Understanding constexpr and Constant Expressions in C++

Introduction

In the realm of C++, understanding the nuances of constant expressions and constexpr variables is crucial. This article aims to shed light on these concepts and their applications.

Constant Expressions

Constant expressions are expressions whose values cannot change and can be evaluated at compile time. Examples include literals (e.g., 10), and const objects initialized by constant expressions (e.g., const int max_files = 20).

constexpr Variables

The new standard (C++11) introduces constexpr to explicitly declare a variable as a constant expression. The benefits of using constexpr are twofold:

• The compiler verifies that the initializer is indeed a constant expression.
• It improves code readability by clearly stating the intent.

The syntax for declaring a constexpr variable is constexpr int mf = 20;. It’s important to note that constexpr variables are implicitly const. However, the initializer must be a constant expression itself and cannot use ordinary functions unless they are defined as constexpr functions.

Examples:

constexpr int mf = 20; // 20 is a constant expression

constexpr int limit = mf + 1; // mf + 1 is a constant expression

constexpr int sz = size(); // ok only if size is a constexpr function        

Although we cannot use an ordinary function as an initializer for a constexpr

variable, the new standard lets us define certain functions as constexpr. Such functions must be simple enough that the compiler can evaluate them at compile time. We can use constexpr functions in the initializer of a constexpr variable.

Literal Types

Literal types are suitable for constexpr declarations because they are simple enough for compile-time evaluation. Examples include arithmetic types, references, and pointers (with limitations). Non-literal types include classes and library types.

Example:

constexpr int *q = nullptr;  // q is a constexpr pointer        

Pointers and constexpr

When applied to pointers, constexpr applies to the pointer itself, not the pointed-to type. This creates two distinct types:

• const int *p: A pointer to a const int (low-level const)
• constexpr int *q: A constant pointer to int (top-level const)

Note that constexpr pointers can point to const or non-const types.        

Initialization of constexpr Pointers and References

Initializing constexpr pointers and references has certain limitations:

• Literal values: You can initialize with nullptr or the literal (constant expression) 0.
• Fixed-address objects: You can point to (or bind to) objects with a fixed memory location throughout the program. This includes global variables (objects defined outside any function) and static local variables (special local variables within functions that persist across function calls).
• Restrictions: Local variables inside functions generally don’t have fixed addresses and cannot be used with constexpr pointers/references.

Example:

constexpr int *np = nullptr; // np is a constexpr pointer initialized with nullptr

static int obj = 0; // obj is a static local variable

constexpr int *p = &obj; // p is a constexpr pointer pointing to a static local variable        

Key Points

To summarize:

• Use constexpr for variables intended as constant expressions.
• Literal types ensure compile-time evaluation for constexpr declarations.
• Understand the distinction between top-level and low-level const with constexpr pointers.
• Be aware of initialization limitations for constexpr pointers and references.

For further reading, check out these articles:

• Sharing Non-Constant Expression Initializers for const in C++
• Constant Folding in C++
• Demystifying Top-Level vs. Low-Level const