Using C++ base class constructors?

Question

While working with templates I ran into a need to make a base class constructors accessible from inherited classes for object creation to decrease copy/paste operations. I was thinking to do this through using keyword in same manner with functions case, but that not work.

class A { public:      A(int val) {} };  class B : public A { };  class C : public A { public:     C(const string &val) {} };  class D : public A { public:     D(const string &val) {}     using A::A;              // g++ error: A::A names constructor };  void main() {     B b(10);                // Ok.   (A::A constructor is not overlapped)     C c(10);                // error: no matching function to call to 'C::C(int)' } 

So my question: Is there any way to import a base class constructors after new ones in inherited class been declared?

Or there is only one alternative to declare new constructors and call a base ones from initializer list?

Answer 1

Yes, Since C++11:

struct B2 {     B2(int = 13, int = 42); }; struct D2 : B2 {     using B2::B2; // The set of inherited constructors is // 1. B2(const B2&) // 2. B2(B2&&) // 3. B2(int = 13, int = 42) // 4. B2(int = 13) // 5. B2()  // D2 has the following constructors: // 1. D2() // 2. D2(const D2&) // 3. D2(D2&&) // 4. D2(int, int) <- inherited // 5. D2(int) <- inherited }; 

For additional information see http://en.cppreference.com/w/cpp/language/using_declaration

Answer 2

Prefer initialization:

class C : public A { public:     C(const string &val) : A(anInt) {} }; 

In C++11, you can use inheriting constructors (which has the syntax seen in your example D).

Update: Inheriting Constructors have been available in GCC since version 4.8.

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If you don't find initialization appealing (e.g. due to the number of possibilities in your actual case), then you might favor this approach for some TMP constructs:

class A { public:      A() {}     virtual ~A() {}     void init(int) { std::cout << "A\n"; } };  class B : public A { public:     B() : A() {}     void init(int) { std::cout << "B\n"; } };  class C : public A { public:     C() : A() {}     void init(int) { std::cout << "C\n"; } };  class D : public A { public:     D() : A() {}     using A::init;     void init(const std::string& s) { std::cout << "D -> " << s << "\n"; } };  int main() {     B b; b.init(10);     C c; c.init(10);     D d; d.init(10); d.init("a");      return 0; }