The type ‘TArray<…>’ cannot be used as a value in a TMap (Unreal Engine Fix)

Table of Contents

What this tutorial covers

  1. Why Unreal Engine throws this error
  2. The underlying restriction in UPROPERTY and reflection
  3. Correct ways to use TArray inside TMap
  4. Working fixes with USTRUCT wrapping
  5. Alternative patterns when nesting containers fails

The problem

You may encounter an error like:

 
The type ‘ArrayProperty’ can not be used as a value in a TMap
 

Or when writing code like:

 
UPROPERTY()
TMap<int32, TArray<int32>> MyMap;
 

This fails even though both TMap and TArray are valid Unreal containers.

Why this happens

This is not a C++ limitation.
It is a Unreal Header Tool (UHT) and reflection system restriction.

Key rule:

UPROPERTY does not support nested container types directly.

Specifically:

  • TMap<Key, Value> is supported
  • TArray<Type> is supported
  • ❌ TMap<Key, TArray<Type>> is not supported as a reflected property

This happens because:

  • Unreal’s reflection system represents containers using FProperty types
  • TArray becomes FArrayProperty
  • TMap becomes FMapProperty

But:

Unreal does not allow FArrayProperty to be used as the value property inside FMapProperty

So UHT throws:

 
ArrayProperty cannot be used as a value in a TMap

Important distinction

Important distinction

This restriction only applies when using:

UPROPERTY()

or from Blueprint authoured maps.


This works (no reflection):
TMap<int32, TArray<int32>> MyMap;


This fails (reflection enabled):

UPROPERTY()
TMap<int32, TArray<int32>> MyMap;

 

The correct solution: Wrap the array in a USTRUCT

Instead of storing TArray directly, wrap it in a struct.

Step 1 — Define a struct

				
					USTRUCT()
struct FIntArrayWrapper
{
    GENERATED_BODY()

    UPROPERTY()
    TArray<int32> Values;
};

Step 2 — Use it inside TMap

				
					UPROPERTY()
TMap<int32, FIntArrayWrapper> MyMap;

Why this works

Now Unreal sees:

  • TMap<Key, Struct> → valid
  • Struct contains TArray → valid

This avoids the unsupported container-inside-container reflection case.

Common variations of this error

You may also see:

  • ArrayProperty cannot be used as a value in a TMap
  • Nested containers are not supported
  • Errors when using TSet<TArray<...>>
  • Errors when exposing nested containers to Blueprints

They all stem from the same restriction.

Summary

  • This is a reflection system limitation, not a C++ issue
  • TMap<Key, TArray<Value>> is invalid with UPROPERTY
  • The correct fix is to wrap the array inside a USTRUCT
  • Alternatively, remove UPROPERTY if reflection is not needed
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