functions
112 Client server examples
113 Header files
114 Const correctness
115 Refactoring techniques
116 Parameter packs
117 Iteration
118 type deduction
119 C++ 11 memory model
120 Build systems
121 Concurrency with OpenMP
122 Type inference
123 Resource management
124 Storage class specifiers
125 Alignment
126 Inline variables
127 Linkage specifications
128 Curiusly recurring template pattern
129 Using declaration
130 Typedef and type aliases
131 Layout of object types
132 C incompatibilities
133 Optimization
134 Semaphore
135 Thread synchronization
136 Debugging
137 Futures and promises
138 More undefined behaviors
139 Mutexes
140 Recursive mutex
141 Unit testing
142 decltype
143 Digit separators
144 C++ Containers
145 Arithmetic meta-programming
146 Contributors