Prevent Multiple Inputs on CIN

Question

Relevant Info: This is a Visual Studio C++ Console Application

I was doing some basic C++ to keep somewhat fresh when I experienced an issue. This is the entirety of my cpp file:

#include <iostream>
#include <string>

double CelsiusToFahrenheit(double);
double FahrenheitToCelsius(double);
void GetUserInput();

int main() {

    GetUserInput();

}

double CelsiusToFahrenheit(double celsiusNum) {
    return ((celsiusNum * (9.0 / 5.0)) + 32.0);
}

double FahrenheitToCelsius(double fahrenheitNum) {
    return ((fahrenheitNum - 32.0) * (5.0/9.0));
}

void GetUserInput() {
    char uiChoice;
    double uiNumber;

    std::cout << "Please Select an Option: " << std::endl;
    std::cout << "Enter C for Celsius to Fahrenheit conversion, Enter F for Fahrenheit to Celsius conversion: ";
    std::cin >> uiChoice;

    if (uiChoice == 'C') {
        std::cout << "Please enter the degrees in Celsius: ";
        std::cin >> uiNumber;

        std::cout << std::endl << uiNumber << " degrees C in Fahrenheit is " << CelsiusToFahrenheit(uiNumber) << " degrees F." << std::endl;
    }

    else if (uiChoice == 'F') {
        std::cout << "Please enter the degrees in Fahrenheit: ";
        std::cin >> uiNumber;

        std::cout << std::endl << uiNumber << " degrees F in Celsius is " << FahrenheitToCelsius(uiNumber) << " degrees C." << std::endl;
    }

    else {
        std::cout << "Invalid choice, please try again." << std::endl;
        GetUserInput();
    }

    return;
}

Inside of GetUserInput(), when running the line std::cin >> uiChoice;, if a user enters more than 1 character for the input, I get multiple executions of the else statement.

Example output:

I was wondering, how can I either

a) Prevent multiple inputs

b) Only fire the else once when multiple inputs are entered

ALSO

I have tried initializing uiChoice as a string and using compare(), but the results have been... weird? An example of that function:

void GetUserInput() {
    std::string uiChoice;
    double uiNumber;

    std::cout << "Please Select an Option: " << std::endl;
    std::cout << "Enter C for Celsius to Fahrenheit conversion, Enter F for Fahrenheit to Celsius conversion: ";
    std::cin >> uiChoice;

    if (uiChoice.compare("C")) {
        std::cout << "Please enter the degrees in Celsius: ";
        std::cin >> uiNumber;

        std::cout << std::endl << uiNumber << " degrees C in Fahrenheit is " << CelsiusToFahrenheit(uiNumber) << " degrees F." << std::endl;
    }

    else if (uiChoice.compare("F")) {
        std::cout << "Please enter the degrees in Fahrenheit: ";
        std::cin >> uiNumber;

        std::cout << std::endl << uiNumber << " degrees F in Celsius is " << FahrenheitToCelsius(uiNumber) << " degrees C." << std::endl;
    }

    else {
        std::cout << "Invalid choice, please try again." << std::endl;
        GetUserInput();
    }

    return;
}

In the following output, you can see that any character entered will satisfy the criteria for if (uiChoice.compare("C")).

output:

I don't have much experience using the compare() function, so this very well could be me interpreting the uses of compare() wrong.

EDIT:

Updated GetUserInput() with std::getline()

void GetUserInput() {
    std::string uiChoice;
    double uiNumber;

    std::cout << "Please Select an Option: " << std::endl;
    std::cout << "Enter C for Celsius to Fahrenheit conversion, Enter F for Fahrenheit to Celsius conversion: ";
    std::getline(std::cin, uiChoice);

    if (uiChoice.compare("C")) {
        std::cout << "Please enter the degrees in Celsius: ";
        std::cin >> uiNumber;

        std::cout << std::endl << uiNumber << " degrees C in Fahrenheit is " << CelsiusToFahrenheit(uiNumber) << " degrees F." << std::endl;
    }

    else if (uiChoice.compare("F")) {
        std::cout << "Please enter the degrees in Fahrenheit: ";
        std::cin >> uiNumber;

        std::cout << std::endl << uiNumber << " degrees F in Celsius is " << FahrenheitToCelsius(uiNumber) << " degrees C." << std::endl;
    }

    else {
        std::cout << "Invalid choice, please try again." << std::endl;
        GetUserInput();
    }

    return;
}

Output:

Answer 1

You are misunderstanding the compare method. The simple way to do what you want is to use ==

if (uiChoice == "C") {

And has already been said getline is the answer to your other problem. But please read this very important issue when mixing getline with reading numbers using >>

Why does std::getline() skip input after a formatted extraction?

This issue catches everybody out at first.