是否有任何指数函数在math.h标头中返回IEEE_FLOAT64？

Question

I want to use exponential function which returns IEEE_FLOAT64 value

Currently I am using expf function, but still I am getting lots of warnings.

value = IEEEPosOne - (IEEE_FLOAT64)expf(value1);

Answer 1

From man 3 exp :

NAME
       exp, expf, expl - base-e exponential function

SYNOPSIS
       #include <math.h>

       double exp(double x);
       float expf(float x);
       long double expl(long double x);

       Link with -lm.

So just use exp() .

Answer 2

//c++
#include <cmath>
double x = 7.0;//float64
auto y = std::exp(x);//exp(float64);

C++ standard provides appropriate overloads. No need to reflect operand type in function name.

Answer 3

This does answer does not apply to C only for C++
For C see @iBug's answer.

The C++ standard does not require an implementation to use the IEEE standard. Though this is usually the easiest floating point implementation to use as the chips are relatively standard in modern machines.

The standard provides a way to check using std::numeric_limits .

So if it is a requirement then you should validate.

#include <limits>
#include <iostream>

int main()
{
    static_assert(sizeof(double) == 64);
    static_assert(std::numeric_limits<double>::is_iec559());

    // If the above compiles your double is IEEE 64 bit value.
    // Or IEEE_754 compliant https://en.wikipedia.org/wiki/IEEE_754_revision
}

Now that you have established you are using IEEE values you can look at the cmath header to see that the functions there all take and return a double value.

• exp
• log
• etc....

Note: You should note that Windows machines (usually) use an 80 bit floating point register (not 64). So things can get super whaky if you need strict compliance.

Note: Do NOT use :

• expf() for float,
• expl() for long double

These are for C library users where the language does not do the correct type checking. In C++ the language uses overloading to use the correct version of the function. If you look at the standard for exp :

Defined in header <cmath>

* float       exp( float arg );
* double      exp( double arg );
* long double exp( long double arg );
* double      exp( Integral arg );

The above are all in the standard namespace.

std::cout << std::exp(100000.1) << "\n";

Notice that exp() can take any floating point type float , double or long double and generate the appropriate result type.