Challenge Library

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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All levels Beginner Intermediate Hard

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5,893 challenges total

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
Code AnalysisBeginner
Lambda capture by reference outliving its scope
lambdacapturedangling
Code AnalysisBeginner
Missing virtual destructor deleted through base
virtual-destructorpolymorphismub
Code AnalysisBeginner
Unsigned underflow in a countdown loop
unsigned-underflowloopsout-of-bounds
Code AnalysisBeginner
Range-for over a temporary container
string-viewrange-fordangling
Code AnalysisBeginner
Memory leak from an early return
raiileakraw-pointer
Code AnalysisBeginner
Slicing when passing derived by value
object-slicingpolymorphismpass-by-value
Code AnalysisIntermediate
Iterator invalidation while erasing in a vector loop
iterator-invalidationvectorerase
Code AnalysisIntermediate
Dangling pointer to a local vector's data
dangling-pointervectorlifetime
Code AnalysisIntermediate
Attempting to copy a unique_ptr
unique-ptrmove-onlycompile-error
Code AnalysisIntermediate
Cross-thread visibility with a relaxed flag
memory-orderatomicsdata-race
Code AnalysisIntermediate
Self-deadlock from locking a non-recursive mutex twice
deadlockmutexconcurrency
Code AnalysisHard
Use-after-move of a std::string argument
use-after-movestringrvalue
Code AnalysisHard
Strict aliasing violation through reinterpret_cast
strict-aliasingreinterpret-castub
Code AnalysisHard
Uninitialized read causing sanitizer failure
uninitializedsanitizerub
Code AnalysisHard
Condition variable lost wakeup with a missing lock
condition-variableracelost-wakeup
Code AnalysisIntermediate
Missing std::forward loses value category
perfect-forwardingstd-forwardvalue-category
Code AnalysisBeginner
relaxed store load reordering output
atomicsmemory-modelcode-analysis
Code AnalysisBeginner
false sharing loop output
false-sharingperformancecode-analysis
Code AnalysisBeginner
initializer_list dangling output
initializer-listlifetimecode-analysis
Code AnalysisBeginner
const char8_t mismatch compile
char8_tunicodecode-analysis
Code AnalysisBeginner
span over temporary output
spandanglingcode-analysis
Code AnalysisBeginner
scope guard throw double output
RAIIexceptionscode-analysis
Code AnalysisIntermediate
atomic double CAS loop output
atomicsfloating-pointcode-analysis
Code AnalysisIntermediate
flat_map sort key output
flat_mapcontainerscode-analysis
Code AnalysisIntermediate
structured binding reference output
structured-bindingsrange-forcode-analysis
Code AnalysisIntermediate
pimpl unique_ptr inline dtor output
pimplunique_ptrincomplete-typecode-analysis
Code AnalysisIntermediate
to_chars buffer too small output
to_charsnumericcode-analysis
Code AnalysisHard
seqlock torn read output
seqlockdata-racecode-analysis
Code AnalysisHard
ADL swap recursion output
ADLname-lookupcode-analysis
Code AnalysisHard
friend injection capture output
friend-injectionmetaprogrammingcode-analysis
Code AnalysisHard
constexpr virtual dispatch output
constexprvirtualcode-analysis
Code AnalysisHard
dependent name template disambiguation output
two-phase-lookuptemplate-disambiguationcode-analysis
Code AnalysisBeginner
Function-Local Static Guard Output
staticmagic-staticsinit
Code AnalysisBeginner
std::atomic wait Missing Predicate
atomicwaitrace
Code AnalysisBeginner
Latch Misused as Reusable
latchreusesync
Code AnalysisBeginner
DCLP Missing Release Ordering
dclpsingletonmemory-order
Code AnalysisBeginner
union Type Punning UB
uniontype-punningub
Code AnalysisBeginner
std::function Allocation Cost
std_functionsboperformance
Code AnalysisBeginner
CRTP Static Dispatch Output
crtpstatic-dispatchtemplates
Code AnalysisBeginner
constinit with Dynamic Initializer
constinitstatic-initcompile-error
Code AnalysisIntermediate
Coroutine Frame Dangling Reference
coroutineslifetimedebugging
Code AnalysisIntermediate
Suspended Coroutine Parameter Lifetime
coroutinesframesparameters
Code AnalysisIntermediate
Barrier Completion Function Deadlock
barrierdeadlockphases
Code AnalysisIntermediate
Semaphore Release Overrun
semaphoreubcounters
Code AnalysisIntermediate
atomic_ref Misalignment
atomic_refalignmentlayout
Code AnalysisHard
Lock-Free Stack ABA Root Cause
lock-freeabacas
Code AnalysisHard
Hazard Pointer Ordering Bug
hazard-pointersmemory-orderrace
Code AnalysisHard
Static Init Order Fiasco
static-initodrtranslation-unit

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

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

Lambda capture by reference outliving its scope

Missing virtual destructor deleted through base

Unsigned underflow in a countdown loop

Range-for over a temporary container

Memory leak from an early return

Slicing when passing derived by value

Iterator invalidation while erasing in a vector loop

Dangling pointer to a local vector's data

Attempting to copy a unique_ptr

Cross-thread visibility with a relaxed flag

Self-deadlock from locking a non-recursive mutex twice

Use-after-move of a std::string argument

Strict aliasing violation through reinterpret_cast

Uninitialized read causing sanitizer failure

Condition variable lost wakeup with a missing lock

Missing std::forward loses value category

relaxed store load reordering output

false sharing loop output

initializer_list dangling output

const char8_t mismatch compile

span over temporary output

scope guard throw double output

atomic double CAS loop output

flat_map sort key output

structured binding reference output

pimpl unique_ptr inline dtor output

to_chars buffer too small output

seqlock torn read output

ADL swap recursion output

friend injection capture output

constexpr virtual dispatch output

dependent name template disambiguation output

Function-Local Static Guard Output

std::atomic wait Missing Predicate

Latch Misused as Reusable

DCLP Missing Release Ordering

union Type Punning UB

std::function Allocation Cost

CRTP Static Dispatch Output

constinit with Dynamic Initializer

Coroutine Frame Dangling Reference

Suspended Coroutine Parameter Lifetime

Barrier Completion Function Deadlock

Semaphore Release Overrun

atomic_ref Misalignment

Lock-Free Stack ABA Root Cause

Hazard Pointer Ordering Bug

Static Init Order Fiasco

Practice now, dominate the arena