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gast128@hotmail.com · Guest

Dear all,

item 24 of Scott Meyers (excellent) effective STL book is about the
performance difference of operator[] compared to map insert. In short
he warns about the possible performance loss of operator[] due to an
extra copy.

Our code uses a lot of this constructs and therefore I profiled his
efficient update algorithm with a std::map<int, std::sting>.

I come to the following profile results (Visual Studio 2003 release
builds):

only adds:
operator[] 15.1
Scott 12.5
(+/- 20%)

dominating inserts:
operator[] 26.4
Scott 25.0
(+/- 4%)

Thus my guess will be that indeed only a very heavy object with
dominating adds in a container will make a difference.

code used:
namespace
{
typedef std::map<int, std::string> Int2StringMap;

const size_t g_nMax = 100000;
const size_t g_nTimes = 10;
//const size_t g_nTimes = 1;
const std::string g_str1 = _T("Hello");
const std::string g_str2 = _T("Hello world");

template <typename MapType, typename KeyType, typename ValueType>
typename MapType::iterator map_add_or_update(MapType& rMap, const
KeyType& rKey, const ValueType& rValue)
{
typename MapType::iterator it = rMap.lower_bound(rKey);
if (it != rMap.end() && (rMap.key_comp()(rKey, it->first)) ==
false)
{
//update
it->second = rValue;
return it;
}
else
{
return rMap.insert(it, MapType::value_type(rKey, rValue));
}
}
}

//profiler does not see static fct's
void Prf1aImpl(Int2StringMap* pMap);
void Prf1bImpl(Int2StringMap* pMap);

//----------------------------------------------------------------------------
// Function Prf1
//----------------------------------------------------------------------------
// Description : prf1
//----------------------------------------------------------------------------
void Prf1()
{
Int2StringMap mp1a;
Int2StringMap mp1b;

Prf1aImpl(&mp1a);
Prf1bImpl(&mp1b);

_ASSERT(mp1a == mp1b);
}

//----------------------------------------------------------------------------
// Function Prf1aImpl
//----------------------------------------------------------------------------
// Description : use operator[]
//----------------------------------------------------------------------------
void Prf1aImpl(Int2StringMap* pMap)
{
for (int n = 0; n < g_nTimes; ++n)
{
const std::string& cr = ((n % 2 == 0) ? g_str1 : g_str2);

for (int i = 0; i < g_nMax; ++i)
{
(*pMap)[i] = cr;
}
}
}

//----------------------------------------------------------------------------
// Function Prf1bImpl
//----------------------------------------------------------------------------
// Description : use Scott Meyers impl.
//----------------------------------------------------------------------------
void Prf1bImpl(Int2StringMap* pMap)
{
for (int n = 0; n < g_nTimes; ++n)
{
const std::string& cr = ((n % 2 == 0) ? g_str1 : g_str2);

for (int i = 0; i < g_nMax; ++i)
{
map_add_or_update(*pMap, i, cr);
}
}
}

Wkr me.

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Bo Persson · Guest

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