How to resolve `no viable overloaded operator[] for type 'std::map<AudioTypes, const char *>`? [duplicate]

Question

So I followed this SO answer for my purposes, and I really do not know how to resolve this error. When I try to access audioType[wav] or audioType[mp3], the error below comes up. Looking up the internet was not helpful for me.

error: no viable overloaded operator[] for type 'const std::map<AudioTypes, const char *>

note: candidate function not viable: 'this' argument has type 'const std::map<AudioTypes, const char *>', but method is not marked const mapped_type& operator[](const key_type& __k);

// WAVFile.h
class WAVFile {
    // ...
    private:
        enum AudioTypes: int;
        static std::map<AudioTypes, const char*> audioType;
    // ...
}

// WAVFile.cpp
enum AudioTypes: int {
    wav,
    mp3
};

static map<AudioTypes, const char*> audioType = {
    {wav, "wav"},
    {mp3, "mp3"}
};

bool WAVFile::exportAudio(const char* filename, const char* filetype) {
    // temporary test code    
    cout << audioType[wav] << endl;
    return true;
}

// main.cpp
int main() {
    file.exportAudio("export_i ran.wav","wav") ? cout << "Exported\n" : cout << "Failed\n";

    return 0;
}

Answer 1

As already pointed out, the reason for that error is - as the error message explains - that operator[] of std::map is not a const qualified member function. (I'm a bit surprised by this, I thought there was a const overload)

A possible fix is to use the member function at, which has this const qualified overload, and should thus be working with const qualified std::maps.

Also note that in the code you showed there's a possible issue:

class WAVFile {
        static std::map<AudioTypes, const char*> audioType;
}

and

static map<AudioTypes, const char*> audioType = {
    {wav, "wav"},
    {mp3, "mp3"}
};

are not the same variable. You should add the class scope before the definition and get rid of the static:

map<AudioTypes, const char*> WAVFile::audioType = {
    {wav, "wav"},
    {mp3, "mp3"}
};

Moreover, you should either move the definition of enum AudioTypes to the header, or possible provide a forward declaration.

Answer 2

After fixing a few issues, in particular the missing WAVFile:: before audioType and AudioTypes and the static before the definition of audioType, this code builds:

#include <map>
#include <iostream>

using namespace std;

class WAVFile {
    public:
        bool exportAudio(const char* filename, const char* filetype);

    private:
        enum AudioTypes: int;
        static std::map<AudioTypes, const char*> audioType;
};

enum WAVFile::AudioTypes: int {
    wav,
    mp3
};

map<WAVFile::AudioTypes, const char*> WAVFile::audioType = {
    {wav, "wav"},
    {mp3, "mp3"}
};

bool WAVFile::exportAudio(const char* filename, const char* filetype) {
    cout << audioType[wav] << endl;
    return true;
}

int main() {
    WAVFile file;

    file.exportAudio("export_i ran.wav","wav") ? cout << "Exported\n" : cout << "Failed\n";

    return 0;
}