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Trauma
Asked: 2020-05-15 01:15:12 +0800 CST 2020-05-15 01:15:12 +0800 CST

Avoid temporary copies in call to lambdas

  • 772

I have the following class:

class Utf8Writer {
  struct ZoneData {
    uint32_t begin; // codepoint inicial.
    uint32_t end; // codepoint final.
  };

  const ZoneData *findCodepoint( uint32_t cp, FontId fid );
};

const ZoneData *Utf8Writer::findCodepoint( uint32_t cp, FontId fid ) {
  auto iter = std::find_if( fid->cbegin( ), fid->cend( ), [cp] ( ZoneData i ) { return ( cp >= i.begin ) && ( cp <= i.end ); } );

  return ( iter == fid->cend( ) ) ? nullptr : &( *iter );
}

It works perfectly, except that it is a bug . What I really wanted to do is:

const ZoneData *Utf8Writer::findCodepoint( uint32_t cp, FontId fid ) {
  auto iter = std::find_if( fid->cbegin( ), fid->cend( ), [cp] ( const ZoneData &i ) { return ( cp >= i.begin ) && ( cp <= i.end ); } );

  return ( iter == fid->cend( ) ) ? nullptr : &( *iter );
}

In fact, if we look at the doc of std::find_if( ):

template< class InputIt, class UnaryPredicate > InputIt find_if( InputIt first, InputIt last, UnaryPredicate p );

being UnaryPredicate->bool pred( const Type &a );

I understand that, in the first version, the compiler creates a temporary, calling the default-copy constructor . It turns out that the class ZoneDatais more complex than shown: it includes std::vectorand other things , and its copy is not trivial (neither in time nor in memory usage).

  • Is it possible to avoid creating temporary lambda calls , for future occasions, without touching the class ZoneData?
c++

1 Answers

  1. Best Answer

    It might be a good idea to use a static_assertthat fails if the lambda parameter is not a reference, to achieve this we need to use the header <type_traits>and its utility std::is_reference:

    This code will fail:

    const ZoneData *Utf8Writer::findCodepoint( uint32_t cp, FontId fid ) {
  auto iter = std::find_if( fid->cbegin( ), fid->cend( ), [cp] ( ZoneData i )
//          'i' es una instancia, podría ser copia o movido ---> ~~~~~~~~~~
{
    // La aserción falla pues 'i' no es una referencia
    static_assert(std::is_reference_v<decltype(i)>, "Evita copiar el parametro!");
    return ( cp >= i.begin ) && ( cp <= i.end );
} );

    While this one should work without a problem:

    const ZoneData *Utf8Writer::findCodepoint( uint32_t cp, FontId fid ) {
  auto iter = std::find_if( fid->cbegin( ), fid->cend( ), [cp] ( const ZoneData &i )
//                     'i' es una referencia, no será copia ---> ~~~~~~~~~~~~~~~~~
{
    // La aserción NO falla pues 'i' no es una referencia
    static_assert(std::is_reference_v<decltype(i)>, "Evita copiar el parametro!");
    return ( cp >= i.begin ) && ( cp <= i.end );
} );
    • 6