Constexpr doubt :/

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I may have got this wrong, but I thought that stuff marked with the keyword constexpr had to be known at compile time.

Then why in my example below, where I pass to the constexpr function a value that is only know at run time, I don't get any errors?! :S

size_t fib(size_t n)
{
	if (n <= 1)
	{
		return 1;
	}
	return fib(n - 1) + fib(n - 2);
}

constexpr size_t fib_constexpr(size_t n)
{
	if (n <= 1) 
	{ 
		return 1;
	}
	return fib(n - 1) + fib(n - 2);
}

int main()
{
	int n;
	cin >> n;
	cout << fib(n) << endl;
	cout << fib_constexpr(n) << endl;
	return 0;
}

 

Edited by MarcusAseth

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Objects/values that you declare with constexpr have to be evaluated at compile time.

Functions that you declare with constexpr can be evaluated at compile time, if all their arguments are also constexpr (or other compile time constants).

It's confusing - welcome to modern C++. ;)

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Really recommend reading: Scott Meyers' "Effective Modern C++: 42 Specific Ways to Improve Your Use of C++11 and C++14", best and most interesting C++11/14 "2nd book" (i.e. the author has not the intention of teaching the basics, but rather focuses on best practices while diving straight into the content) available.

More particularly, it tackles a.o. constexpr and () vs {} initializers ;) .

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Declaring a function constexpr means that it is "certified" for compile-time use, to compute constants, in addition to being compiled and callable like a non-constexpr function.

In your program there is no call with constant arguments (which would be optimized away) and the two functions are basically identical. See them on Godbolt: https://godbolt.org/g/JJ4LsQ

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