{"id":13393,"date":"2026-04-13T05:48:24","date_gmt":"2026-04-13T03:48:24","guid":{"rendered":"https:\/\/store.algosyntax.com\/?post_type=asx-lms-tutorial-cpt&p=13393"},"modified":"2026-04-13T05:58:07","modified_gmt":"2026-04-13T03:58:07","slug":"the-type-tarray-cannot-be-used-as-a-value-in-a-tmap-unreal-engine-fix","status":"publish","type":"asx-lms-tutorial-cpt","link":"https:\/\/store.algosyntax.com\/tutorials\/unreal-engine\/the-type-tarray-cannot-be-used-as-a-value-in-a-tmap-unreal-engine-fix\/","title":{"rendered":"The type ‘TArray<\u2026>’ cannot be used as a value in a TMap (Unreal Engine Fix)"},"content":{"rendered":"\t\t
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What this tutorial covers<\/h2>
  1. Why Unreal Engine throws this error<\/li>
  2. The underlying restriction in UPROPERTY<\/code> and reflection<\/li>
  3. Correct ways to use TArray<\/code> inside TMap<\/code><\/li>
  4. Working fixes with USTRUCT<\/code> wrapping<\/li>
  5. Alternative patterns when nesting containers fails<\/li><\/ol>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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    The problem<\/h2>

    You may encounter an error like:<\/p>

    \u00a0<\/div><\/div><\/div>
    The<\/span> type <\/span>‘ArrayProperty’<\/span> can not<\/span> be used<\/span> as a<\/span> value in<\/span> a TMap<\/span><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div>
    \u00a0<\/div><\/div><\/div><\/div><\/div>

    Or when writing code like:<\/p>

    \u00a0<\/div><\/div><\/div>
    UPROPERTY()
    TMap<\/span><<\/span>int32<\/span>, TArray<\/span><<\/span>int32<\/span>>><\/span> MyMap<\/span>;<\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div>
    \u00a0<\/div><\/div><\/div><\/div><\/div>

    This fails even though both TMap<\/code> and TArray<\/code> are valid Unreal containers.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

    \n\t\t\t\t\t
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    Why this happens<\/h2>

    This is not a C++ limitation<\/strong>.
    It is a Unreal Header Tool (UHT) and reflection system restriction<\/strong>.<\/p>

    Key rule:<\/h3>

    UPROPERTY<\/code> does not support nested container types directly.<\/p><\/blockquote>

    Specifically:<\/p>

    • TMap<Key, Value><\/code> is supported<\/li>
    • TArray<Type><\/code> is supported<\/li>
    • \u274c TMap<Key, TArray<Type>><\/code> is not supported as a reflected property<\/strong><\/li><\/ul>

      This happens because:<\/p>

      • Unreal\u2019s reflection system represents containers using FProperty<\/code> types<\/li>
      • TArray<\/code> becomes FArrayProperty<\/code><\/li>
      • TMap<\/code> becomes FMapProperty<\/code><\/li><\/ul>

        But:<\/p>

        Unreal does not allow FArrayProperty<\/code> to be used as the value property inside FMapProperty<\/code><\/strong><\/p><\/blockquote>

        So UHT throws:<\/p>

        \u00a0<\/div>
        ArrayProperty cannot be used as a value in a TMap<\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
        \n\t\t\t\t\t
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        Important distinction<\/h2>

        Important distinction<\/p>

        This restriction only applies when using:<\/p>

        UPROPERTY()

        <\/strong>or from Blueprint authoured maps.<\/p>


        This works (no reflection):<\/strong>
        TMap<int32, TArray<int32>> MyMap;<\/p>


        This fails (reflection enabled):<\/strong><\/p>

        UPROPERTY()<\/strong>
        TMap<int32, TArray<int32>> MyMap;<\/p>

        \u00a0<\/div><\/div><\/div><\/div><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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        The correct solution: Wrap the array in a USTRUCT<\/h2>

        Instead of storing TArray<\/code> directly, wrap it in a struct.<\/p>

        Step 1 \u2014 Define a struct<\/h3>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t
        \n\t\t\t\t\t\t\t
        \n\t\t\t
        \n\t\t\t\t\n\t\t\t\t\tUSTRUCT()\r\nstruct FIntArrayWrapper\r\n{\r\n    GENERATED_BODY()\r\n\r\n    UPROPERTY()\r\n    TArray<int32> Values;\r\n};<\/xmp>\n\t\t\t\t<\/code>\n\t\t\t<\/pre>\n\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-b5783ac elementor-widget elementor-widget-text-editor\" data-id=\"b5783ac\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t\t\t\t\t\t<h2 data-section-id=\"hdo5o1\" data-start=\"1857\" data-end=\"1909\">Step 2 \u2014 Use it inside TMap<\/h2>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-f50741d elementor-widget elementor-widget-code-highlight\" data-id=\"f50741d\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"code-highlight.default\">\n\t\t\t\t\t\t\t<div class=\"prismjs-default copy-to-clipboard \">\n\t\t\t<pre data-line=\"\" class=\"highlight-height language-cpp line-numbers\">\n\t\t\t\t<code readonly=\"true\" class=\"language-cpp\">\n\t\t\t\t\t<xmp>UPROPERTY()\r\nTMap<int32, FIntArrayWrapper> MyMap;<\/xmp>\n\t\t\t\t<\/code>\n\t\t\t<\/pre>\n\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-6083603 e-flex e-con-boxed e-con e-parent\" data-id=\"6083603\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-41ccf00 elementor-widget elementor-widget-text-editor\" data-id=\"41ccf00\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t\t\t\t\t\t<h2 data-section-id=\"dhunqv\" data-start=\"2220\" data-end=\"2237\">Why this works<\/h2><p data-start=\"2239\" data-end=\"2255\">Now Unreal sees:<\/p><ul data-start=\"2257\" data-end=\"2321\"><li data-section-id=\"uzod2i\" data-start=\"2257\" data-end=\"2286\"><code data-start=\"2259\" data-end=\"2278\">TMap&lt;Key, Struct&gt;<\/code> \u2192 valid<\/li><li data-section-id=\"ho7tp1\" data-start=\"2287\" data-end=\"2321\">Struct contains <code data-start=\"2305\" data-end=\"2313\">TArray<\/code> \u2192 valid<\/li><\/ul><p data-start=\"2323\" data-end=\"2398\">This avoids the unsupported <strong data-start=\"2351\" data-end=\"2397\">container-inside-container reflection case<\/strong>.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-76923ed e-flex e-con-boxed e-con e-parent\" data-id=\"76923ed\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-52b3c37 elementor-widget elementor-widget-text-editor\" data-id=\"52b3c37\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t\t\t\t\t\t<h2 data-section-id=\"1c82j19\" data-start=\"3637\" data-end=\"3671\">Common variations of this error<\/h2><p data-start=\"3673\" data-end=\"3690\">You may also see:<\/p><ul data-start=\"3692\" data-end=\"3880\"><li data-section-id=\"q47vjt\" data-start=\"3692\" data-end=\"3745\"><code data-start=\"3694\" data-end=\"3745\">ArrayProperty cannot be used as a value in a TMap<\/code><\/li><li data-section-id=\"wj58ho\" data-start=\"3746\" data-end=\"3785\"><code data-start=\"3748\" data-end=\"3785\">Nested containers are not supported<\/code><\/li><li data-section-id=\"1swobc4\" data-start=\"3786\" data-end=\"3825\">Errors when using <code data-start=\"3806\" data-end=\"3825\">TSet&lt;TArray&lt;...&gt;&gt;<\/code><\/li><li data-section-id=\"2u2na0\" data-start=\"3826\" data-end=\"3880\">Errors when exposing nested containers to Blueprints<\/li><\/ul><p data-start=\"3882\" data-end=\"3922\">They all stem from the same restriction.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-ec64d3b e-flex e-con-boxed e-con e-parent\" data-id=\"ec64d3b\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-474e8c8 elementor-widget elementor-widget-text-editor\" data-id=\"474e8c8\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t\t\t\t\t\t<h2 data-section-id=\"1k5q749\" data-start=\"3929\" data-end=\"3939\">Summary<\/h2><ul data-start=\"3941\" data-end=\"4193\"><li data-section-id=\"152b3od\" data-start=\"3941\" data-end=\"4004\">This is a <strong data-start=\"3953\" data-end=\"3985\">reflection system limitation<\/strong>, not a C++ issue<\/li><li data-section-id=\"5m30bq\" data-start=\"4005\" data-end=\"4065\"><code data-start=\"4007\" data-end=\"4033\">TMap&lt;Key, TArray&lt;Value&gt;&gt;<\/code> is <strong data-start=\"4037\" data-end=\"4063\">invalid with UPROPERTY<\/strong><\/li><li data-section-id=\"br7dma\" data-start=\"4066\" data-end=\"4127\">The correct fix is to <strong data-start=\"4090\" data-end=\"4125\">wrap the array inside a USTRUCT<\/strong><\/li><li data-section-id=\"5cu692\" data-start=\"4128\" data-end=\"4193\">Alternatively, remove <code data-start=\"4152\" data-end=\"4163\">UPROPERTY<\/code> if reflection is not needed<\/li><\/ul>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t","protected":false},"featured_media":0,"template":"","asx-lms-tutorial-categories":[86,45],"asx-lms-tutorial-tags":[134,135],"class_list":["post-13393","asx-lms-tutorial-cpt","type-asx-lms-tutorial-cpt","status-publish","hentry","asx-lms-tutorial-categories-unreal-api","asx-lms-tutorial-categories-unreal-engine","asx-lms-tutorial-tags-tarray","asx-lms-tutorial-tags-unrealengine","entry"],"acf":[],"_links":{"self":[{"href":"https:\/\/store.algosyntax.com\/asx-rest\/wp\/v2\/asx-lms-tutorial-cpt\/13393","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/store.algosyntax.com\/asx-rest\/wp\/v2\/asx-lms-tutorial-cpt"}],"about":[{"href":"https:\/\/store.algosyntax.com\/asx-rest\/wp\/v2\/types\/asx-lms-tutorial-cpt"}],"version-history":[{"count":2,"href":"https:\/\/store.algosyntax.com\/asx-rest\/wp\/v2\/asx-lms-tutorial-cpt\/13393\/revisions"}],"predecessor-version":[{"id":13401,"href":"https:\/\/store.algosyntax.com\/asx-rest\/wp\/v2\/asx-lms-tutorial-cpt\/13393\/revisions\/13401"}],"wp:attachment":[{"href":"https:\/\/store.algosyntax.com\/asx-rest\/wp\/v2\/media?parent=13393"}],"wp:term":[{"taxonomy":"asx-lms-tutorial-categories","embeddable":true,"href":"https:\/\/store.algosyntax.com\/asx-rest\/wp\/v2\/asx-lms-tutorial-categories?post=13393"},{"taxonomy":"asx-lms-tutorial-tags","embeddable":true,"href":"https:\/\/store.algosyntax.com\/asx-rest\/wp\/v2\/asx-lms-tutorial-tags?post=13393"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}