# Pointer error, related to variable scope
**URL:** https://discourse.julialang.org/t/pointer-error-related-to-variable-scope/41910
**Category:** General Usage
**Tags:** question, package
**Created:** [June 23, 2020, 3:33am UTC](https://discourse.julialang.org/t/pointer-error-related-to-variable-scope/41910 "2020-06-23T03:33:26Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@zhangliye](https://discourse.julialang.org/u/zhangliye)
#### Post date: [June 23, 2020, 3:33am UTC](https://discourse.julialang.org/t/pointer-error-related-to-variable-scope/41910/1 "2020-06-23T03:33:27Z")
I create a struct as a global variable named `g_preload_data ` which contains a member variable `hotspot_area` in `preload.jl`. The member variable `hotspot_area` is initialized by creating a ArchGDAL geometry object.
When I use this variable `g_preload_data.hotspot_area` in a function in `main.jl`, there is error. If i create the ArchGDAL object locally, there is no error.
‘preload.jl’
```julia
using ArchGDAL
mutable struct SystemPreloadData
hotspot_area
function SystemPreloadData()
self = new()
self.hotspot_area = load_area()
return self
end
end
function load_area()
d = ArchGDAL.read(hotspot_path)
layer = ArchGDAL.getlayer(d, 0)
fs = ArchGDAL.getfeature(layer, 1) do f
ArchGDAL.getgeom(f)
end
return fs
end
g_preload_data = SystemPreloadData()
```
1. Got errors when use the global variable in main.jl as follows
```julia
ArchGDAL.contains( g_preload_data .hotspot_area,
ArchGDAL.createpoint( 102.1, 0.4) ) # error here
```
The error message is as follows
```julia
ERROR: LoadError: LoadError: GDALError (CE_Failure, code 10):
Pointer 'hThis' is NULL in 'OGR_G_Contains'.
Stacktrace:
[1] gdaljl_errorhandler(::GDAL.CPLErr, ::Int32, ::Cstring) at /home/liye/.julia/packages/GDAL/yPmFi/src/error.jl:36
[2] ogr_g_contains at /home/liye/.julia/packages/GDAL/yPmFi/src/ogr_api.jl:941 [inlined]
[3] contains(::ArchGDAL.IGeometry, ::ArchGDAL.IGeometry) at /home/liye/.julia/packages/ArchGDAL/ohMpc/src/ogr/geometry.jl:605
[4] #read_trj_json#21(::Bool, ::typeof(Hotspot.read_trj_json), ::String) at /mnt/c/julia_dev/mt/hotspot/src/detect_hotspot_json_file.jl:26
[5] read_trj_json at /mnt/c/julia_dev/mt/hotspot/src/detect_hotspot_json_file.jl:13 [inlined]
[6] macro expansion at ./util.jl:288 [inlined]
[7] #detect_hotspot_json_file#26(::Int64, ::Int64, ::Int64, ::String, ::typeof(Hotspot.detect_hotspot_json_file), ::String) at /mnt/c/julia_dev/mt/hotspot/src/detect_hotspot_json_file.jl:105
[8] (::Hotspot.var"#kw##detect_hotspot_json_file")(::NamedTuple{(:out_dir_base,),Tuple{String}}, ::typeof(Hotspot.detect_hotspot_json_file), ::String) at ./none:0
[9] top-level scope at util.jl:288
[10] top-level scope at /mnt/c/julia_dev/mt/hotspot/test/test_debug_data.jl:24
[11] include at ./boot.jl:328 [inlined]
[12] include_relative(::Module, ::String) at ./loading.jl:1105
[13] include(::Module, ::String) at ./Base.jl:3
```
1. If call load data function locally, there is no error
```julia
hotspot_area = load_area()
ArchGDAL.contains( hotspot_area,
ArchGDAL.createpoint( 102.1, 0.4) ) # works well
```
It would be caused by something related to variable scope. Can any one help? Thanks
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [June 23, 2020, 5:04am UTC](https://discourse.julialang.org/t/pointer-error-related-to-variable-scope/41910/2 "2020-06-23T05:04:09Z")
```julia
g_preload_data = SystemPreloadData()
```
Does this line happen within a `module`? If so, it’s probably being precompiled, which will not work if that data structure stores any raw pointers. Instead, you need to construct the pointer in your ` __init__ ` function, as discussed in [Modules · The Julia Language](https://docs.julialang.org/en/v1/manual/modules/index.html#Module-initialization-and-precompilation-1)
---
### Author:  [@zhangliye](https://discourse.julialang.org/u/zhangliye)
#### Post date: [June 23, 2020, 6:34am UTC](https://discourse.julialang.org/t/pointer-error-related-to-variable-scope/41910/3 "2020-06-23T06:34:45Z")
Your suggestion works. It is solved by putting the code into ` __init__ ()`. Thank you so much!