CODEWITHSUNDEEP
c++
simon
simon

Reputation: 579

define variable according to string

How to define variable according to string. I had defined many classes.But I want to creat variable of this class according to some string.

The code looks like this.

class AA {};
class BB {};
class CC {
    CC(void *pt);
    virtual ~CC();
};
......

void test(char *ss,void *pt=NULL) {
    //??????How to do?

}

int main() {
    a1=test("AA");    //a1=new AA();
    a2=test("AA");    //a2=new AA();
    b1=test("BB");    //b1=new BB();
    c1=test("CC",pt); //c1=new CC(pt);
}

Orther,you can consider this as URL and handle function.The std::map is common method to get instance of class according to string.But can't create a new instance to variable. I hope get a new instance according to string.

Upvotes: 0

Views: 245

Answers (3)

PiotrNycz
PiotrNycz

Reputation: 24382

Maybe instead of using string names of types - use types as they are. To do this - use templates.

class AA {};
class BB {};
class CC {
public:
    CC(void *pt) {}
    virtual ~CC() {}
};

template <class T>    
T* test() {
    return new T();

}
template <class T>    
T* test(void *pt) {
    return new T(pt);

}

int main() {
    void* pt;
    AA* a1=test<AA>();    //a1=new AA();
    AA* a2=test<AA>();    //a2=new AA();
    BB* b1=test<BB>();    //b1=new BB();
    CC* c1=test<CC>(pt); //c1=new CC(pt);
}

Upvotes: 0

Dietmar K&#252;hl
Dietmar K&#252;hl

Reputation: 153820

You probably want you function to return something, either void* or, preferably, a [smart] pointer to a common base. The string should probably be passed as char const* or as std::string const&. Within the function you either directly compare the argument and you call the appropriate allocation or you create a std::map<std::string, FactoryFunction> to look up a factory function based on the string.

Upvotes: 1

Luchian Grigore
Luchian Grigore

Reputation: 258568

C++ is a strongly typed language, so this isn't possible as you have it now.

Best case, you'd use a common base class for AA, BB, CC and then use a factory. You can't just write:

a1=test("AA");    //a1=new AA();
a2=test("AA");    //a2=new AA();
b1=test("BB");    //b2=new BB();
c1=test("CC",pt); //b2=new CC(pt);

without defining a type for the variables.

For example:

class Base{};
class AA : public Base {};
class BB : public Base {};

Base* create(const std::string& what)
{
   if (what == "AA")
       return new AA;
   if (what == "BB")
       return new BB;
   return NULL;
}

int main()
{
    Base* a;
    a = create("AA");
}

Alternitively, you should use smart pointers. If you don't you'll have to manage the memory yourself.

Upvotes: 5

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