18

I have a class called AppSettings where I have an Array with a range of note frequencies. I'm getting several errors with the code below and I'm not sure what the problem is.

The error messages are:

  • static data member of type 'const float [36] must be initialized out of line
  • A brace enclosed initializer is not allowed here before '{' token
  • Invalid in-class initialization of static data member of non-integral type

And the code:

class AppSettings{

public:
    static const float noteFrequency[36] = {
    //  C       C#      D       D#      E       F       F#      G       G#      A       A#      B
        130.81, 138.59, 146.83, 155.56, 164.81, 174.61, 185.00, 196.00, 207.65, 220.00, 223.08, 246.94,
        261.63, 277.18, 293.66, 311.13, 329.63, 349.23, 369.99, 392.00, 415.30, 440.00, 466.16, 493.88,
        523.25, 554.37, 587.33, 622.25, 659.25, 698.46, 739.99, 783.99, 830.61, 880.00, 932.33, 987.77
    };

};

As the name suggests this is just a header file with some settings and values I need throughout the app.

Improve this question
4
  • 3
    Note frequencies are simply calculated by mathematical formula. Do it dynamically at startup. Commented Jul 27, 2012 at 18:44
  • That's a good idea, I'll have a look Commented Jul 27, 2012 at 20:55
  • @Rok Kralj, why waste time calculating static data that will never change. Commented Jan 2, 2015 at 4:08
  • @bazz: Precision. However - this example is so small it doesn't really matter. But if it were a lot bigger, I'd prefer 0,01ms slower program startup to 200k bigger executable size. Commented Jan 2, 2015 at 8:57

3 Answers 3

Reset to default
30

You can't define the value of static class members within the class. You need to have a line like this in the class:

class AppSettings
{
public:
    static const float noteFrequency[];

And then in an implementation file for the class (AppSettings.cpp perhaps):

const float AppSettings::noteFrequency[] = { /* ... */ };

Also, you don't need to specify the number within the [] here, because C++ is smart enough to count the number of elements in your initialization value.

Improve this answer
Sign up to request clarification or add additional context in comments.

5 Comments

-1 As I'm writing this the advice is just to place the initializer after the class. With that one would in general break the One Definition Rule. I.e. for the general case the linker will protest strongly.
@Cheersandhth.-Alf That has nothing to do with the question at hand. This answer is well written and concise. Not putting implementation in header files is just common sense for the language. I could mark down half the solutions on SO because people leave things out to be concise, it doesn't make it wrong. Though it might make your answer better than this one if you provided extra content.
@woote: of course the correctness of the answer matters. on the other hand, your point about how well written it is, is in my opinion irrelevant.
@woote: I agree with Cheers and hth on this, actually.
@Cheersandhth.-Alf: I actually misread the OP: I thought he say that this was all in an implementation file, not all in a header file. Changing my answer to state this explicitly.
13

This works just fine in C++11

class AppSettings{

public:
    static constexpr float noteFrequency[36] = {
//  C       C#      D       D#      E       F       F#      G       G#      A       A#      B
    130.81, 138.59, 146.83, 155.56, 164.81, 174.61, 185.00, 196.00, 207.65, 220.00, 223.08, 246.94,
    261.63, 277.18, 293.66, 311.13, 329.63, 349.23, 369.99, 392.00, 415.30, 440.00, 466.16, 493.88,
    523.25, 554.37, 587.33, 622.25, 659.25, 698.46, 739.99, 783.99, 830.61, 880.00, 932.33, 987.77
    };

};
Improve this answer

1 Comment

In case constexpr can't be used, you can use inline static const float noteFrequency since C++17.
7

C++03 doesn't support in-class definitions of complex data like arrays of constants.

To place such a definition at namespace scope in a header file, and avoid breaking the One Definition Rule, you can leverage a special exemption for template classes, as follows:

#include <iostream>
using namespace std;

//----------------------------------------- BEGIN header file region
template< class Dummy >
struct Frequencies_
{
    static const double noteFrequency[36];
};

template< class Dummy >
double const Frequencies_<Dummy>::noteFrequency[36] =
{
    //  C       C#      D       D#      E       F       F#      G       G#      A       A#      B
    130.81, 138.59, 146.83, 155.56, 164.81, 174.61, 185.00, 196.00, 207.65, 220.00, 223.08, 246.94,
    261.63, 277.18, 293.66, 311.13, 329.63, 349.23, 369.99, 392.00, 415.30, 440.00, 466.16, 493.88,
    523.25, 554.37, 587.33, 622.25, 659.25, 698.46, 739.99, 783.99, 830.61, 880.00, 932.33, 987.77
};

class AppSettings
    : public Frequencies_<void>
{
public:
};
//----------------------------------------- END header file region

int main()
{
    double const a = AppSettings::noteFrequency[21];

    wcout << a << endl;
}

There are also some other techniques that can be used:

  • An inline function producing a reference to the array (or used as indexer).

  • Placing the definition in a separately compiled file.

  • Simply computing the numbers as needed.

Without more information I wouldn’t want to make the choice for you, but it shouldn’t be a difficult choice.

Improve this answer

4 Comments

@MooingDuck: good idea, but it gives me multiple definition linking error? after full specialization the class is is a real class. and i think, no special ODR exemption for it then.
@MooingDuck: it should compile, but it duplicates the data: once for false instance and once for true instance. perhaps compiler smart enough to optimize that away. but if one wants to get rid of possible inadvertent data duplication then i think a custom enum with no defined elements would be the way go.
@MooingDuck: both visual c++ and g++ optimize that away. but each instantiation of the template is its own class. i think that to the degree that data is not duplicated here, it's not duplicated by the template in my answer either?
Once I thought it through, my code is stupid. Of course it duplicates the data, I had the data being a member of every instance of the class. Dunno what I was thinking.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.