`if constexpr()` vs. `if()`: Compile-Time vs. Runtime Conditional Evaluation?

Difference Between "if constexpr()" and "if()"

In C , the "if constexpr()" and "if()" statements provide conditional evaluation during compilation and runtime, respectively.

Key Difference:

The main difference between "if constexpr()" and "if()" lies in their evaluation time:

• if constexpr(): Evaluated during compile time
• if(): Evaluated during runtime

Usage and Applications:

if constexpr()

• Used for constant expressions that can be determined at compile time.
• Allows compilers to discard unreachable code paths, leading to optimizations.
• Useful for selecting template specializations and optimizing branch predictions.

if()

• Used for conditional evaluation during runtime.
• Checks conditions that may change during program execution.
• Typically used for branching based on user input, function calls, or runtime data.

Example:

Consider the following code snippet that calculates the length of a value based on its type:

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

• If the type of value is an integer, the if constexpr evaluates to true at compile time, and the branch that returns value is taken.
• If the type of value is a string, the if constexpr evaluates to false, and the branch that returns value.length() is taken.

By using if constexpr, the compiler can eliminate the branch for the other type, leading to efficient code generation.

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