Ch7.1: constexpr Functions

Overview

A constexpr function is a function that may be evaluated at compile time. This chapter focuses only on how to define such a function correctly and what rules apply to its definition.

A constexpr function is also implicitly inline. This allows you to place its definition in a header without causing duplicate symbol errors across translation units.

Defining a constexpr function

To define a constexpr function, place the keyword constexpr before the return type:

constexpr int add(int a, int b)
{
    return a + b;
}

A constexpr function must follow the rules of constant expressions. In practice, this means:

• the function must return a value
• it cannot perform I/O
• it cannot modify global variables

Because constexpr implies inline, defining such functions in header files is safe and avoids multiple-definition errors.

Example

constexpr int square(int x)
{
    return x * x;
}

This function is valid in constant expressions and safe to place in a header file.

Key takeaways

• constexpr defines a function that may run at compile time.
• constexpr functions are implicitly inline.
• They must follow the rules of constant expressions.
• They are safe to define in header files.