Use if constexpr to simplify compile-time decisions
An if constexpr(condition) statement is used where code needs to be executed based on a compile-time condition. The condition may be any constexpr expression of type bool.
How to do it…
Consider the case where you have a template function that needs to operate differently depending upon the type of the template parameter.
template<typename T>
auto value_of(const T v) {
if constexpr (std::is_pointer_v<T>) {
return *v; // dereference the pointer
} else {
return v; // return the value
}
}
int main() {
int x{47};
int* y{&x};
cout << format("value is {}\n", value_of(x)); // value
cout << format("value is {}\n", value_of(y));
// pointer
return 0;
}
Output:
value is 47 value is 47
The type of the template parameter T is available at compile time. The constexpr if statement allows the code to easily distinguish between a pointer and a value.
How it works…
The constexpr if statement works like a normal if statement except it's evaluated at compile time. The runtime code will not contain any branch statements from a constexpr if statement. Consider our branch statement from above:
if constexpr (std::is_pointer_v<T>) {
return *v; // dereference the pointer
} else {
return v; // return the value
}
The condition is_pointer_v<T> tests a template parameter, which is not available at runtime. The constexpr keyword tells the compiler that this if statement needs to evaluate at compile time, while the template parameter <T> is available.
This should make a lot of meta programming situations much easier. The if constexpr statement is available in C++17 and later.
