c++ - Why is it better to use std::make_* instead of the constructor? -

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there functions in stl start make_ prefix std::make_pair, std::make_shared, std::make_unique etc. why better practice use them instead of using constructor ?

auto pair2 = std::pair< int, double >( 1, 2.0 ); auto pair3 = std::make_pair( 1, 2.0 );  std::shared_ptr< int > pointer1 = std::shared_ptr< int >( new int( 10 ) ); std::shared_ptr< int > pointer2 = std::make_shared< int >( 10 );
  • i see these functions make code little shorter, ?
  • are there other advantages ?
  • are these functions safer use ?

aside benefit of enabling argument deduction (as mentioned in other answers), there other benefits.

std::make_pair<t1, t2> takes care not return std::pair<t1, t2>. if pass in value using std::ref, returned pair won't store std::reference_wrapper, store reference.

std::make_shared can combine allocations. shared_ptr needs place hold things refcount, weak pointer list, etc. cannot stored in shared_ptr directly. these can combined object being created, in 1 larger block rather in 2 separate blocks.

std::make_shared , std::make_unique both make sure no object gets left behind if exceptions thrown. if call function f(std::shared_ptr<int>(new int), std::shared_ptr<int>(new int)), it's possible compiler first allocates 2 int objects, , constructs 2 shared_ptr<int> objects. if second allocation fails, , no shared_ptr<int> object set yet release memory on destruction, have memory leak. std::make_shared , std::make_unique combine allocation of int , construction of std::shared_ptr<int> in function call, , other allocation of int , other construction of std::shared_ptr<int> in function call. function calls cannot overlap, if second allocation fails, there shared pointer destroyed, undoing first allocation well.


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