conditional template based code execution c++ -

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so have been battling time , first post here. wanted know, if passed in container forward container, defined in boost, how can write code in way further checks see if code either backinsertionsequence or frontinsertionsequence , executes depending on type is? 

template <class sequentialcontainer> void myclass<sequentialcontainer>::saysomething() {       boost_concept_assert((boost::forwardcontainer<sequentialcontainer>));       if (boost::backinsertionsequence<sequentialcontainer> == true) {           //do       } else {           //say else       }   }

if did if check have there, both branches have compiled regardless of fact 1 branch ever executed particular type. severely limits usefulness of simple if - need construct such 1 of branches compiles.

there 2 common ways of doing this. tag dispatch:

template <class sequentialcontainer> void myclass<sequentialcontainer>::saysomething() {       saysomething<sequentialcontainer>(         is_back_insertion_sequence<sequentialcontainer>{}     ); }  template <class sequentialcontainer> void saysomething(std::true_type /* insertion sequence */ ) { ... }  template <class sequentialcontainer> void saysomething(std::false_type /* not insertion sequence */ ) { ... }

and sfinae:

template <class sequentialcontainer> typename std::enable_if<     is_back_insertion_sequence<sequentialcontainer>::value >::type saysomething() {     /* insertion sequence */ }  template <class sequentialcontainer> typename std::enable_if<     !is_back_insertion_sequence<sequentialcontainer>::value >::type saysomething() {     /* not insertion sequence */ }

there advantages , disadvantages both, if it's 1 flag you're checking - it's matter of opinion.


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