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How to Print C Enum Values as Text Without Using if/switch Statements?

Patricia Arquette

Release： 2024-11-29 22:55:12

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How to Print C Enum Values as Text Without Using if/switch Statements?

Printing Enum Values as Text in C Without if/switch

In C , enums provide a convenient way to represent a set of named constants. However, when printing an enum value, the default behavior is to display its numeric representation.

Suppose we have an enum like this:

enum Errors {
    ErrorA = 0,
    ErrorB,
    ErrorC,
};

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If we try to print an enum value using std::cout:

Errors anError = ErrorA;
std::cout << anError; // 0 will be printed

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We will get the numeric value 0 instead of the text representation "ErrorA". This issue arises because std::cout lacks built-in support for converting enums to strings.

Solutions

1. Using a Map

One approach is to create a map that associates each enum value with its text representation:

#include <map>
#include <string_view>

std::ostream& operator<<(std::ostream& out, const Errors value) {
    static const auto strings = [] {
        std::map<Errors, std::string_view> result;
        #define INSERT_ELEMENT(p) result.emplace(p, #p);
        INSERT_ELEMENT(ErrorA);     
        INSERT_ELEMENT(ErrorB);     
        INSERT_ELEMENT(ErrorC);             
        #undef INSERT_ELEMENT
        return result;
    };

    return out << strings[value];
}

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2. Using an Array of Structures with Linear Search

Another option is to use an array of structures, each containing an enum value and its text representation, and then perform a linear search:

#include <string_view>

std::ostream& operator<<(std::ostream& out, const Errors value) {
#define MAPENTRY(p) {p, #p}
    const struct MapEntry {
        Errors value;
        std::string_view str;
    } entries[] = {
        MAPENTRY(ErrorA),
        MAPENTRY(ErrorB),
        MAPENTRY(ErrorC),
        {ErrorA, 0} //doesn't matter what is used instead of ErrorA here...
    };
#undef MAPENTRY
    const char* s = 0;
    for (const MapEntry* i = entries; i->str; i++) {
        if (i->value == value) {
            s = i->str;
            break;
        }
    }

    return out << s;
}

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3. Using switch/case

Finally, we can also use a switch/case statement:

#include <string>

std::ostream& operator<<(std::ostream& out, const Errors value) {
    return out << [value] {
    #define PROCESS_VAL(p) case(p): return #p;
        switch(value) {
            PROCESS_VAL(ErrorA);     
            PROCESS_VAL(ErrorB);     
            PROCESS_VAL(ErrorC);
        }
    #undef PROCESS_VAL
    };
}

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Testing the Solutions:

To test these solutions, we can create an executable using the following code:

#include <iostream>

int main(int argc, char** argv) {
    std::cout << ErrorA << std::endl << ErrorB << std::endl << ErrorC << std::endl;
    return 0;   
}

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Running this executable should output the text representations of the enum values:

ErrorA
ErrorB
ErrorC

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