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