C ++：为全局类变量赋值

Question

Consider the class MyClass that has no default constructor.

I want to write a code that looks like the following:

MyClass instance;

void init_system() {
    instance = MyClass(parameters, of, the, constructor);
}

The code I wrote above of course fails with the error MyClass has no C'tor that takes no arguments.

Is there any correct way to do it, or I must implement a workaround, eg using shared pointers?

Answer 1

Well, either a default object of your class can sensibly exist, or it cannot.
In the latter case, you might be interested in std::optional ( boost::optional before C++17) to defer the construction of the object.

Answer 2

You could move the initialization of the object into your init_system() function:

MyClass& init_system()
{
   static MyClass instance(parameters, of, the, constructor);
   return instance;
}

You may want to look up the singleton pattern, too, and read the extensive discussions about it ;)

And yes, another solution could be to use a unique_ptr<> or shared_ptr<> .

Answer 3

There are 2 ways of achieving this....

MyClass instance(parameters, of, the, constructor);

Would initialize MyClass with the correct parameters.

A singleton pattern

MyClass & MyClass::getInstance(){
   static MyClass instance( parameters, of, constructor );
   return instance;
}

with getInstance returned at the time of calling.

The second pattern improves control over when the object is constructed.