Modern C++: const and constexpr

This blog post is part of the series Modern C++.

Introduction

In the realm of C++, ensuring data integrity and optimizing performance are paramount. Two keywords that play a pivotal role in achieving these goals are const and constexpr. While they might seem similar at first glance, they have distinct purposes and usage scenarios. Let's dive into the nuances of these keywords and how they empower modern C++ development.

const: The Guardian of Immutability

The const keyword is your trusted ally when it comes to enforcing immutability. When you declare a variable as const, you're essentially making a promise that its value won't change after initialization. This has several benefits:

• Protection Against Accidental Modification: Mistakenly altering a critical value can lead to unexpected behavior. const prevents such mishaps, safeguarding the integrity of your data.
• Self-Documenting Code: When you see a const variable, you immediately know that its value is fixed, making your code easier to understand and maintain.
• Compiler Optimizations: The compiler can often perform optimizations on const variables, as it knows their values won't change during execution.

constexpr: Compile-Time Constant Expressions

While const guarantees immutability, constexpr goes a step further. It signifies that an expression can be evaluated at compile time. This opens up a world of possibilities:

• Performance Boost: Calculations performed at compile time eliminate the need for runtime computation, leading to faster execution.
• Array Size Determination: You can use constexpr to define array sizes, allowing the compiler to allocate memory statically.
• Template Metaprogramming: constexpr is a cornerstone of template metaprogramming, enabling complex computations and type manipulations at compile time.

Key Differences: const vs. constexpr

const

• Evaluated at: Compile time or runtime
• Usage: Variables, functions, objects
• Initialization: Can be initialized at runtime

constexpr

• Evaluated at: Always compile time
• Usage: Variables, functions, literals
• Initialization: Must be initialized with literals

Examples

// const: Runtime initialization is allowed
const int runtime_value = calculateValue(); // Value calculated at runtime
 
// constexpr: Compile-time initialization is required
constexpr double pi = 3.14159;  // Value known at compile time
constexpr int array_size = 10; // Array size determined at compile time
 
// constexpr function (since C++11)
constexpr int factorial(int n) {
    return (n <= 1) ? 1 : n * factorial(n - 1);
}
 
int main() {
    constexpr int result = factorial(5);  // Calculated at compile time
    std::cout << result << std::endl;   // Output: 120
}

Best Practices and Considerations

• Prefer constexpr When Possible: If you know a value at compile time, use constexpr. This unlocks potential optimizations and enhances type safety.
• Combine const and constexpr: For maximum flexibility, declare variables as const and initialize them with constexpr expressions.
• Be Mindful of C++ Standards: The capabilities of constexpr have evolved with different C++ standards. Ensure you're using the correct features for your chosen standard.

Takeaway

const and constexpr are indispensable tools in the modern C++ arsenal. They enhance code safety, readability, and performance. By understanding their nuances and using them judiciously, you'll elevate your C++ code to new heights of efficiency and maintainability.