How to change attribute of object in C++ from static method

问题

How to change object attribute from static method in C++? My method must be static.

Code:

class Wrapper
{
    private:
        double attribute; //attribute which I want to change
    public:
        Wrapper();
        ~Wrapper();
        static void method(double x);
}

I tried:

std::string Wrapper::method(double x)
{
    attribute = x;
}

But:

error: invalid use of member ‘Wrapper::attribute’ in static member function

回答1:

A static member function cannot access a non-static member because there is no object whose member you'd be referring to. Sure, you could pass a reference to an object as a parameter as suggested, but that's just silly because you might just as well make the function a non-static member.

It is a kind of object which is and will be created only once.

The simplest solution is to make the member static. static members can be accessed without an object because static members are shared for all instances.

tried to make attribute static, but I got error: undefined reference to `Wrapper::attribute

That means you forgot to define the variable.

回答2:

Maybe this way:

std::string Wrapper::method(double x, Wrapper& obj)
{
    obj.attribute = x;
}

But if you have such a problem, you should rethink your design. Methods referring to instance of a class have no reson to be static.

Static methods are not associated with any instance of the class but with the class itself. Compiler has no idea, that there is only one instance of this class. There are many possible solutions for your problem, but the correct one depends on what you want to achieve in a bigger scale.

回答3:

method is a class method and attribute is an instance variable. There is no instance and therefore no attribute when method is called in your current design. The only way to change an instance variable (like attribute) is to provide an instance of Wrapper to method. There are several possible solutions. Some ideas:

an instance as a parameter to method (see Estiny's answer)
a global instance of Wrapper (not advised)
make Wrapper a singleton (in general not advised, but singletons can be a solutions in some situations)

来源：https://stackoverflow.com/questions/32478983/how-to-change-attribute-of-object-in-c-from-static-method

标签

c++

static-methods