c++之如何将整数转换为 float 并向零舍入

当整数转换为浮点数且该值不能直接由目标类型表示时，通常会选择最接近的值(IEEE-754要求)。
如果整数值不能直接由浮点类型表示，我想将整数转换为浮点并向零舍入。
例:

int i = 2147483647; 
float nearest = static_cast<float>(i);  // 2147483648 (likely) 
float towards_zero = convert(i);        // 2147483520 

请您参考如下方法:

从C++ 11开始，可以使用fesetround()，即浮点环境rounding direction管理器。有四个标准rounding directions，并且允许实现添加其他舍入方向。

#include <cfenv> // for fesetround() and FE_* macros 
#include <iostream> // for cout and endl 
#include <iomanip> // for setprecision() 
 
#pragma STDC FENV_ACCESS ON 
 
int main(){ 
    int i = 2147483647; 
 
    std::cout << std::setprecision(10); 
 
    std::fesetround(FE_DOWNWARD); 
    std::cout << "round down         " << i << " :  " << static_cast<float>(i) << std::endl; 
    std::cout << "round down        " << -i << " : " << static_cast<float>(-i) << std::endl; 
 
    std::fesetround(FE_TONEAREST); 
    std::cout << "round to nearest   " << i << " :  " << static_cast<float>(i) << std::endl; 
    std::cout << "round to nearest  " << -i << " : " << static_cast<float>(-i) << std::endl; 
 
    std::fesetround(FE_TOWARDZERO); 
    std::cout << "round toward zero  " << i << " :  " << static_cast<float>(i) << std::endl; 
    std::cout << "round toward zero " << -i << " : " << static_cast<float>(-i) << std::endl; 
 
    std::fesetround(FE_UPWARD); 
    std::cout << "round up           " << i << " :  " << static_cast<float>(i) << std::endl; 
    std::cout << "round up          " << -i << " : " << static_cast<float>(-i) << std::endl; 
 
    return(0); 
} 

round down         2147483647 :  2147483520 
round down        -2147483647 : -2147483648 
round to nearest   2147483647 :  2147483648 
round to nearest  -2147483647 : -2147483648 
round toward zero  2147483647 :  2147483520 
round toward zero -2147483647 : -2147483520 
round up           2147483647 :  2147483648 
round up          -2147483647 : -2147483520 

float convert(int i, int direction = FE_TOWARDZERO){ 
    float retVal = 0.; 
    int prevdirection = std::fegetround(); 
    std::fesetround(direction); 
    retVal = static_cast<float>(i); 
    std::fesetround(prevdirection); 
    return(retVal); 
}