What It Means & How to Fix It

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🧠 Why You’re Seeing This Error

C2057 means you’re trying to define something that requires a constant expression, but what you provided isn’t known at compile time. This most often shows up when declaring C-style arrays where the size must be a constant.

In UE5, this happens when:

• You declare arrays like int MyArray[Size], where Size isn’t a compile-time constant
• You use int32 or float variables to size arrays instead of constexpr or #define
• You’re trying to use runtime values (like user input or function return values) to size a static array

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💥 Example Error Message

error C2057: expected constant expression

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🛠️ Example Code

int32 Size = 5;
int32 MyArray[Size]; // ❌ C2057: Size must be constant

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✅ How to Fix C2057 in UE5 – Step-by-Step

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✔️ 1. Use constexpr or const for Compile-Time Values

If the size is known at compile time, make sure it’s marked as such.

Fix:

constexpr int32 Size = 5;
int32 MyArray[Size]; // ✅ C++11 or later allows constexpr array size

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✔️ 2. Use TArray Instead of C-Style Arrays in UE5

Best practice in Unreal: avoid raw arrays. Use UE’s dynamic container.

Fix:

TArray MyArray;
MyArray.SetNum(5); // ✅ Runtime-sized array, no C2057

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✔️ 3. Avoid Using Runtime Values in Array Declarations

Wrong:

int32 Size = GetArraySizeFromSomewhere();
int32 Data[Size]; // ❌ Not allowed

Fix:

TArray Data;
Data.SetNum(GetArraySizeFromSomewhere()); // ✅

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✔️ 4. Avoid #define Hacks Unless Truly Necessary

Old C++ style might use:

#define SIZE 5
int32 MyArray[SIZE]; // ✅ But not recommended in modern UE5

Instead, prefer constexpr or TArray.

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✅ Summary: How to Fix C2057 in UE5

Cause	Fix Example
Array size not constant	Use constexpr or const int
Runtime variable used as array size	Switch to TArray for dynamic sizing
Using function return for size	Use SetNum() or vector-like containers