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Showing content from https://en.cppreference.com/w/cpp/algorithm/../ranges/../numeric/math/expm1.html below:

std::expm1, std::expm1f, std::expm1l - cppreference.com

(1) float       expm1 ( float num );

double      expm1 ( double num );

long double expm1 ( long double num ); (until C++23)

/*floating-point-type*/
            expm1 ( /*floating-point-type*/ num );

(since C++23)
(constexpr since C++26)

float       expm1f( float num );

(2) (since C++11)
(constexpr since C++26)

long double expm1l( long double num );

(3) (since C++11)
(constexpr since C++26) template< /*math-floating-point*/ V >

constexpr /*deduced-simd-t*/<V>

            expm1 ( const V& v_num ); (S) (since C++26)

template< class Integer >
double      expm1 ( Integer num );

(A) (constexpr since C++26) 1-3)

Computes the

e

(

Euler's number

,

2.7182818...

) raised to the given power

num

, minus

1.0

. This function is more accurate than the expression

std::exp(num) - 1.0

if

num

is close to zero.

The library provides overloads of std::expm1 for all cv-unqualified floating-point types as the type of the parameter.(since C++23)

A) Additional overloads are provided for all integer types, which are treated as double.

(since C++11) [edit] Parameters num - floating-point or integer value [edit] Return value

If no errors occur enum
-1 is returned.

If a range error due to overflow occurs, +HUGE_VAL, +HUGE_VALF, or +HUGE_VALL is returned.

If a range error occurs due to underflow, the correct result (after rounding) is returned.

[edit] Error handling

Errors are reported as specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

[edit] Notes

The functions std::expm1 and std::log1p are useful for financial calculations, for example, when calculating small daily interest rates: (1+x)n
-1 can be expressed as std::expm1(n * std::log1p(x)). These functions also simplify writing accurate inverse hyperbolic functions.

For IEEE-compatible type double, overflow is guaranteed if 709.8 < num.

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::expm1(num) has the same effect as std::expm1(static_cast<double>(num)).

[edit] Example 
#include <cerrno>
#include <cfenv>
#include <cmath>
#include <cstring>
#include <iostream>
// #pragma STDC FENV_ACCESS ON
 
int main()
{
    std::cout << "expm1(1) = " << std::expm1(1) << '\n'
              << "Interest earned in 2 days on $100, compounded daily at 1%\n"
              << "    on a 30/360 calendar = "
              << 100 * std::expm1(2 * std::log1p(0.01 / 360)) << '\n'
              << "exp(1e-16)-1 = " << std::exp(1e-16) - 1
              << ", but expm1(1e-16) = " << std::expm1(1e-16) << '\n';
 
    // special values
    std::cout << "expm1(-0) = " << std::expm1(-0.0) << '\n'
              << "expm1(-Inf) = " << std::expm1(-INFINITY) << '\n';
 
    // error handling
    errno = 0;
    std::feclearexcept(FE_ALL_EXCEPT);
 
    std::cout << "expm1(710) = " << std::expm1(710) << '\n';
 
    if (errno == ERANGE)
        std::cout << "    errno == ERANGE: " << std::strerror(errno) << '\n';
    if (std::fetestexcept(FE_OVERFLOW))
        std::cout << "    FE_OVERFLOW raised\n";
}

Possible output:

expm1(1) = 1.71828
Interest earned in 2 days on $100, compounded daily at 1%
    on a 30/360 calendar = 0.00555563
exp(1e-16)-1 = 0, but expm1(1e-16) = 1e-16
expm1(-0) = -0
expm1(-Inf) = -1
expm1(710) = inf
    errno == ERANGE: Result too large
    FE_OVERFLOW raised
[edit] See also returns e raised to the given power (\({\small e^x}\)ex)
(function) [edit] returns 2 raised to the given power (\({\small 2^x}\)2x)
(function) [edit] natural logarithm (to base e) of 1 plus the given number (\({\small\ln{(1+x)}}\)ln(1+x))
(function) [edit]

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