The Subtle Distinction: "if constexpr()" vs "if()"

In the realm of C programming, the control flow statements "if constexpr()" and "if()" share a common purpose: conditional execution of code segments. However, a fundamental difference distinguishes them: the timing of evaluation.

Compile-Time Versus Runtime Evaluation

"if constexpr()" differs from "if()" in that its condition is evaluated at compile time rather than runtime. This means that if the condition evaluates to "true," the corresponding code block is guaranteed to execute. Conversely, if the condition is "false," the code block is discarded and not generated in the compiled executable.

Practical Implications

The compile-time evaluation of "if constexpr()" has several implications:

• Optimized Code: By discarding unreachable code blocks, "if constexpr()" helps reduce compilation time and executable size, resulting in more efficient and optimized code.
• Branch Prediction: Since the condition is resolved at compile time, the compiler can better optimize branching decisions and eliminate unnecessary jumps, resulting in faster execution.

Use Cases

1. Constant Expressions: "if constexpr()" is particularly useful for evaluating constant expressions that can be determined at compile time, such as determining the size of an array or checking for valid input.

2. Compile-Time Branching: When multiple code paths can be determined based on compile-time information, "if constexpr()" allows for conditional compilation, reducing duplication and improving code maintainability.

3. Compiler Diagnostics: "if constexpr()" can be used to provide more informative error messages and warnings by checking conditions at compile time and reporting errors before execution.

Example:

Consider the following code snippet:

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

This code calculates the length of a generic type T. The "if constexpr()" version of the code would eliminate the need for duplicate code and ensure compile-time evaluation of the type information:

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

By leveraging the compile-time evaluation of "if constexpr()", the code becomes more efficient and easier to maintain.