如何从无序的集合中准确地得到一个元素?

begin()

if (incompletePieces.size() > 0)
    auto containedPiece = *(incompletePieces.begin());

incompletePieces

for (auto piece : incompletePieces) {
  // process piece
}
incompletePieces.clear();

auto it = incompletePieces.begin();
while (it != incompletePieces.end()) {
  // process *it
#if C++11
  it = incompletePieces.erase(it);
#else
  auto prev = it++;
  incompletePieces.erase(prev);
#endif
}

*unordered_set::begin() unordered_set::front()

while (incompletePieces.size()){
    for (auto containedPiece : incompletePieces){
        //  .... doing some stuff with the contained piece
        incompletePieces.erase(containedPiece);
        break;
    }
}

for (auto* containedPiece : incompletePieces){
    //  .... doing some stuff with the contained piece
}
incompletePieces.clear();

for(auto* containedPiece : incompletePieces){
     //Process the set contents
}
//Clear entire set in one go 
incompletePieces.clear();

 auto it = incompletePieces.begin(); //Take the pointer to first element of set
 for( ; it !=incompletePieces.end() ; it++){ 
     incompletePieces.erase(*it); //Erase one element at a time
 }