Initializing a static const array of const strings in C++

Question

I am having trouble initializing a constant array of constant strings.

From week.h (showing only relevant parts):

class Week {
  private:
    static const char *const *days = { "mon", "tue", "wed", "thur",
                                       "fri", "sat", "sun" };
};

When I compile I get the error "excess elements in scalar initializer". I tried making it type const char **, thinking I messed up the 2nd const placement, but I got the same error. What am I doing wrong?

Not really sure at all, but does static const char * days[] = { help or will it break your strings from being const? — ssube
– ssube, Commented Jun 30, 2011 at 15:03
@Tim : If a C-string is a char*, then a C-array of C-strings must be char**. — ildjarn
– ildjarn, Commented Jun 30, 2011 at 15:05
"An array of C-strings is type char**" No it's not. You are confusing arrays with pointers. As is ildjarn, apparently. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Oct 31, 2014 at 14:19

EvilTeach · Accepted Answer · 2019-05-07 19:14:53Z

56

First of all, you need an array, not a pointer.

static const char * const days[] = {"mon", "tue", "wed", "thur",
                                       "fri", "sat", "sun"};

Second of all, you can't initialize that directly inside the class definition. Inside the class definition, leave only this:

static const char * const days[]; //declaration

Then, in the .cpp file, write the definition

const char * const Week::days[] = {"mon", "tue", "wed", "thur",
                                       "fri", "sat", "sun"};

Update for C++11 Now you can initialize members directly in the class definition:

const char * const days[] = {"mon", "tue", "wed", "thur",
                                       "fri", "sat", "sun"};

edited May 7, 2019 at 19:14

EvilTeach

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answered Jun 30, 2011 at 15:03

Armen Tsirunyan

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2 Comments

Jaffa Over a year ago

Actually you can update your answer for C++11, so you can do it in the class declaration.

Kyle Delaney Over a year ago

Is a static array like this what you'd recommend for a set of strings like these weekday names? Is there a best practice for this sort of situation?

marni · Accepted Answer · 2014-10-31 14:13:51Z

19

For C++11, you can make the initialisation inside your class declaration, in your .h file. However, you will need to include constexpr in your .cpp file too. Example for the case above:

In your week.h file, write:

class Week {
    public:        
       static const constexpr char* const days[] = 
           { "mon", "tue", "wed", "thur","fri", "sat", "sun" };
};

In your week.cpp file, write somewhere:

constexpr const char* const Week::days[];

Make sure you enable C++11, e.g. compile with

g++ -std=c++11 week.cpp

answered Oct 31, 2014 at 14:13

marni

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2 Comments

Nir Friedman Over a year ago

This was very helpful. Do you know the fundamental reason why the bit in the .cpp file is necessary? This looks very strange and unlike typical c++ (the declaration looks like a definition and vice versa). My best guess is that the in-class initialization is initializing the elements of the array, but not the array itself, and that's what the .cpp part is doing, but I have no real faith in this. In general in C++11, I thought it was possible to initialize static constexpr's of any time purely in the class definition so I don't see why the .cpp part is necessary at all.

scx Over a year ago

The first const is redundant I believe.

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