如何使编译器在C ++中创建默认构造函数？

Question

Is there a way to make the compiler create the default constructors even if I provide an explicit constructor of my own?

Sometimes I find them very useful, and find it a waste of time to write eg the copy constructor, especially for large classes.

Answer 1

You can't - the compiler turns off some of its auto-generated default constructor when you provide your own, so you can prevent default-constructing certain classes that way. However, I think C++0x will allow you to explicitly state a default compiler implementation, eg:

MyClass() = default;  // 'delete' also allowed by upcoming standard to disable

I don't think any compilers support this yet - C++0x (as the next standard has been known) is not yet final, so you'll just have to make do typing out your default constructors for now. It's not much code! MyClass() {} will do as long as all the members are themselves default constructible.

Answer 2

The copy constructor is provided whether you define any other constructors or not. As long as you don't declare a copy constructor, you get one.

The no-arg constructor is only provided if you declare no constructors. So you don't have a problem unless you want a no-arg constructor, but consider it a waste of time writing one.

IIRC, C++0x has a way of delegating construction to another constructor. I can't remember the details, but it would allow you to define a no-arg constructor by specifying another constructor, plus the argument(s) to pass to it. Might save typing some data member initializers in some cases. But the default no-arg constructor wouldn't have provided those initializers either.

Answer 3

Compiler will generate default copy constructor always, unless you provide your own definition of copy constructor. Your problem is only with default no-arg constructor, which is not generated if there is any constructor definition present. But it's not so hard to provide no-arg constructor which behaves exactly like generated one:

class yourClass
{
    public:
       yourClass(){}
}