Ask in C++ code to promp user for function expression input

C++ code to prompt user for function expression input.

I try to make a code where after the RUN the interface to ask me to write the expression of function like pow(x,2)+5 and the program to read it and understand it with its mathematical expression.

So, instead to declare from the beginning:

double f(double x) {   pow(x,2)+5;
} 

I want something like this:

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

int main()
{
    int n,i;        //n is for subintervals and i is for loop
    double a,b,h,sum=0,integral;  
    double f(double x)  
    cout<<"Enter the expression of the function f(x)=";  // I will answer writing "pow(x,2)+5"
    cin>>f;

How I can do that ?

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