Modern C++ : constexpr

Following my series from less known features of Modern C++.

Modern C++: An Introduction

Swastik Baranwal ・ Dec 23 '19

#cpp #beginners #tutorial #codenewbie

constexpr means constant at compile time. It was introduced in C++ 11 and later improved in C++14. It is generally used to save time and get the value at compile time.

Syntax

// Declaring variables
constexpr type name = value;

// Declaring functions
constexpr type func_name(...) {
  // body
}

// Declaring classes and methods
class Foo {
   int foo;
 public:
    constexpr Foo(int i) : foo(i){}
    constexpr int GetSize() { return foo; }
}

Difference between const and constexpr?

• const can be used for runtime and compile time but constexpr can only be used for compile time values.
• const is just a constant whereas constexpr is a constant expression.
• const can only be used for non-static function members but not for functions but constexpr can used anywhere but the return type should be a literal.
• All constexpr values are const but not vice versa.

When to use constexpr?

• Size for arrays
• Optimization
• Constant Expression

Rules

1. Variables

• It must be Literal Type
• Initialized at compile time

1. Functions

• Return type must be Literal
• Parameters must be Literal
• Must not contain goto
• No Definition of non literal type

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