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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.

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  • 2
    Luckily you cannot do stuff like this in C++. It's not PHP and does not encourage bad things. Commented Sep 24, 2012 at 11:40

3 Answers 3

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5

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.

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7 Comments

This class like AA,BB can't know.I hope encapsulation the function of "create" .Order people will make class like AA,BB.I only write base class and create function.
Maybe,this class will write more like "CC","DD","EE"......I don't want modify the function of "create" .
@user1641913 then you'll need a map of callbacks to functions that create a specific class and extend that map when you add a new class.
std::map don't help define variable....This is common method to get instance of class according to string...But can't create a new instance to variable.
@user1641913 do you know what a callback is? I said you should have a map of strings to callbacks - i.e. functions that generate instances of the class.
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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.

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4 Comments

std::map don't help define variable....This is common method to get instance of class according to string...But can't create a new instance to variable.
Read my answer again: the map would hold a mapping from string to factory function. You'd call the found function to create your instance (assuming one is found; if none is found you'signal an error, e.g., by throwing an exception).
You are right.But I have to write function for each class.Perhaps the only way！
You need to write two functions: one for the flavor passing the pointer and one for the others: template <typename T> T* createSimple(void*) { return new T(); } and similarly a version using the argument. You can then register it using &createSimple<AA>.
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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);
}
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