C++ fegetround()用法及代碼示例

fegetround() 函數在<cfenv> 頭文件中定義。

fegetround()原型

int fegetround();

參數：

• None

返回：

• 成功時，fegetround() 函數返回說明當前舍入方向的浮點舍入宏。
• 失敗時返回負值。

舍入宏值

宏	說明
FE_DOWNWARD	向下舍入
FE_TONEAREST	四舍五入到最接近的值
FE_TOWARDZERO	向零舍入
FE_UPWARD	向上舍入

示例：fegetround() 函數如何工作？

#include <iostream>
#include <cmath>
#include <cfenv>
#pragma STDC FENV_ACCESS ON
using namespace std;

void print_current_rounding_direction()
{
	cout << "Current rounding method: ";
	switch (fegetround()) {
		case FE_TONEAREST:
			cout << "FE_TONEAREST";
			break;
		case FE_DOWNWARD:
			cout << "FE_DOWNWARD";
			break;
		case FE_UPWARD:
			cout << "FE_UPWARD";
			break;
		case FE_TOWARDZERO:
			cout << "FE_TOWARDZERO";
			break;
		default:
			cout << "unknown";
	};
	cout << endl;
}

int main(void)
{
	print_current_rounding_direction();
	cout << "6.2 -> " << rint(6.2) << endl;
	cout << "18.7 -> " << rint(18.7) << endl;

	fesetround(FE_UPWARD);
	print_current_rounding_direction();
	cout << "6.2 -> " << rint(6.2) << endl;
	cout << "19.7 -> " << rint(19.7) << endl;
	
	fesetround(FE_DOWNWARD);
	print_current_rounding_direction();
	cout << "6.2 -> " << rint(6.2) << endl;
	cout << "19.7 -> " << rint(19.7) << endl;

	return 0;
}

運行程序時，輸出將是：

Current rounding method: FE_TONEAREST
6.2 -> 6
18.7 -> 19
Current rounding method: FE_UPWARD
6.2 -> 7
19.7 -> 20
Current rounding method: FE_DOWNWARD
6.2 -> 6
19.7 -> 19