I have a slightly complex data member of a class, as noted in what follows:
class BranchOutputRow
{
...
}
class Foo
{
public:
// Slightly complex data member here
std::map<boost::multiprecision::cpp_int, std::set<BranchOutputRow>> hits;
void DoLoop1()
{
// This loop calls the std::pair<> constructor
std::for_each(hits.cbegin(), hits.cend(),
[&](std::pair<boost::multiprecision::cpp_int,
std::set<BranchOutputRow>> const & hit)
{
...
}
}
void DoLoop2()
{
// This loop does NOT call the std::pair<> constructor
for (std::map<boost::multiprecision::cpp_int,
std::set<BranchOutputRow>>::const_iterator hitsPtr
= hits.cbegin();
hitsPtr != hits.cend();
++hitsPtr)
{
...
}
}
}
int main()
{
Foo foo;
foo.hits[1] = std::set<BranchOutputRow>();
foo.hits[1].insert(BranchOutputRow());
foo.DoLoop1(); // direct access to map object is not available
foo.DoLoop2(); // direct access to map object is available
}
As noted, I find that Loop #1 calls the std::pair
constructor, despite the fact that the lambda function accepts its argument by reference. Therefore, in Loop 1, I do not have direct access to the object in the map, but only a copy. In my actual program, I need direct access; therefore, I must use the version indicated by Loop 2.
(In fact, Loop 2 does not call the std::pair
constructor - not a surprise - and does provide direct access to the object in the map.)
I would think that std::for_each
would have been carefully designed to provide the same semantics as a for
loop such as Loop 2, and therefore not call the std::pair
constructor, instead allowing direct access to the object in the map.
Why does Loop 1 call the std::pair
constructor, despite the fact that the lambda function accepts its argument by reference?
The value_type
of std::map<K,V>
is std::pair<const K, V>
. Your lambda takes std::pair<K,V>
. Note the difference in constness.
The conversion is done through this constructor:
template< class U1, class U2 >
pair( const pair<U1, U2>& p );
(see (4) on this reference page )
The result of the conversion is a temporary, and temporaries can bind to const references, so your code works.
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