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c++structconstructor

C++ struct initialization using own type

Using custom constructor, a struct can be initialized using own type.

struct Foo {
    int x, y;
    Foo(int X, int Y) : x(X), y(Y) {}
    Foo(const Foo& f) : Foo(f.x, f.y) {}
};
Foo foo1(1, 2);
Foo foo2 = Foo(foo1);

However, I found that declaring the constructor wasn't necessary.

struct Foo {
    int x, y;
    Foo(int X, int Y) : x(X), y(Y) {}
    // Foo(const Foo& f) : Foo(f.x, f.y) {} <-- This can be omitted.
};
Foo foo1(1, 2);
Foo foo2 = Foo(foo1);

I wonder how is this possible.

Solution

  • From https://en.cppreference.com/w/cpp/language/copy_constructor:

    If no user-defined copy constructors are provided for a class type, the compiler will always declare a copy constructor as a non-explicit inline public member of its class. This implicitly-declared copy constructor has the form T::T(const T&)...