c++,arrays,dll,linker , holding constructed static arrays in memory for multiple files c++


holding constructed static arrays in memory for multiple files c++

Question:

Tag: c++,arrays,dll,linker

I have a few arrays that I need to hold in memory for the duration of my program. The arrays are used as look up references by different files so I thought I should make a DLL to hold them in.

The main problem I seem to run into is that the files have to be constructed at the beginning of the program. The arrays hold a few thousand values each and future ones may hold millions, so hard coding the arrays isn't an option.

Here is my best attempt:

First, I made the Dll header file. I read about making static constructors, which is what I am trying to do here to hold the arrays. I put the export only on the NumCalc class (correct?).

// TablesDll.h
#ifndef TABLESDLL_EXPORTS
#define TABLESDLL_EXPORTS

#ifdef TABLESDLL_EXPORTS
    #define TABLESDLL_API __declspec(dllexport) 
#else
#define TABLESDLL_API __declspec(dllimport) 

#endif

namespace tables
{
    class Arrays
    {
    public:
        static const int* arr;

    private:
        static int* SetNums();

    };

    class TABLESDLL_API NumCalc
    {
    public:
        static Arrays arrays;
    };
}


#endif

Now the definitions:

// TablesDll.cpp
#include "stdafx.h"
#include "TablesDll.h"
#include <stdexcept>   (<- I don't know why this is here...)

namespace tables
{   
    const int* Arrays::arr = SetNums();

    int* Arrays::SetNums()
    {
        int* arr= new int[2000];
        /* set the numbers*/
        return arr;
    }
}

It compiles fine. I take the files and stick them into a test program as so:

// TestTablesDll
#include "stdafx.h"
#include "TablesDll.h"
using namespace tables;

int _tmain(int argc, _TCHAR* argv[])
{
    for(int i=0; i<299; i++)
        printf("arr[%i] = %d/n", i, NumCalc::arrays::arr[i]);

    return 0;
}

This doesn't even compile unfortunately.

error C3083: 'arrays': the symbol to the left of a '::' must be a type

My previous attempt didn't use the static constructor. There was no class Arrays. NumCalc was the only class containing

static TABLESDLL_API const int* arr

and the private function

static const int* SetNums().

This yielded a LNK2001 compiler error when run in the TestTablesDll

I'm pretty sure there's an issue with the function not running at compile time, leaving the arr variable undefined.

How can I do this?


Answer:

In TablesDll.h you should put TABLESDLL_API to the Arrays class too. Otherwise you will not be able to use the parts of NumCalc that depend on Arrays.

Also you should have this Arrays NumCalc::arrays; in TablesDll.cpp even though Arrays is an empty class - arrays has to be defined somewhere (and not just declared in the class definition).

EDIT: There were more problems.

arr should be accessed like this: NumCalc::arrays.arr - with . and not with ::

Also the header always exports symbols beacuse you define TABLESDLL_EXPORTS and right after that you check if it's defined. This is how it should be:

#ifndef TABLESDLL_HEADER_GUARD
#define TABLESDLL_HEADER_GUARD

#ifdef TABLESDLL_EXPORTS
    #define TABLESDLL_API __declspec(dllexport) 
#else
    #define TABLESDLL_API __declspec(dllimport) 
#endif

and in TablesDll.cpp you should define TABLESDLL_EXPORTS before including the header - so that only the dll exports the symbols and the executable imports them. Like this:

#define TABLESDLL_EXPORTS
#include "TablesDll.h"

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