C++ fegetenv()用法及代码示例

fegetenv() 函数在<cfenv> 头文件中定义。

fegetenv()原型

int fegetenv( fenv_t* envp );

此函数尝试将浮点环境存储在指针对象 envp 中。

参数：

• envp：指向存储浮点环境状态的 fenv_t 类型对象的指针。

返回：

• 成功时，fegetenv() 函数返回 0。
• 失败时，它返回非零。

示例：fegetenv() 函数如何工作？

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

void print_exceptions()
{
	cout << "Raised exceptions: ";
	if(fetestexcept(FE_ALL_EXCEPT))
	{
		if(fetestexcept(FE_DIVBYZERO))
			cout << "FE_DIVBYZERO ";
		if(fetestexcept(FE_INEXACT))
			cout << "FE_INEXACT ";
		if(fetestexcept(FE_INVALID))
			cout << "FE_INVALID ";
		if(fetestexcept(FE_OVERFLOW))
			cout << "FE_OVERFLOW ";
		if(fetestexcept(FE_UNDERFLOW))
			cout << "FE_UNDERFLOW ";
	}
	else
		cout << "None";

	cout << endl;
}

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;
}

void print_environment()
{
	print_exceptions();
	print_current_rounding_direction();
}

int main(void)
{
	cout << "Initial environment " << endl;
	print_environment();
	fenv_t envp;

	/* Save current environment */
	fegetenv(&envp);
	feraiseexcept(FE_INVALID|FE_DIVBYZERO);
	fesetround(FE_DOWNWARD);
	cout << "After changing environment " << endl;
	print_environment();

	/* Restores previous environment */
	fesetenv(&envp);
	cout << "Restoring initial environment " << endl;
	print_environment();
	
	return 0;
}

运行程序时，输出将是：

Initial environment
Raised exceptions: None
Current rounding method: FE_TONEAREST
After changing environment
Raised exceptions: FE_DIVBYZERO FE_INVALID
Current rounding method: FE_DOWNWARD
Restoring initial environment
Raised exceptions: None
Current rounding method: FE_TONEAREST