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c++initialization-list

Usage of Initilizer list with default keyword

Why I cannot use default keyword after initialization list

class classA
{
    int num;
public:
    classA():num(3) = default;
};
Solution

  • = default provides a definition of the constructor. Note that it doesn't provide the body, it provides a definition. The definition of a constructor includes both the mem-initialiser-list and the body. So if you want your own mem-initialiser-list, you must provide the entire definition yourself.

    Also note that there is zero problem with doing that. Just write {} instead of = default. A default constructor defined with = default performs exactly the same operations as one defined with {}.

    The only difference between these is that a constructor defined with = default right at its declaration is not considered user-provided and thus allows the class to be a trivial class. But since you want something non-trivial to happen in the constructor, you get exactly what you want by using {}.