C++ – How to Call One Constructor from Another

c++constructor

class A{
  A(int a = 5){
    DoSomething();
    A();
  }
  A(){...}
}

Can the first constructor call the second one?

Best Answer

Not before C++11.

Extract the common functionality into a separate function instead. I usually name this function construct().

The "so-called" second call would compile, but has a different meaning in C++: it would construct a new object, a temporary, which will then be instantly deleted at the end of the statement. So, no.

A destructor, however, can be called without a problem.

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C++ Constructor Chaining – How to Call a Constructor from Another Constructor

C++11: Yes!

C++11 and onwards has this same feature (called delegating constructors).

The syntax is slightly different from C#:

class Foo {
public: 
  Foo(char x, int y) {}
  Foo(int y) : Foo('a', y) {}
};

C++03: No

Unfortunately, there's no way to do this in C++03, but there are two ways of simulating this:

You can combine two (or more) constructors via default parameters:

class Foo {
public:
  Foo(char x, int y=0);  // combines two constructors (char) and (char, int)
  // ...
};

Use an init method to share common code:

class Foo {
public:
  Foo(char x);
  Foo(char x, int y);
  // ...
private:
  void init(char x, int y);
};

Foo::Foo(char x)
{
  init(x, int(x) + 7);
  // ...
}

Foo::Foo(char x, int y)
{
  init(x, y);
  // ...
}

void Foo::init(char x, int y)
{
  // ...
}

See the C++FAQ entry for reference.

Best Answer

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C++ Constructor Chaining – How to Call a Constructor from Another Constructor

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