[Boost-users] Understanding how enable_if works.

From: Michael Marcin (mmarcin_at_[hidden])
Date: 2006-08-10 19:28:29

Hi,

I'm new to boost / generic programming and I'm trying to understand how
enable_if works because I don't like using things I don't understand.

I made a simple example which I thought would have the same behavior as 
using
enable_if but doesn't.

I've attached the test could someone take a look and explain why enable_if
works and the other doesn't.

Thanks

begin 666 main.cpp
M(VEN8VQU9&4@/&)O;W-T+W5T:6QI='DO96YA8FQE7VEF+FAP<#X-"B-I;F-L
M=61E(#QI;W-T<F5A;3X-"@T*;F%M97-P86-E(%1E<W1"<F]K96X-"GL-"@ET
M96UP;&%T93P_at_8VQA<W,@5" ^#0H)<W1R=6-T(%1E<W14<F%I= T*"7L-"@E]
M.PT*#0H)=&5M<&QA=&4\/@T*"7-T<G5C="!497-T5')A:70\:6YT/@T*"7L-
M"@D)='EP961E9B!I;G0_at_96YA8FQE7W1Y<&4[#0H)?3L-"@T*"71E;7!L871E
M/"!C;&%S<R!4+"!C;&%S<R!%;F%B;&4@/2!V;VED(#X-"@ES=')U8W0_at_16YA
M8FQE5&5S= T*"7L-"@D)16YA8FQE5&5S="@I#0H)"7L-"@D)"7-T9#HZ8V]U
M=" \/" B1V5N97)I8R!%;F%B;&4_at_5&5S="!C;VYS=')U8W1E9"(@/#P@<W1D
M.CIE;F1L.PT*"0E]#0H)?3L-"@T*"71E;7!L871E/"!C;&%S<R!4(#X-"@ES
M=')U8W0_at_16YA8FQE5&5S=#P_at_5"P@='EP96YA;64_at_5&5S=%1R86ET/%0^.CIE
M;F%B;&5?='EP92 ^#0H)>PT*"0E%;F%B;&5497-T*"D-"@D)>PT*"0D)<W1D
M.CIC;W5T(#P\(")3<&5C:6%L:7IE9"!%;F%B;&4_at_5&5S="!C;VYS=')U8W1E
M9"(@/#P@<W1D.CIE;F1L.PT*"0E]#0H)?3L-"GT-"@T*;F%M97-P86-E(%1E
M<W17;W)K:6YG#0I[#0H)=&5M<&QA=&4\(&-L87-S(%0@/@T*"7-T<G5C="!I
M<U]I;G0-"@E[#0H)"7-T871I8R!C;VYS="!B;V]L('9A;'5E(#T_at_9F%L<V4[
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M<W0_at_5V]R:VEN9R ]/3TB(#P\('-T9#HZ96YD;#L-"@ES=&0Z.F-O=70@/#P@
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:;#X@=&5M<#M]#0H-"@ER971U<FX@,#L-"GT`
`
end