# FE_DFL_ENV

< cpp‎ | numeric‎ | fenv

定义于头文件 `<cfenv>` |  |
#define FE_DFL_ENV /*implementation defined*/ |  | (C++11 起)

宏常量 FE_DFL_ENV 展开成 const std::fenv_t* 类型表达式，它指向默认浮点环境，即在程序加载时的浮点环境的完整副本。

实现亦可支持附加的以 `FE_` 后随大写字母，并拥有 const std::fenv_t* 类型的宏。

### 示例

    #include <iostream>
    #include <cfenv>

    #pragma STDC FENV_ACCESS ON

    void show_env()
    {
        int e = std::fetestexcept(FE_ALL_EXCEPT);
        if(e & FE_DIVBYZERO) std::cout << "division by zero is raised\n";
        if(e & FE_INEXACT)   std::cout << "inexact is raised\n";
        if(e & FE_INVALID)   std::cout << "invalid is raised\n";
        if(e & FE_UNDERFLOW) std::cout << "underflow is raised\n";
        if(e & FE_OVERFLOW)  std::cout << "overflow is raised\n";
        int r = std::fegetround();
        switch(r)
        {
            case FE_DOWNWARD: std::cout << "rounding down\n"; break;
            case FE_TONEAREST: std::cout << "rounding to nearest\n"; break;
            case FE_TOWARDZERO: std::cout << "rounding to zero\n"; break;
            case FE_UPWARD: std::cout << "rounding up\n"; break;
        }
    }

    int main()
    {
        std::cout << "On startup: \n";
        show_env();

        std::feraiseexcept(FE_UNDERFLOW | FE_OVERFLOW);
        std::fesetround(FE_UPWARD);

        std::cout << "\nBefore restoration: \n";
        show_env();

        std::fesetenv(FE_DFL_ENV);

        std::cout << "\nAfter reset to default: \n";
        show_env();
    }

输出：

    On startup:
    rounding to nearest

    Before restoration:
    underflow is raised
    overflow is raised
    rounding up

    After reset to default:
    rounding to nearest

### 参阅

fegetenv fesetenv (C++11) | 保存或恢复当前浮点环境 (函数)
feupdateenv (C++11) | 恢复浮点环境并引发之前引发的异常 (函数)