- make test fails to find testxgboost, if a build directory is created

[sriramch]

on which cmake is run. this appends the build directory to the test binary

• make the assignments of HostDeviceVector exception safe
• rm storing a dummy GPUDistribution instance for CPU based code

[rongou]

@RAMitchell

[sriramch]

#3720 reference to bad_alloc

[codecov-io]

Codecov Report

Merging #4301 into master will increase coverage by <.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #4301      +/-   ##
==========================================
+ Coverage   67.82%   67.82%   +<.01%     
==========================================
  Files         132      132              
  Lines       12202    12203       +1     
==========================================
+ Hits         8276     8277       +1     
  Misses       3926     3926

Impacted Files	Coverage Δ
src/common/host_device_vector.h	75% <ø> (ø)	⬆️
src/common/host_device_vector.cc	64.47% <85.71%> (+0.96%)	⬆️
src/tree/updater_histmaker.cc	64.62% <0%> (-0.1%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6d5b34d...f800657. Read the comment docs.

[trivialfis]

@sriramch Your assignment operator is not exception safe since new can throw. https://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom might help a little. But I would leave it to another PR. ;-)

[sriramch]

@sriramch Your assignment operator is not exception safe since new can throw. https://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom might help a little. But I would leave it to another PR. ;-)

@trivialfis what this patch is attempting to do is that when a target hostvectorimpl (other) is assigned to a source (*this), any intermediary exception that may occur during this assignment does not alter the initial state of the source. it is not trying to make the assignment operator not throw any exceptions at all. this entails simply doing the copy work on the side and when that operation is deemed successful, simply swaps the guts of it with the source. if the side work fails ofr some reason (say due to new throwing as you state), source object (*this) isn't impacted in any way

[trivialfis]

@sriramch Ah, I see. Thanks for the explanation. Looks good to me.