13.4 — Converting an enumeration to and from a string

In the prior lesson (13.3 -- Unscoped enumerator integral conversions), we showed an example like this:

#include <iostream>

enum Color
{
    black, // 0
    red,   // 1
    blue,  // 2
};

int main()
{
    Color shirt{ blue };

    std::cout << "Your shirt is " << shirt << '\n';

    return 0;
}

This prints:

Your shirt is 2

Because operator<< doesn’t know how to print a Color, the compiler will implicitly convert Color into an integral value and print that instead.

Most of the time, printing an enumeration as an integral value (such as 2) isn’t what we want. Instead, we typically want to print the name of whatever the enumerator represents (e.g. blue). C++ doesn’t come with an out-of-the-box way to do this, so we’ll have to find a solution ourselves. Fortunately, that’s not very difficult.

Getting the name of an enumerator

The typical way to get the name of an enumerator is to write a function that allows us to pass in an enumerator and returns the enumerator’s name as a string. But that requires some way to determine which string should be returned for a given enumerator.

There are two common ways to do this.

In lesson 8.5 -- Switch statement basics, we noted that a switch statement can switch on either an integral value or an enumerated value. In the following example, we use a switch statement to select an enumerator and return the appropriate color string literal for that enumerator:

#include <iostream>
#include <string_view>

enum Color
{
    black,
    red,
    blue,
};

constexpr std::string_view getColorName(Color color)
{
    switch (color)
    {
    case black: return "black";
    case red:   return "red";
    case blue:  return "blue";
    default:    return "???";
    }
}

int main()
{
    constexpr Color shirt{ blue };

    std::cout << "Your shirt is " << getColorName(shirt) << '\n';

    return 0;
}

This prints:

Your shirt is blue

In the above example, we switch on color, which holds the enumerator we passed in. Inside the switch, we have a case-label for each enumerator of Color. Each case returns the name of the appropriate color as a C-style string literal. This C-style string literal gets implicitly converted into a std::string_view, which is returned to the caller. We also have a default case which returns "???", in case the user passes in something we didn’t expect.

The function is constexpr so that we can use the color’s name in a constant expression.

While this lets us get the name of an enumerator, if we want to print that name to the console, having to do std::cout << getColorName(shirt) isn’t quite as nice as std::cout << shirt. We’ll teach std::cout how to print an enumeration in upcoming lesson 13.5 -- Introduction to overloading the I/O operators.

The second way to solve the program of mapping enumerators to strings is to use an array. We cover this in lesson 17.6 -- std::array and enumerations.

Unscoped enumerator input

Now let’s take a look at an input case. In the following example, we define a Pet enumeration. Because Pet is a program-defined type, the language doesn’t know how to input a Pet using std::cin:

#include <iostream>

enum Pet
{
    cat,   // 0
    dog,   // 1
    pig,   // 2
    whale, // 3
};

int main()
{
    Pet pet { pig };
    std::cin >> pet; // compile error: std::cin doesn't know how to input a Pet

    return 0;
}

One simple way to work around this is to read in an integer, and use static_cast to convert the integer to an enumerator of the appropriate enumerated type:

#include <iostream>

enum Pet
{
    cat,   // 0
    dog,   // 1
    pig,   // 2
    whale, // 3
};

constexpr std::string_view getPetName(Pet pet)
{
    switch (pet)
    {
    case cat:   return "cat";
    case dog:   return "dog";
    case pig:   return "pig";
    case whale: return "whale";
    default:    return "???";
    }
}

int main()
{
    std::cout << "Enter a pet (0=cat, 1=dog, 2=pig, 3=whale): ";

    int input{};
    std::cin >> input; // input an integer

    if (input < 0 || input > 3)
        std::cout << "You entered an invalid pet\n";
    else
    {
        Pet pet{ static_cast<Pet>(input) }; // static_cast our integer to a Pet
        std::cout << "You entered: " << getPetName(pet) << '\n';
    }

    return 0;
}

While this works, it’s a bit awkward. Also note that we should only static_cast<Pet>(input) once we know input is in range of the enumerator.

Getting an enumeration from a string

Instead of inputting a number, it would be nicer if the user could type in a string representing an enumerator (e.g. “pig”), and we could convert that string into the appropriate Pet enumerator. However, doing this requires us to solve a couple of challenges.

First, we can’t switch on a string, so we need to use something else to match the string the user passed in. The simplest approach here is to use a series of if-statements.

Second, what Pet enumerator should we return if the user passes in an invalid string? One option would be to add an enumerator to represent “none/invalid”, and return that. However, a better option is to use std::optional here.

#include <iostream>
#include <optional> // for std::optional

enum Pet
{
    cat,   // 0
    dog,   // 1
    pig,   // 2
    whale, // 3
};

constexpr std::string_view getPetName(Pet pet)
{
    switch (pet)
    {
    case cat:   return "cat";
    case dog:   return "dog";
    case pig:   return "pig";
    case whale: return "whale";
    default:    return "???";
    }
}

constexpr std::optional<Pet> getPetFromString(std::string_view sv)
{
    if (sv == "cat")   return cat;
    if (sv == "dog")   return dog;
    if (sv == "pig")   return pig;
    if (sv == "whale") return whale;
    
    return {};
}

int main()
{
    std::cout << "Enter a pet: cat, dog, pig, or whale: ";
    std::string s{};
    std::cin >> s;
        
    std::optional<Pet> pet { getPetFromString(s) };

    if (!pet)
        std::cout << "You entered an invalid pet\n";
    else
        std::cout << "You entered: " << getPetName(*pet) << '\n';

    return 0;
}

In the above solution, we use a series of if-else statements to do string comparisons. If the user’s input string matches an enumerator string, we return the appropriate enumerator. If none of the strings match, we return {}, which means “no value”.

#include <algorithm> // for std::transform
#include <cctype>    // for std::tolower

// This function returns a std::string that is the lower-case version of the std::string_view passed in.
// Only 1:1 character mapping can be performed by this function
constexpr std::string toASCIILowerCase(std::string_view sv)
{
    std::string upper{};
    std::transform(sv.begin(), sv.end(), std::back_inserter(upper), 
        [](unsigned char c){ return std::tolower(c); });
    return upper;
}

Similar to the output case, it would be better if we could just std::cin >> pet. We’ll cover this in upcoming lesson 13.5 -- Introduction to overloading the I/O operators.