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This is related to some other questions, such as: this, and some of my other questions.

In this question, and others, we see we can declare and initialise string arrays in one nice step, for example:

const char* const list[] = {"zip", "zam", "bam"}; //from other question

This can be done in the implementation of a function with no bother, or in the body of a .cpp file, outside any scope.

What I want to do is to have an array like this as as member of a class I am using, something like this:

class DataProvider : public SomethingElse { const char* const mStringData[] = {"Name1", "Name2", "Name3", ... "NameX"}; public: DataProvider(); ~DataProvider(); char* GetData() { int index = GetCurrentIndex(); //work out the index based on some other data return mStringData[index]; //error checking and what have you omitted } };

But, the compiler complains and I can't seem to work out why. Is it possible to declare and initialise an array like this in one step in a class definition? Are there alternatives that are better?

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  • "implimentation" should be spelt "implementation" Commented Nov 13, 2008 at 10:22

3 Answers 3

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Use the keyword static and external initialization to make the array a static member of the class:

In the header file:

class DataProvider : public SomethingElse { static const char* const mStringData[]; public: DataProvider(); ~DataProvider(); const char* const GetData() { int index = GetCurrentIndex(); //work out the index based on some other data return mStringData[index]; //error checking and what have you omitted } };

In the .cpp file:

const char* const DataProvider::mStringData[] = {"Name1", "Name2", "Name3", ... "NameX"};
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8 Comments

You need to give the array a size in the header declaration.
Are you sure? It work fine for me (Visual C++ 2005) and I have been using it previously a few times. Unless it is undefined behaviour in the standard (which I will not look up now), I believe it will work.
You are fine while using the initialiser. The compiler will calculate the size based on the number of items in the initialiser.
Yeah i think it's valid to omit the size, the array is incomplete then. But in the header, you won't know the size of the array then indeed. so no sizeof on it possible too -.-
Well, since the OP is using a (member) function, GetCurrentIndex(), it can use the sizeof operator if it is defined in the same .cpp file.
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The reason you can't declare your array like that (const char* []) is that:

  • you can't have initializers in the class declaration, and so
  • the syntax const char* [] does not state how much space the compiler needs to allocate for each instance (your array is declared as instance variable).

Besides, you probably want to make that array static, since it is in essence a constant value.

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Actually, const char* [] does state how much space the compiler needs to allocate for each instance - just a pointer to memory. This is really why static array declaration is not allowed for non-static variables; every new instance requires extra memory allocation, and that kind of processing overhead is traditionally handled explicitly in constructors.
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This is not possible in C++. You cannot directly initialize the array. Instead you have to give it the size it will have (4 in your case), and you have to initialize the array in the constructor of DataProvider:

class DataProvider { enum { SIZEOF_VALUES = 4 }; const char * values[SIZEOF_VALUES]; public: DataProvider() { const char * const v[SIZEOF_VALUES] = { "one", "two", "three", "four" }; std::copy(v, v + SIZEOF_VALUES, values); } };

Note that you have to give up on the const-ness of the pointers in the array, since you cannot directly initialize the array. But you need to later set the pointers to the right values, and thus the pointers need to be modifiable.

If your values in the array are const nevertheless, the only way is to use a static array:

/* in the header file */ class DataProvider { enum { SIZEOF_VALUES = 4 }; static const char * const values[SIZEOF_VALUES]; }; /* in cpp file: */ const char * const DataProvider::values[SIZEOF_VALUES] = { "one", "two", "three", "four" };

Having the static array means all objects will share that array. Thus you will have saved memory too.

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"Having the static array means all objects will share that array" Which also means you have likely just compromised the fundamental principles of object-oriented programming.

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