Bug report #14713

Return duplicated elements in SnapWithResultsWithinTolerances mode

Added by Yun Lin over 5 years ago. Updated over 2 years ago.

Affected QGIS version:2.12.3 Regression?:No
Operating System: Easy fix?:No
Pull Request or Patch supplied:No Resolution:end of life
Crashes QGIS or corrupts data:No Copied to github as #:22674


I used QGIS recently and found out the there is certain situation i got two same snapping results from QgsSnapper. And i debugged a little, and found out a bug in the following place:

File : src/core/qgssnapper.cpp

Function : int QgsSnapper::snapMapPoint( const QgsPoint& mapCoordPoint, QList<QgsSnappingResult>& snappingResult, const QList<QgsPoint>& excludePoints )

The code :

QgsSnappingResult returnResult = evalIt.value();
for ( evalIt = snappingResultList.begin(); evalIt != snappingResultList.end(); ++evalIt ) {
if ( evalIt.value().snappedVertexNr != -1 ) {
returnResult = evalIt.value();
snappingResultList.erase( evalIt );
//We return the preferred result
snappingResult.push_back( returnResult );
if ( mSnapMode  QgsSnapper::SnapWithOneResult )
//return only a single result, nothing more to do
else if ( mSnapMode QgsSnapper::SnapWithResultsForSamePosition ) {
//take all snapping results within a certain tolerance because rounding differences may occur
double tolerance = 0.000001;
for ( evalIt = snappingResultList.begin(); evalIt != snappingResultList.end(); ++evalIt )
if ( returnResult.snappedVertex.sqrDist( evalIt.value().snappedVertex ) < tolerance*tolerance ) {
snappingResult.push_back( evalIt.value() );
else //take all results
for ( evalIt = snappingResultList.begin(); evalIt != snappingResultList.end(); ++evalIt ) {
snappingResult.push_back( evalIt.value() );

The problem occurs when var 'snappingResultList' has no elements satisfied the condition :

if ( evalIt.value().snappedVertexNr != -1 )

And since the following code :

snappingResult.push_back( returnResult );

will push the first result(by QgsSnappingResult returnResult = evalIt.value();) anyway, and
when we under 'SnapWithResultsWithinTolerances' snap mode, we will get two duplicated elements!
And since the problem is obvious, so no test case provided.


#1 Updated by Yun Lin over 5 years ago

OK. It's also affected the mode 'SnapWithResultsForSamePosition' too

#2 Updated by Giovanni Manghi over 4 years ago

  • Easy fix? set to No
  • Regression? set to No

#3 Updated by Giovanni Manghi over 2 years ago

  • Resolution set to end of life
  • Status changed from Open to Closed

End of life notice: QGIS 2.18 LTR


QGIS 3.4 has recently become our new Long Term Release (LTR) version. This is a major step in our history – a long term release version based on the massive updates, library upgrades and improvements that we carried out in the course of the 2.x to 3x upgrade cycle.

We strongly encourage all users who are currently using QGIS 2.18 LTR as their preferred QGIS release to migrate to QGIS 3.4. This new LTR version will receive regular bugfixes for at least one year. It also includes hundreds of new functions, usability improvements, bugfixes, and other goodies. See the relevant changelogs for a good sampling of all the new features that have gone into version 3.4

Most plugins have been either migrated or incorporated into the core QGIS code base.

We strongly discourage the continued use of QGIS 2.18 LTR as it is now officially unsupported, which means we’ll not provide any bug fix releases for it.

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