visual stdio 2017 ][ constexpr

visual stdio 2017 ][ constexpr

■ constexpr

 

	#include <iostream>
	using namespace std;
	// Pass by value
	constexpr float exp(float x, int n)
	{
	return n == 0 ? 1 :
	n % 2 == 0 ? exp(x * x, n / 2) :
	exp(x * x, (n - 1) / 2) * x;
	};
	// Pass by reference
	constexpr float exp2(const float& x, const int& n)
	{
	return n == 0 ? 1 :
	n % 2 == 0 ? exp2(x * x, n / 2) :
	exp2(x * x, (n - 1) / 2) * x;
	};
	// Compile time computation of array length
	template<typename T, int N>
	constexpr int length(const T(&ary)[N])
	{
	return N;
	}
	// Recursive constexpr function
	constexpr int fac(int n)
	{
	return n == 1 ? 1 : n*fac(n - 1);
	}
	// User-defined type
	class Foo
	{
	public:
	constexpr explicit Foo(int i) : _i(i) {}
	constexpr int GetValue()
	{
	return _i;
	}
	private:
	int _i;
	};
	int main()
	{
	//foo is const:
	constexpr Foo foo(5);
	// foo = Foo(6); //Error!
	//Compile time:
	constexpr float x = exp(5, 3);
	constexpr float y { exp(2, 5) };
	constexpr int val = foo.GetValue();
	constexpr int f5 = fac(5);
	const int nums[] { 1, 2, 3, 4 };
	const int nums2[length(nums) * 2] { 1, 2, 3, 4, 5, 6, 7, 8 };
	//Run time:
	cout << "The value of foo is " << foo.GetValue() << endl;
	}

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