Initialize sparse static array

Question

I need to initialize a static array. Not all of the values are sequential.

Something like this works fine for a sequential array:

class Foo {

  public:

  static const char * name[];

}

const char * Foo::name[] = { "Sun", "Moon" };

How can I assign values at arbitrary positions in the array? I need to do something like this (pseudocode):

const char * Foo::name[] = { 67: "Sun", 68: "Moon" };

The array will never be bigger than 255; the indices come from byte values.

I found part of a thread where someone gives an example of something similar to what I want, but I couldn't get anything like this to work.

type array[SIZE] = {[SIZE-4]=1, 2, 3, 4};

[SIZE-4]=1 are called designated intializers and are only available in C and illegal in C++ (however, in gcc they offer it as an extension). However, non-trivial (i.e. your example) are not supported. — Jesse Good
– Jesse Good, Commented Jul 4, 2012 at 4:14

Naveen · Accepted Answer · 2012-07-04 03:59:00Z

4

I would suggest you to use std::map<int, std::string> (or unordered_map if you have C++11 support) instead of the array. You can then insert into this map with the code : m[67] = "Sun" and retrieve items using std::string s = m[67];.

answered Jul 4, 2012 at 3:59

Naveen

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4 Comments

Dagg Nabbit Over a year ago

Thanks, but I'd like to do it without importing any more libraries if possible.

Naveen Over a year ago

@GGG: It is part of the C++ standard library..no additional libraries are required.

Dagg Nabbit Over a year ago

This will be different than the rest of the code. Why do you suggest using the std::strings instead of const char *? The code is written in a c-like way, encapsulated in classes.

Naveen Over a year ago

@GGG: You can use use const char*, I suggested std::string as people generally forget the const part of the char* and use it as a non char* pointer with functions like strcpy. That will cause the program to crash.

Vaughn Cato · Accepted Answer · 2012-07-04 04:06:55Z

1

Here's one old-school approach:

class NameArray {
  public:
    NameArray()
    {
      array[67] = "Sun";
      array[68] = "Moon";
    }

    const char *operator[](size_t index) const
    {
      assert(index<256);
      return array[index];
    }


  private:
    const char * array[256];
};

class Foo {
  public:
    static NameArray name;
};

NameArray Foo::name;

By wrapping the array in a class, you can make sure it gets constructed with the values that you want. You can also do bounds checking.

answered Jul 4, 2012 at 4:06

Vaughn Cato

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5 Comments

Dagg Nabbit Over a year ago

Should NameArray be a singleton if I'm going to do this?

Vaughn Cato Over a year ago

@GGG: If by singleton you mean removing the ability to copy or assign, sure if that is what you want.

Dagg Nabbit Over a year ago

I mean, this doesn't really have the advantage of being static, I was thinking a singleton pattern could fix that, but it seems like overkill. I really just want a plain static sparse array. Is there no way to do that?

Vaughn Cato Over a year ago

@GGG: I'm not sure what you mean. It is still declared static in the Foo class.

Dagg Nabbit Over a year ago

Hmm, good point, I was thinking other things would need to instantiate NameArrays, but they would just look at Foo::name.

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