[英]Return Generic Type data from function
我寫了下面的代碼。 函數func()打印標題和數據。
class ICell
{
public:
wstring header;
virtual void Fetch() = 0;
};
template <class T>
class Cell : public ICell
{
public:
//wstring header;
T data;
void Fetch()
{
wcout<< header << L": ";
cout<<data<<endl;
}
// implementation of cell methods
};
class Row
{
public:
vector <ICell *> cells;
};
有沒有辦法在函數內返回數據而不是打印? 如果是這樣,應修改代碼的哪一部分? 提前致謝。
int main()
{
Cell<int>c1;
Cell<double>c2;
c1.header = L"Roll", c1.data = 100;
c2.header = L"CGPA", c2.data = 3.5;
Row r;
r.cells.push_back(&c1);
r.cells.push_back(&c2);
vector <ICell *>::iterator it;
for(it=r.cells.begin();it!=r.cells.end();it++)
{
//checkt type of it wherther it points Cell<int> or Cell<double>
}
return 0;
}
我在這里改變了我的問題。 在循環內部的main()中,如何檢查“ it”指向的對象類型?
謝謝大家的耐心和幫助:)
最簡單的方法是使用dynamic_cast
:
vector <ICell *>::iterator it;
for(it=r.cells.begin();it!=r.cells.end();it++)
{
Cell<int>* cell_i= dynamic_cast<Cell<int>*>(*it);
if(cell_i)
{
do_something(cell_i->data);
continue;
}
Cell<double>* cell_d= dynamic_cast<Cell<double>*>(*it);
if(cell_d)
{
do_something(cell_d->data);
continue;
}
}
更好的方法是使用訪客模式:
class ICellVisitor; //declaration for ICell to understand ICell* pointer
class ICell
{ public:
~ICell(){}; // important
std::wstring header;
virtual void visit( ICellVisitor* v ) = 0;
};
template <class T> class Cell; // for Cell<T>* pointer
class ICellVisitor
{ public:
virtual void visit( Cell<int>* c ) = 0;
virtual void visit( Cell<double>* c ) = 0;
virtual void visit( Cell<float>* c ) = 0;
virtual void visit( Cell<long long>* c ) = 0;
};
template <class T> class Cell : public ICell
{ public:
//wstring header;
T data;
void visit( ICellVisitor* v )
{
std::wcout<< header << L": ";
v->visit(this);
}
// implementation of cell methods
};
class Row
{ public:
std::vector <ICell *> cells;
};
現在我們需要定義具體的訪問者,以保留每種類型的算法:
class MyCellVisitor: public ICellVisitor
{ public:
void visit( Cell<int>* c ){
std::wcout<<"(int)"<<c->data<<std::endl;
}
void visit( Cell<double>* c ){
std::wcout<<"(double)"<<c->data<<std::endl;
}
void visit( Cell<float>* c ){
std::wcout<<"(float)"<<c->data<<std::endl;
}
void visit( Cell<long long>* c ){
std::wcout<<"(long long)"<<c->data<<std::endl;
}
};
int main()
{
Cell<int>c1;
Cell<double>c2;
c1.header = L"Roll", c1.data = 100;
c2.header = L"CGPA", c2.data = 3.5;
Row r;
r.cells.push_back(&c1);
r.cells.push_back(&c2);
MyCellVisitor visitor;
std::vector <ICell *>::iterator it;
for(it=r.cells.begin();it!=r.cells.end();it++)
{
(*it)->visit( &visitor );
}
return 0;
}
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