cin C++ If Statement

// cin1.cpp   // Filename
#include <iostream>  //including the dll? Biblothek? iostream
using namespace std;  /* The following content uses The Namespace std, else we would have to wirte std::cout everytime */

int main(void) { /*Main Function, which has to be a number (int=integer) (i forgot what void meant would be nice if someone could explain it fast) */
    float wert; /* defines the Variable wert as a float (means a number with decimal point) */
    cout << "Bitte eine Fließkommazahl eingeben : "; /*writes the Text between the "*" out and then rests there */ 
    cin >> wert;  /* now it waits there for an input with cin for the variable of wert. It already checked that wert has to be a float or it will error */

    if (!(cin >> wert)) { /*New If stance where it checks something would be nice if you would explain it to me i dont entirly got it */
        cerr << "Fehler bei der Eingabe!\n"; /* it checked if cin is a float or not (whats the matter with the "!" ??  what does "cin >> wert" do? 
So atm it works if i wirte something else then a floatnumber it says : "Fehler bei der Eingabe!" Which means something like: "Wrong Input". So it checks if its a floutnumber and if not it says yo its wrong buddy*/
    }
    else { /* else statement beginnning */
    cout << "Die Eingabe war " << wert << "\n"; /* Gives out: "Die Eingabe war " which means something like: "Input was: ", if it is a floatnumber (was checked in IF statement). And after that it gives out the variable: "wert" which must be a floatnumber at this point. */
    }
    return 0; /*Ends the Process */
    }

int main(void) {

main(void) {

float v;
bool bCin = cin >> v;
if(bCin == false) {
    cout << "FAIL";
}

cin >> var; //doesn't save return value
if(!cin >> var) //uses cin again, and compares to return value

boolean flag;
flag = cin >> var; //saves return value in flag
if(!flag) //tests if flag is false

flag = cin >> var;
if(!flag) {
....
}

flag = cin >> var; //saves return value in flag

float var;
std::cin >> var;
if (!var)
{
    std::cout << "var = false";
}

//same as:
float var;
std::cin >> var;
if (var == false)
{
    std::cout << "var = false";
}

	
 float val;
 bool flag;
 flag = !(std::cin >> val); //equals flag = (std::cin >> val).fail();
 //also possible
 //std::cin >> val;
 //flag  = !std::cin; or flag = std::cin.fail();
 if(flag)
 {
    std::cout << "Fail" <<std::endl;
 }

float val;
if(!(std::cin >> val)) //or if((std::cin >> val).fail())

float var;
std::cout << (std::cin >> var);

float var; 
std::cin >> var; //save the input to var !  "var = std::cin >> var" would save something else...
if (!var)  //Tests if var is equal to zero
{
     std::cout << "var = 0 (false)"
}

//Now the wrong example:
float var; 
if (!(std::cin >> var))  //Tests if the return of cin is equal to zero...
{
     std::cout << "return of std::cin = 0 (false)"
}

//Example for unexpected input
	float var, var_2;

	std::cin >> var;
	std::cin >> var_2;

	std::cout << std::endl << var << " " << var_2;
//Run...
Input 1: "1 text"
Input 2: takes "text" from the input buffer and saves it to var_2 (not wanted)

//Another example:
	float var, var_2;

	std::cin >> var;
	fflush(stdin);  //flush the input buffer

	std::cin >> var_2;
        fflush(stdin);

	std::cout << std::endl << var << " " << var_2;
//Run..
Input 1: "5 text"
-> whole input buffer gets deleted, so the user is able to write another value
Input 2: "6"

Output: "56"

float var;
std::cin >> var;
//Input: g, failbit is set because of type mismatch, if 456 was entered failbit wouldn't be set.
if(!std::cin) //equals cin.fail() -> true

//Same
float var;
if(!(std::cin >> var))