As a C# developer I'm used to running through constructors:
class Test {
public Test() {
DoSomething();
}
public Test(int count) : this() {
DoSomethingWithCount(count);
}
public Test(int count, string name) : this(count) {
DoSomethingWithName(name);
}
}
Is there a way to do this in C++?
I tried calling the Class name and using the 'this' keyword, but both fail.
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.
Foo:Foo(char x) { Foo(x, int(x)+7); } What would this do? –
Spotted init; the hacky C++03 approach and the C++11 approach improve on that by allowing you to construct the correct final version exactly once (which is of course why they work with const and references). –
Mcleod new (this) Foo(...), this should work. –
Undrape Yes and No, depending on which version of C++.
In C++03, you can't call one constructor from another (called a delegating constructor).
This changed in C++11 (aka C++0x), which added support for the following syntax:
(example taken from Wikipedia)
class SomeType
{
int number;
public:
SomeType(int newNumber) : number(newNumber) {}
SomeType() : SomeType(42) {}
};
SomeType(string const &s) { /*...*/ } SomeType(char const *pc) : SomeType(string(pc)) { /*...*/ } –
Aholah I believe you can call a constructor from a constructor. It will compile and run. I recently saw someone do this and it ran on both Windows and Linux.
It just doesn't do what you want. The inner constructor will construct a temporary local object which gets deleted once the outer constructor returns. They would have to be different constructors as well or you would create a recursive call.
new syntax. This usually isn't what you want, though. (And it doesn't do anything to allow you to explicitly provide template arguments.) –
Celestecelestia new would still create a new object, albeit at the same memory location. But a different object nevertheless, and it is possible to put together the code that proves this. –
Groundage It is worth pointing out that you can call the constructor of a parent class in your constructor e.g.:
class A { /* ... */ };
class B : public A
{
B() : A()
{
// ...
}
};
But, no, you can't call another constructor of the same class.
In C++11, a constructor can call another constructor overload:
class Foo {
int d;
public:
Foo (int i) : d(i) {}
Foo () : Foo(42) {} //New to C++11
};
Additionally, members can be initialized like this as well.
class Foo {
int d = 5;
public:
Foo (int i) : d(i) {}
};
This should eliminate the need to create the initialization helper method. And it is still recommended not calling any virtual functions in the constructors or destructors to avoid using any members that might not be initialized.
If you want to be evil, you can use the in-place "new" operator:
class Foo() {
Foo() { /* default constructor deliciousness */ }
Foo(Bar myParam) {
new (this) Foo();
/* bar your param all night long */
}
};
Seems to work for me.
edit
As @ElvedinHamzagic points out, if Foo contained an object which allocated memory, that object might not be freed. This complicates things further.
A more general example:
class Foo() {
private:
std::vector<int> Stuff;
public:
Foo()
: Stuff(42)
{
/* default constructor deliciousness */
}
Foo(Bar myParam)
{
this->~Foo();
new (this) Foo();
/* bar your param all night long */
}
};
Looks a bit less elegant, for sure. @JohnIdol's solution is much better.
would expect them to do that operation twice with that code. –
Quickfreeze this->~Foo();, before new (this) Foo(); –
Eunuchize const or references. –
Eichhorn *this = Foo(); –
Mcilroy Simply put, you cannot before C++11.
C++11 introduces delegating constructors:
Delegating constructor
If the name of the class itself appears as class-or-identifier in the member initializer list, then the list must consist of that one member initializer only; such constructor is known as the delegating constructor, and the constructor selected by the only member of the initializer list is the target constructor
In this case, the target constructor is selected by overload resolution and executed first, then the control returns to the delegating constructor and its body is executed.
Delegating constructors cannot be recursive.
class Foo { public: Foo(char x, int y) {} Foo(int y) : Foo('a', y) {} // Foo(int) delegates to Foo(char,int) };
Note that a delegating constructor is an all-or-nothing proposal; if a constructor delegates to another constructor, the calling constructor isn't allowed to have any other members in its initialization list. This makes sense if you think about initializing const/reference members once, and only once.
No, in C++ you cannot call a constructor from a constructor. What you can do, as warren pointed out, is:
Note that in the first case, you cannot reduce code duplication by calling one constructor from another. You can of course have a separate, private/protected, method that does all the initialization, and let the constructor mainly deal with argument handling.
Another option that has not been shown yet is to split your class into two, wrapping a lightweight interface class around your original class in order to achieve the effect you are looking for:
class Test_Base {
public Test_Base() {
DoSomething();
}
};
class Test : public Test_Base {
public Test() : Test_Base() {
}
public Test(int count) : Test_Base() {
DoSomethingWithCount(count);
}
};
This could get messy if you have many constructors that must call their "next level up" counterpart, but for a handful of constructors, it should be workable.
In Visual C++ you can also use this notation inside constructor: this->Classname::Classname(parameters of another constructor). See an example below:
class Vertex
{
private:
int x, y;
public:
Vertex(int xCoo, int yCoo): x(xCoo), y(yCoo) {}
Vertex()
{
this->Vertex::Vertex(-1, -1);
}
};
I don't know whether it works somewhere else, I only tested it in Visual C++ 2003 and 2008. You may also call several constructors this way, I suppose, just like in Java and C#.
P.S.: Frankly, I was surprised that this was not mentioned earlier.
Vertex(-1, -1). The only difference is use of qualified lookup instead of unqualified, but it still names the same class. –
Eichhorn this-> though? –
Ardolino this-> making no sense. This is not valid C++, period. –
Ardolino c.iterator it = c.begin() instead of std::vector<Blah>::iterator it = c.begin(); –
Eichhorn this->typename T varname; would be very convenient for accessing a dependent name in CRTP. –
Eichhorn This approach may work for some kinds of classes (when the assignment operator behaves 'well'):
Foo::Foo()
{
// do what every Foo is needing
...
}
Foo::Foo(char x)
{
*this = Foo();
// do the special things for a Foo with char
...
}
I would propose the use of a private friend method which implements the application logic of the constructor and is the called by the various constructors. Here is an example:
Assume we have a class called StreamArrayReader with some private fields:
private:
istream * in;
// More private fields
And we want to define the two constructors:
public:
StreamArrayReader(istream * in_stream);
StreamArrayReader(char * filepath);
// More constructors...
Where the second one simply makes use of the first one (and of course we don't want to duplicate the implementation of the former). Ideally, one would like to do something like:
StreamArrayReader::StreamArrayReader(istream * in_stream){
// Implementation
}
StreamArrayReader::StreamArrayReader(char * filepath) {
ifstream instream;
instream.open(filepath);
StreamArrayReader(&instream);
instream.close();
}
However, this is not allowed in C++. For that reason, we may define a private friend method as follows which implements what the first constructor is supposed to do:
private:
friend void init_stream_array_reader(StreamArrayReader *o, istream * is);
Now this method (because it's a friend) has access to the private fields of o. Then, the first constructor becomes:
StreamArrayReader::StreamArrayReader(istream * is) {
init_stream_array_reader(this, is);
}
Note that this does not create multiple copies for the newly created copies. The second one becomes:
StreamArrayReader::StreamArrayReader(char * filepath) {
ifstream instream;
instream.open(filepath);
init_stream_array_reader(this, &instream);
instream.close();
}
That is, instead of having one constructor calling another, both call a private friend!
If I understand your question correctly, you're asking if you can call multiple constructors in C++?
If that's what you're looking for, then no - that is not possible.
You certainly can have multiple constructors, each with unique argument signatures, and then call the one you want when you instantiate a new object.
You can even have one constructor with defaulted arguments on the end.
But you may not have multiple constructors, and then call each of them separately.
When calling a constructor it actually allocates memory, either from the stack or from the heap. So calling a constructor in another constructor creates a local copy. So we are modifying another object, not the one we are focusing on.
Would be more easy to test, than decide :) Try this:
#include <iostream>
class A {
public:
A( int a) : m_a(a) {
std::cout << "A::Ctor" << std::endl;
}
~A() {
std::cout << "A::dtor" << std::endl;
}
public:
int m_a;
};
class B : public A {
public:
B( int a, int b) : m_b(b), A(a) {}
public:
int m_b;
};
int main() {
B b(9, 6);
std::cout << "Test constructor delegation a = " << b.m_a << "; b = " << b.m_b << std::endl;
return 0;
}
and compile it with 98 std: g++ main.cpp -std=c++98 -o test_1
you will see:
A::Ctor
Test constructor delegation a = 9; b = 6
A::dtor
so :)
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thisORautoin the referred context would be interesting keywords for future refactoring purposes. – Archimage