Projection | Unreal Engine 5.7 Documentation

Description

Projection() is a related function to Invoke(), in that it can be used to invoke an object with a set of arguments. However, it works by transforming something invocable into something callable, i.e. can be called with a normal parenthesized set of arguments.

// These are equivalent: Projection(I)(Args...) Invoke(I, Args...).

... with the same advantages:

// Can accept member function pointers Invoke(&FObjType::Member, Obj); // equivalent to Obj.Member Invoke(&FObjType::Func, Obj, Args...); // equivalent to Obj.Func(Args...) Projection(&FObjType::Member)(Obj); // equivalent to Obj.Member Projection(&FObjType::Func)(Obj, Args...); // equivalent to Obj.Func(Args...)

// Can operate on pointers, including smart pointers Invoke(&FObjType::Member, ObjPtr); // equivalent to ObjPtr->Member Invoke(&FObjType::Func, ObjPtr, Args...); // equivalent to ObjPtr->Func(Args...) Projection(&FObjType::Member)(ObjPtr); // equivalent to ObjPtr->Member Projection(&FObjType::Func)(ObjPtr, Args...); // equivalent to ObjPtr->Func(Args...)

However, Projection() has some additional advantages:

Projection(I) returns I unchanged if it is already callable, meaning no redundant stepping in and out of many Invoke() calls in the debugger.
Projection(...) is variadic and can transform a sequence of invocables into a callable that invokes them one by one:

Projection(A, B, C)(Args...) Invoke(C, Invoke(B, Invoke(A, Args...)))

This allows users to pass a sequence of projections to an algorithm that takes a single projection:

struct FInner { FString Name; }; struct FOuter { FInner Inner; };

// Sort array of outers by the names of inner, Algo::SortBy(ArrayOfOuters, Projection(&FOuter::Inner, &FInner::Name));


Name	Projection
Type	function
Header File	/Engine/Source/Runtime/Core/Public/Templates/Projection.h
Include Path	#include "Templates/Projection.h"

template<typename Invocable0Type, typename... InvocableTypes>  
AUTORTFM_INFER constexpr auto Projection  
(  
    Invocable0Type && Invocable0,  
    InvocableTypes &&... Invocables  
)

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