Difference between if constexpr() and if()

Q: Difference between if constexpr() and if()

The if constexpr is mainly used where decisions need to be made at compile time based on type traits or other constant expressions. Here are some key applications:

Application of if constexpr

Features	if constexpr()	if()
Execution	Compile-time, the code branch that is not satisfying the condition is discarded at compile time.	Runtime, the code branch that is not satisfying the is not executed during runtime.
Condition	Condition must be a constant expressions	Condition can be any expression
Syntax Flexibility	Compile-Time Conditions	Runtime Conditions
Branching	Conditional code generation	Multiple paths at runtime
Error Reporting	Early Errors, errors related to branch that is discarded are reported during compilation.	Errors related to both discarded and non discarded branches may be reported during compilation and runtime.
Constant evaluation	Evaluated Immediately	Evaluation may be deferred
Performance	Optimized Execution	Runtime Overhead
Compile-time checks	Yes it checks at compile time	No compile time checks

if constexpr in C++ 17

In C++17, if constexpr feature was introduced to allow compile-time branching based on constant expressions. Unlike regular if statements, which are evaluated at runtime, the if constexpr allows the compiler to discard branches of code that do not apply. It means only the branch of code for which the condition is true is compiled and other code branch is discarded during compilation.