What's the difference between `if constexpr()` and `if()` in C ?

Difference between "if constexpr()" vs "if()"

Introduction

In C , the "if" statement is commonly employed for conditional branching based on the outcome of a certain condition. However, for compile-time conditional evaluation, the "if constexpr()" statement serves as a potent tool. This article explores the distinctions between "if constexpr()" and "if()", highlighting their use cases and applicability.

Defining the Differences

The fundamental distinction between "if constexpr()" and "if()" lies in their timing of evaluation. "if constexpr()" evaluates the expression within its parentheses at compile-time, whereas "if()" evaluates its condition at runtime. This difference carries significant implications:

• Compile-Time vs. Runtime: "if constexpr()" enables the compiler to determine the path to take at compile-time, allowing for the elimination of code branches that are known to be unreachable or unnecessary. "if()", on the other hand, evaluates conditions only when the program is actually running.

Practical Applications

Case 1: Branching Based on Type Trait

Consider a scenario where you have a function, "length", that requires different logic for determining the length of numbers and objects with a ".length()" function. Using "ifconstexpr()", you can handle both cases with a single function, as it allows you to evaluate the type of the value at compile-time. Here's an example:

template<typename T>
auto length(const T&amp; value) noexcept {
    if constexpr (std::is_integral<T>::value) { // is number
        return value;
    else
        return value.length();
}

Case 2: Avoiding Illegal Operations

Another use case of "if constexpr()" is to bypass potential errors arising from illegal operations. For instance, if you decide to invoke a member function on a value that may not have it, you can use "if constexpr()" to test the type and execute a different path if the function is unavailable:

template<typename T>
bool contains(const T&amp; value, const std::string&amp; key) noexcept {
    if constexpr (has_find<T>::value) { // has std::find function
        return std::find(value.begin, value.end, key) != value.end();
    else
        return false;
}

When to Use Each Statement

Generally, "if constexpr()" should be employed when you need to make decisions based on the outcome of a condition during compilation. This allows the compiler to optimize the code and eliminate unnecessary or unreachable code. "if()", on the other hand, is appropriate when the condition's outcome is known only at runtime.

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