Description
Resizes the array to exactly NewNum elements, default-constructing any new elements if growing, and destructing trailing elements if shrinking. Analogous to std::vector::resize.
Caveats & Gotchas
- • When growing, new elements are default-constructed — not zeroed. For POD types with no default constructor the new elements contain garbage. Use SetNumZeroed() if you need zero-initialized new slots.
- • When shrinking, SetNum calls RemoveAt internally which may trigger a reallocation depending on EAllowShrinking. Pass EAllowShrinking::No in tight loops to avoid repeated shrink-reallocations.
- • The bool bAllowShrinking overload is deprecated in UE 5.6 — use EAllowShrinking::No or EAllowShrinking::Yes.
- • SetNum(0) is a valid but unusual call — use Reset() or Empty() instead, as those better express intent and avoid the overhead of the size comparison.
Signature
void SetNum(SizeType NewNum, EAllowShrinking AllowShrinking = UE::Core::Private::AllowShrinkingByDefault<AllocatorType>())Parameters
| Name | Type | Description | Default |
|---|---|---|---|
| NewNum | SizeType | The desired number of elements after the call. | — |
| AllowShrinking | EAllowShrinking | Controls whether memory is released when reducing the count. | UE::Core::Private::AllowShrinkingByDefault<AllocatorType>() |
Return Type
void Example
Pre-size an array for direct index assignment C++
TArray<FVector> Positions;
Positions.SetNum(64); // 64 default-constructed FVectors
for (int32 i = 0; i < 64; ++i)
{
Positions[i] = FVector(i * 100.f, 0.f, 0.f);
}Tags
Version History
Introduced in: 4.0
| Version | Status | Notes |
|---|---|---|
| 5.6 | stable | — |
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