# How can I create a TArray\<const struct\> ? (It won't compile)

**URL:** <https://forums.unrealengine.com/t/how-can-i-create-a-tarray-const-struct-it-wont-compile/2071890>\
**Category:** C++\
**Tags:** tarray, unreal-engine, CPP\
**Created:** [October 16, 2024, 11:51pm UTC](https://forums.unrealengine.com/t/how-can-i-create-a-tarray-const-struct-it-wont-compile/2071890 "2024-10-16T23:51:52Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![andyvyking](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/andyvyking/32/641039_2.png) [@andyvyking](https://forums.unrealengine.com/u/andyvyking)\
**Post date:** [October 16, 2024, 11:51pm UTC](https://forums.unrealengine.com/t/how-can-i-create-a-tarray-const-struct-it-wont-compile/2071890/1 "2024-10-16T23:51:52Z")

</div>

I want to create a `TArray` of `const struct`s, but the compiler is failing on my `Add()` calls.

Sample code:

```cpp
USTRUCT(BlueprintType)
struct MULTIPLAYERSESSIONS_API FMultiplayerSessionsSearchResult
{
	GENERATED_BODY()

	FMultiplayerSessionsSearchResult();
	~FMultiplayerSessionsSearchResult();

};

// ...

	auto Results = TArray<const FMultiplayerSessionsSearchResult>();
	Results.Add(FMultiplayerSessionsSearchResult());

```

And here’s the compiler failure:

```auto
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): error C2665: 'operator new': no overloaded function could convert all the argument types
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Misc\MemStack.h(300): note: could be 'void *operator new(size_t,FMemStackBase &,EMemOned,int32)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,FMemStackBase &,EMemOned,int32)': cannot convert argument 2 from 'std::align_val_t' to 'FMemStackBase &'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Misc\MemStack.h(282): note: or 'void *operator new(size_t,FMemStackBase &,EMemZeroed,int32)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,FMemStackBase &,EMemZeroed,int32)': cannot convert argument 2 from 'std::align_val_t' to 'FMemStackBase &'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Misc\MemStack.h(275): note: or 'void *operator new(size_t,std::align_val_t,FMemStackBase &,int32)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,std::align_val_t,FMemStackBase &,int32)': cannot convert argument 3 from 'const FMultiplayerSessionsSearchResult *' to 'FMemStackBase &'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Misc\MemStack.h(268): note: or 'void *operator new(size_t,FMemStackBase &,int32)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,FMemStackBase &,int32)': cannot convert argument 2 from 'std::align_val_t' to 'FMemStackBase &'
2>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.38.33130\INCLUDE\vcruntime_new.h(105): note: or 'void *operator new(size_t,std::align_val_t,const std::nothrow_t &) noexcept'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,std::align_val_t,const std::nothrow_t &) noexcept': cannot convert argument 3 from 'const FMultiplayerSessionsSearchResult *' to 'const std::nothrow_t &'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: Reason: cannot convert from 'const FMultiplayerSessionsSearchResult *' to 'const std::nothrow_t'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'std::nothrow_t::nothrow_t': no overloaded function could convert all the argument types
2>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.38.33130\INCLUDE\vcruntime_new.h(37): note: could be 'std::nothrow_t::nothrow_t(std::nothrow_t &&)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'std::nothrow_t::nothrow_t(std::nothrow_t &&)': cannot convert argument 1 from 'const FMultiplayerSessionsSearchResult *' to 'std::nothrow_t &&'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: Reason: cannot convert from 'const FMultiplayerSessionsSearchResult *' to 'std::nothrow_t'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: Conversion requires a second user-defined-conversion operator or constructor
2>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.38.33130\INCLUDE\vcruntime_new.h(37): note: or 'std::nothrow_t::nothrow_t(const std::nothrow_t &)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'std::nothrow_t::nothrow_t(const std::nothrow_t &)': cannot convert argument 1 from 'const FMultiplayerSessionsSearchResult *' to 'const std::nothrow_t &'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: Reason: cannot convert from 'const FMultiplayerSessionsSearchResult *' to 'const std::nothrow_t'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: Conversion requires a second user-defined-conversion operator or constructor
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: while trying to match the argument list '(const FMultiplayerSessionsSearchResult *)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\ChunkedArray.h(50): note: or 'void *operator new(size_t,TChunkedArray<T,TargetBytesPerChunk,AllocatorType> &)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,TChunkedArray<T,TargetBytesPerChunk,AllocatorType> &)': expects 2 arguments - 3 provided
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\SparseArray.h(39): note: or 'void *operator new(size_t,TSparseArray<T,Allocator> &,int32)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,TSparseArray<T,Allocator> &,int32)': could not deduce template argument for 'TSparseArray<T,Allocator> &' from 'std::align_val_t'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\SparseArray.h(38): note: or 'void *operator new(size_t,TSparseArray<T,Allocator> &)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,TSparseArray<T,Allocator> &)': expects 2 arguments - 3 provided
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(48): note: or 'void *operator new(size_t,TArray<T,AllocatorType> &,TArray<T,AllocatorType>::SizeType)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,TArray<T,AllocatorType> &,TArray<T,AllocatorType>::SizeType)': could not deduce template argument for 'TArray<T,AllocatorType> &' from 'std::align_val_t'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(47): note: or 'void *operator new(size_t,TArray<T,AllocatorType> &)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: 'void *operator new(size_t,TArray<T,AllocatorType> &)': expects 2 arguments - 3 provided
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: while trying to match the argument list '(unsigned __int64, std::align_val_t, const FMultiplayerSessionsSearchResult *)'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2232): note: the template instantiation context (the oldest one first) is
2>I:\Projects\MenuSystem\Plugins\MultiplayerSessions\Source\MultiplayerSessions\Private\MultiplayerSessionsSubsystem.cpp(121): note: see reference to class template instantiation 'TArray<const FMultiplayerSessionsSearchResult,FDefaultAllocator>' being compiled
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2289): note: while compiling class template member function 'int TArray<const FMultiplayerSessionsSearchResult,FDefaultAllocator>::Add(const FMultiplayerSessionsSearchResult &&)'
2>I:\Projects\MenuSystem\Plugins\MultiplayerSessions\Source\MultiplayerSessions\Private\MultiplayerSessionsSubsystem.cpp(123): note: see the first reference to 'TArray<const FMultiplayerSessionsSearchResult,FDefaultAllocator>::Add' in 'UMultiplayerSessionsSubsystem::OnFindSessionComplete'
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2292): note: see reference to function template instantiation 'int TArray<const FMultiplayerSessionsSearchResult,FDefaultAllocator>::Emplace<const FMultiplayerSessionsSearchResult>(const FMultiplayerSessionsSearchResult &&)' being compiled
2>I:\Unreal Engine\UE_5.4\Engine\Source\Runtime\Core\Public\Containers\Array.h(2207): fatal error C1903: unable to recover from previous error(s); stopping compilation

```

Anyone know of a way to get this to work?

---

<div class="post-metadata">

**Author:** ![RecourseDesign](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/recoursedesign/32/1194503_2.png) [@RecourseDesign](https://forums.unrealengine.com/u/RecourseDesign)\
**Post date:** [October 17, 2024, 12:14am UTC](https://forums.unrealengine.com/t/how-can-i-create-a-tarray-const-struct-it-wont-compile/2071890/2 "2024-10-17T00:14:02Z")

</div>

Hi,

try casting it to a const:

```auto
Results.Add((const FMultiplayerSessionsSearchResult&)FMultiplayerSessionsSearchResult());

```

---

<div class="post-metadata">

**Author:** ![andyvyking](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/andyvyking/32/641039_2.png) [@andyvyking](https://forums.unrealengine.com/u/andyvyking)\
**Post date:** [October 22, 2024, 6:09pm UTC](https://forums.unrealengine.com/t/how-can-i-create-a-tarray-const-struct-it-wont-compile/2071890/3 "2024-10-22T18:09:59Z")

</div>

Thanks for the suggestion.

Casting didn’t work, and after replacing the `TArray` with `standard::vector,` I got the following compilation error:

```auto
The C++ Standard forbids containers of const elements because allocator<const T> is ill-formed.

```

So looks like containers of `const` elements are just prohibited in C++.

---

<div class="post-metadata">

**Author:** ![RecourseDesign](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/recoursedesign/32/1194503_2.png) [@RecourseDesign](https://forums.unrealengine.com/u/RecourseDesign)\
**Post date:** [October 22, 2024, 6:17pm UTC](https://forums.unrealengine.com/t/how-can-i-create-a-tarray-const-struct-it-wont-compile/2071890/4 "2024-10-22T18:17:50Z")

</div>

Interesting - You can use const pointers in TArrays fine but not const inline objects.  
Maybe just not have it const?

---

<div class="post-metadata">

**Author:** ![andyvyking](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/andyvyking/32/641039_2.png) [@andyvyking](https://forums.unrealengine.com/u/andyvyking)\
**Post date:** [October 22, 2024, 6:32pm UTC](https://forums.unrealengine.com/t/how-can-i-create-a-tarray-const-struct-it-wont-compile/2071890/5 "2024-10-22T18:32:03Z")

</div>

Yeah in this case I wanted actual structs instead of pointers. Const pointers probably work because the value is const, but the pointer isn’t, so like `const int *` vs `int * const`.

This stack overflow explains the issue with `const` values: [c++ - Vector of structs with const members? - Stack Overflow](https://stackoverflow.com/questions/8467968/vector-of-structs-with-const-members)

---

<div class="post-metadata">

**Author:** ![RecourseDesign](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/recoursedesign/32/1194503_2.png) [@RecourseDesign](https://forums.unrealengine.com/u/RecourseDesign)\
**Post date:** [October 22, 2024, 6:39pm UTC](https://forums.unrealengine.com/t/how-can-i-create-a-tarray-const-struct-it-wont-compile/2071890/6 "2024-10-22T18:39:47Z")

</div>

That explains it nicely.