C ++无法实现默认构造函数

Question

I have the following class:

class Fraction {
    private:
        int x;
        int y;
    public:
    // Constructors
        Fraction(long x = 0, long y = 1);
        Fraction(const Fraction&)=default;//here is the problem
        virtual ~Fraction();
 };

I'm trying to disable default C++ constructor to implement my own (I intend to use it for copy). So, I declared it as a default. But, when I'm trying to implement it:

Fraction::Fraction(const Fraction&){}

Compiler throws the following error at me:

./src/Fraction.cpp:16:1: error: definition of explicitly-defaulted 'Fraction::Fraction(const Fraction&)' Fraction::Fraction(const Fraction&){ ^ In file included from ../src/Fraction.cpp:8:0: ../src/Fraction.h:22:2: error: 'Fraction::Fraction(const Fraction&)' explicitly defaulted here Fraction(const Fraction&)=default;

Is there any way to fix it? What I'm doing wrong ? I found some articles about defaults, but nothing that can help me fix these errors.

Answer 1

= default tells the compiler to use the default implementation. This also means that you cannot provide your own . If you don't want the compiler to create one, simply remove = default :

class Fraction {
// ...
    Fraction(const Fraction&); // NO =default
};

By the way, the "default constructor" is the one that's called when you don't provide any arguments, eg

Fraction my_frac; // default constructor called

If you want to disable the default constructor, use =delete :

class Fraction{
public:
    Fraction() = delete;
}

Keep in mind that your current Fraction(long, long) provides default arguments, so it's eligible as a default constructor.