GDAL C++ test layer capabilities

Question

I'm trying to check if a shapefile has the fast feature count. I have the following C++ code:

#include <iostream>
#include "ogrsf_frmts.h"

int main()
{
    GDALAllRegister();
    GDALDataset *poDS;
    poDS = (GDALDataset*)GDALOpenEx("C:/Users/root/Desktop/2016/GIS Programming/Plugin_new_new/Shapefiles_Karte/Ortsstrasse/temp/buildings - Kopie (2).shp", GDAL_OF_VECTOR, NULL, NULL, NULL);
    if (poDS == NULL)
    {
        printf("Open failed.\n");
        exit(1);
    }
    if (poDS->TestCapability(OLCFastFeatureCount) == true) {
        printf("true\n");
    } else {
        printf("false\n");
    }

    GDALClose(poDS);
}

The code tells me every time "false" even though I am 100% sure the input shapefile has the fast feature count ability.

So I assume that there is a problem in the function call. Isn't this the way you call the TestCapability function?

I am very new to C++ and have only used GDAL with Python so far ..

Answer 1

Ok, by looking at the source code of a python script doing the exact same thing I think I found the error. The working source code is this one:

#include <iostream>
#include "ogrsf_frmts.h"

int main()
{
    GDALAllRegister();
    GDALDataset *poDS;    
    poDS = (GDALDataset*)GDALOpenEx("C:/Users/root/Desktop/2016/GIS Programming/Plugin_new_new/Shapefiles_Karte/Landkreise/Landkreise.shp", GDAL_OF_VECTOR, NULL, NULL, NULL);
    if (poDS == NULL)
    {
        printf("Open failed.\n");
        exit(1);
    }

    bool fastFeatureCount = true;

    for (int i = 0; i < poDS->GetLayerCount(); i++){
       auto layer = poDS->GetLayer(i);
       if (layer->TestCapability(OLCFastFeatureCount) == false) fastFeatureCount = false;
    }

    printf("%d\n", fastFeatureCount);


    GDALClose(poDS);
}

Might be some sloppy coding style in there, but I'm new to c++ and don't know any better ..

Answer 2

    if (poDS->TestCapability(OLCFastFeatureCount) == true) {

This is dangerous: TestCapability returns an int, which is documented to be TRUE or FALSE. Comparing an int to the boolean value true will cause the boolean to be implicitly converted to 1, so if TRUE is defined as -1 or some other value, you'll get false from this comparison.

Instead you should compare to the constant TRUE:

    if (poDS->TestCapability(OLCFastFeatureCount) == TRUE) {

Or just check for nonzero (because any sane library defines FALSE as 0):

    if (poDS->TestCapability(OLCFastFeatureCount)) {