Call a class method from inside the same class If i have file cSpeedOfSound.h:

#ifndef cSpeedOfSound_h
#define cSpeedOfSound_h
#include "Arduino.h"
#include "math.h"
class cSpeedOfSound{
    public:
        cSpeedOfSound(float *i,float *C);
    private:
        float *C;
        float *a;
        float *i;
    };
#endif


and file cSpeedOfSound.cpp:

#include "cSpeedOfSound.h"
cSpeedOfSound::cSpeedOfSound(float *i,float *C){
    //some code...
    *C =roundto(*C,2);}
float cSpeedOfSound::roundto(float x,float dp){
return (round(x*pow(10., dp)) / pow(10., dp));}


where i want to run function roundto inside class, but it doesnt seems to work. So what im making wrong and how to do it properly?

Ps: Im begginer to arduino (want it to learn robotics + C language) and know python (learnd for about 1+ year), so pls do not hate me if i made crucial mistakes.
You need to include roundto() in your class declaration, e.g. something like this:

class cSpeedOfSound {
public:
    cSpeedOfSound(float *i,float *C);

private:
    float roundto(float x, float dp);

    float *C;
    float *a;
    float *i;
};


In C++, class structures are fixed. You can define the body of a function almost anywhere, but all member functions/data must be specified in the original declaration.



As a side note, it's worth pointing out that you have a naming conflict in your code. You have member variables called i and C, and you have totally separate function parameters called i and C. The compiler won't complain about this (the function parameters will simply mask the member variables), but it may not do what you expect.