What is the difference between SFINAE and constexpr in C++?

What is the difference between SFINAE and constexpr in C++?

SFINAE stands for Substitution Failure Is Not An Error, which allows template specialization to fail gracefully. On the other hand, constexpr is used to evaluate expressions at compile time. They serve different purposes in template metaprogramming.

Can you give an example of SFINAE in action?

Sure! Here's a simple example:

template <typename T, typename = decltype(std::declval<T>().foo())>
void func(T t) { /* ... */ }

template <typename T>
void func(T t, std::enable_if_t<!std::is_same<T, int>::value>* = 0) { /* ... */ }

In this case, func will only be instantiated if T has a member function foo().

And how does constexpr work in templates?

constexpr allows you to define functions or variables that can be evaluated at compile time. For example:

constexpr int square(int x) { return x * x; }

You can use square in a template argument, and it will be evaluated during compilation.

When should I use SFINAE over constexpr?

Use SFINAE when you want to conditionally enable or disable template functions based on type traits or properties. Use constexpr when you need compile-time evaluation of functions or values.