8.4 — Constexpr if statements

Normally, the conditional of an if-statement is evaluated at runtime.

However, consider the case where the conditional is a constant expression, such as in the following example:

#include <iostream>

int main()
{
	constexpr double gravity{ 9.8 };

	// reminder: low-precision floating point literals of the same type can be tested for equality
	if (gravity == 9.8) // constant expression, always true
		std::cout << "Gravity is normal.\n";   // will always be executed
	else
		std::cout << "We are not on Earth.\n"; // will never be executed

	return 0;
}

Because gravity is constexpr and initialized with value 9.8, the conditional gravity == 9.8 must evaluate to true. As a result, the else-statement will never be executed.

Evaluating a constexpr conditional at runtime is wasteful (since the result will never vary). It is also wasteful to compile code into the executable that can never be executed.

Constexpr if statements C++17

C++17 introduces the constexpr if statement, which requires the conditional to be a constant expression. The conditional of a constexpr-if-statement will be evaluated at compile-time.

If the constexpr conditional evaluates to true, the entire if-else will be replaced by the true-statement. If the constexpr conditional evaluates to false, the entire if-else will be replaced by the false-statement (if it exists) or nothing (if there is no else).

To use a constexpr-if-statement, we add the constexpr keyword after the if:

#include <iostream>

int main()
{
	constexpr double gravity{ 9.8 };

	if constexpr (gravity == 9.8) // now using constexpr if
		std::cout << "Gravity is normal.\n";
	else
		std::cout << "We are not on Earth.\n";

	return 0;
}

When the above code is compiled, the compiler will evaluate the conditional at compile time, see that it is always true, and keep only the single statement std::cout << "Gravity is normal.\n";.

In other words, it will compile this:

int main()
{
	constexpr double gravity{ 9.8 };

	std::cout << "Gravity is normal.\n";

	return 0;
}

Modern compilers and if statements with constexpr conditionals C++17

For optimization purposes, modern compilers will generally treat non-constexpr if-statements that have constexpr conditionals as if they were constexpr-if-statements. However, they are not required to do so.

A compiler that encounters a non-constexpr if-statement with a constexpr conditional may issue a warning advising you to use if constexpr instead. This will ensure that compile-time evaluation will occur (even if optimizations are disabled).