The `log10()` function is part of the `cmath` library in C++ -- it allows you to take the *logarithm of base 10* of any number. 

**Logarithm** is a math function that finds out `x` by following the equation below.

$$y = 10^{x}$$

If `y` is given, we can find `x` by taking `log10(y)`.

The following table defines the different outputs of the logarithm function. 
|  Input y | Output x  |
| - | - |
|y > 1| positive value   |
|  y = 1 |  zero value |
|  0 > y > 1 |  negative value | 
| y = 0  |  $-\infty$ | 
| y < 0  |  not a number | 

The `log10()` function in `cmath` takes a number of any type in the argument and returns its base, `10 log`. The returned value can be a `float`, `double`, `long double` or `int`.

Let's see different examples of how the `log10()` function can be used. 

#include <iostream>
#include <cmath>
using namespace std;

int main() {
  // input output both of same type
  double x0, y0 = 453.23;
  x0 = log10(y0);
  cout << "Log Base 10 of " << y0 << " is : " << x0 << endl;

  //input output both of different type
  double x1;
  float y1 = 100.45;
  x1 = log10(y1);
  cout << "Log Base 10 of " << y1 << " is : " << x1 << endl;
  
  // output as an integer 
  // notice that the output will discard any decimal places
  int x2;
  long double y2 = 4845645.85;
  x2 = log10(y2);
  cout << "Log Base 10 of " << y2 << " is : " << x2 << endl;

  return 0;
}

How to use log10( ) in C++

The log10() function in C++'s cmath library calculates the base-10 logarithm of a number, returning various output types based on the input value.

Input y	Output x
y > 1	positive value
y = 1	zero value
0 > y > 1	negative value
y = 0	$-\infty$
y < 0	not a number