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Coding Challenges

5,893 curated questions across every arena — browse by topic, then jump into a live 3-minute battle to put them to the test.

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Code AnalysisBeginner
Returning reference to local
cppdanglingreferencescode-analysis
Code AnalysisBeginner
Iterator invalidation in vector loop
cppstlvectoriterator-invalidation
Code AnalysisBeginner
Slicing via by-value base parameter
cppobject-slicingvirtualcode-analysis
Code AnalysisIntermediate
Deleting derived through base pointer
cppvirtualdestructorundefined-behavior
Code AnalysisIntermediate
Capture by reference outliving scope
cpplambdacapturedangling
Code AnalysisIntermediate
Dangling reference in std::async
cppasynclambdacapture
Code AnalysisHard
Dangling string_view from temporary
cppstring_viewdanglingcode-analysis
Code AnalysisHard
Pessimizing std::move on return
cppmove-semanticsrvoanti-pattern
Code AnalysisHard
shared_ptr reference cycle
cppsmart-pointersshared_ptrmemory-leak
Code AnalysisIntermediate
Return local by value
cpprvoreturncode-analysis
Code AnalysisBeginner
Move on const object copies
cppmove-semanticsconst-correctnessgotcha
Code AnalysisHard
Cross-TU static init order
cppstatic-initodrundefined-behavior
Code AnalysisBeginner
remove vs erase semantics
cppalgorithmserase-removecode-analysis
Code AnalysisIntermediate
Lambda returning reference to by-value capture
cpplambdadanglingreferences
Code AnalysisBeginner
unique_ptr copy attempt
cppsmart-pointersunique_ptrcode-analysis
Code AnalysisIntermediate
Virtual call during destruction
cppvirtualdestructorcode-analysis
Code AnalysisIntermediate
unordered_map pointer survives rehash
cppstlunordered_mapiterator-invalidation
Code AnalysisBeginner
Signed integer overflow
cppundefined-behaviorinteger-overflowcode-analysis
Code AnalysisIntermediate
Reference to vector element across growth
cppstlvectordangling
Code AnalysisIntermediate
Brace init narrowing error
cppaggregate-initnarrowingcode-analysis
Code AnalysisIntermediate
const member mutating through pointer
cppconst-correctnesspointersgotcha
Code AnalysisHard
Lifetime extension of temporary subobject binding
cppreferenceslifetimetemporary
Code AnalysisHard
Missing std::forward loses rvalueness
cpptemplatesperfect-forwardingstd-forward
Code AnalysisIntermediate
new[] / delete mismatch
cppundefined-behaviornew-deletecode-analysis
Code AnalysisHard
Exception-unsafe member update
cppexception-safetycode-analysis
Code AnalysisIntermediate
Range-for over returning-by-value container
cpprange-forlifetimecode-analysis
Code AnalysisBeginner
vector<bool> auto deduction
cppstlvector-boolauto
Code AnalysisHard
static_cast vs dynamic_cast downcast
cppcastingundefined-behaviorcode-analysis
Code AnalysisBeginner
dynamic_cast requires polymorphic type
cppcastingrtticode-analysis
Code AnalysisIntermediate
Two-mutex lock-order deadlock
cppthreadingdeadlockmutex
Code AnalysisBeginner
condition_variable wait predicate
cppthreadingcondition-variablespurious-wakeup
Code AnalysisIntermediate
volatile increment not atomic
cppatomicsvolatiledata-race
Code AnalysisHard
Relaxed store publish bug
cppatomicsmemory-ordercode-analysis
Code AnalysisBeginner
reserve prevents reallocation
cppstlvectorreserve
Code AnalysisBeginner
Missing return in non-void function
cppundefined-behaviorcode-analysis
Code AnalysisBeginner
operator[] inserts default element
cppstlmapoperator-subscript
Code AnalysisBeginner
map::at vs operator[] on missing
cppstlmapcode-analysis
Code AnalysisHard
initializer_list in emplace_back
cppstlemplacebraced-init
Code AnalysisIntermediate
Detached thread using this
cppthreadingdanglinglambda
Code AnalysisIntermediate
std::thread destructor terminate
cppthreadingstd-threadcode-analysis
Code AnalysisHard
Self-deadlock non-recursive mutex
cppthreadingmutexdeadlock
Code AnalysisBeginner
unique_lock needed for condition_variable
cppthreadingcondition-variableunique-lock
Code AnalysisBeginner
sizeof empty class
cppsizeofcode-analysis
Code AnalysisIntermediate
sizeof object with vptr
cppsizeofvtablecode-analysis
Code AnalysisHard
Lifetime extension of returned-by-value ref
cppreferenceslifetimetemporary
Code AnalysisHard
Lambda in loop capturing by reference
cpplambdacapturegotcha
Code AnalysisBeginner
Lambda capturing loop var by value
cpplambdacapturecode-analysis
Code AnalysisHard
string_view elements dangle
cppstring_viewdanglinglifetime
Code AnalysisIntermediate
constexpr used as array bound
cppconstexprarraycode-analysis
Code AnalysisHard
Header function missing inline
cppodrinlinelinker
Code AnalysisIntermediate
vector uses copy when move may throw
cppexception-safetynoexceptvector
Code AnalysisBeginner
Copy-init from derived to base object
cppobject-slicingvirtualcode-analysis
Code AnalysisIntermediate
Infinite loop optimized away
cppatomicsundefined-behavioroptimization
Code AnalysisBeginner
Use after move of string
cppmove-semanticsmoved-fromcode-analysis
Code AnalysisBeginner
Returning data() pointer of local vector
cppdanglingstlvector
Code AnalysisIntermediate
Calling empty std::function
cppstd-functioncode-analysis
Code AnalysisHard
weak_ptr use_count check is racy
cppsmart-pointersweak_ptrthread-safety
Code AnalysisHard
Forwarding reference deduction
cpptemplatesforwarding-referencecode-analysis
Code AnalysisHard
decltype(auto) vs auto
cppautodecltypetype-deduction
Code AnalysisIntermediate
No implicit void* conversion
cpptypingvoid-pointercode-analysis

Practice now, dominate the arena

Every challenge here is drawn from the live Code Royale pool. Review a question, then head to the arena to answer under pressure against a real opponent — 3-minute matches, Elo ranking, and badge tiers on the line.

Coding Challenges

Returning reference to local

Iterator invalidation in vector loop

Slicing via by-value base parameter

Deleting derived through base pointer

Capture by reference outliving scope

Dangling reference in std::async

Dangling string_view from temporary

Pessimizing std::move on return

shared_ptr reference cycle

Return local by value

Move on const object copies

Cross-TU static init order

remove vs erase semantics

Lambda returning reference to by-value capture

unique_ptr copy attempt

Virtual call during destruction

unordered_map pointer survives rehash

Signed integer overflow

Reference to vector element across growth

Brace init narrowing error

const member mutating through pointer

Lifetime extension of temporary subobject binding

Missing std::forward loses rvalueness

new[] / delete mismatch

Exception-unsafe member update

Range-for over returning-by-value container

vector<bool> auto deduction

static_cast vs dynamic_cast downcast

dynamic_cast requires polymorphic type

Two-mutex lock-order deadlock

condition_variable wait predicate

volatile increment not atomic

Relaxed store publish bug

reserve prevents reallocation

Missing return in non-void function

operator[] inserts default element

map::at vs operator[] on missing

initializer_list in emplace_back

Detached thread using this

std::thread destructor terminate

Self-deadlock non-recursive mutex

unique_lock needed for condition_variable

sizeof empty class

sizeof object with vptr

Lifetime extension of returned-by-value ref

Lambda in loop capturing by reference

Lambda capturing loop var by value

string_view elements dangle

constexpr used as array bound

Header function missing inline

vector uses copy when move may throw

Copy-init from derived to base object

Infinite loop optimized away

Use after move of string

Returning data() pointer of local vector

Calling empty std::function

weak_ptr use_count check is racy

Forwarding reference deduction

decltype(auto) vs auto

No implicit void* conversion

Practice now, dominate the arena