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POget const or non-const reference type from trait
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Id
2628714
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AcceptedAnswerId
2628794
AnswerCount
4
ClosedDate
CommentCount
1
CommunityOwnedDate
CreationDate
2010-04-13T10:19:13.637
FavoriteCount
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2011-07-23T16:48:52.123
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2011-07-23T16:48:52.123
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ViewCount
2032
LastEditorDisplayName
text
Body
I am writing a functor F which takes function of type void (*func)(T) and func's argument arg. <pre><code>template<typename T> void F(void (*func)(T), WhatTypeHere? arg) { func(arg); } </code></pre> Then functor F calls func with arg. I would like F not to copy arg, just to pass it as reference. But then I cannot simply write "void F(void (*func)(T), T&)" because T could be a reference. So I am trying to write a trait, which allows to get proper reference type of T: <pre><code>T -> T& T& -> T& const T -> const T& const T& -> const T& </code></pre> I come up with something like this: <pre><code>template<typename T> struct type_op { typedef T& valid_ref_type; }; template<typename T> struct type_op<T&> { typedef typename type_op<T>::valid_ref_type valid_ref_type; }; template<typename T> struct type_op<const T> { typedef const T& valid_ref_type; }; template<typename T> struct type_op<const T&> { typedef const T& valid_ref_type; }; template<typename T> void F(void (*func)(T), typename type_op<T>::valid_ref_type arg) { func(arg); } </code></pre> Which doesn't work for example for <pre><code>void a(int x) { std::cout << x << std::endl; } F(&a, 7); </code></pre> Giving error: invalid initialization of non-const reference of type ‘int&’ from a temporary of type ‘int’ in passing argument 2 of ‘void F(void (*)(T), typename type_op::valid_ref_type) [with T = int]’ How to get this trait to work?
Tags
<c++><templates><reference><functor><traits>
Title
get const or non-const reference type from trait
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1. PO
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1. USLightness Races in Orbit
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1. USmaciekp
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1. COI think it's pretty similar to this one: http://stackoverflow.com/questions/2539361/c-template-functor-cannot-deduce-reference-type
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 POget const or non-const reference type from trait
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 USJohannes Schaub - litb

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