C++: explicit constructor is implicitly called by derived class - c++

Why making a constructor explicit does not prevent it to be implicitly called by derived class?
class A{
public:
explicit A(){}
};
class B : public A{
public:
B(){ //Constructor A() is called implicitly
//...
}
}
I had a situation in my program when I'd rather have compiler error in that case, it would save me a lot of time to find a bug. For now I changed default constructor of A to accept a dummy "int" argument to achieve that, but shouldn't "explicit" keyword work for this?
g++-4.8 compiles the code above without any errors or warnings.

Your assumption is wrong on the explicit keyword.
The explicit keyword isn't meant to prevent the constructor from being called from a derived class but rather to prevent implicit conversions like the one in the sample here: https://stackoverflow.com/a/121163/1938163
I'm summarizing the relevant parts here:
class Foo
{
public:
// single parameter constructor, can be used as an implicit conversion
Foo (int foo) : m_foo (foo)
{
}
int GetFoo () { return m_foo; }
private:
int m_foo;
};
Since at most one implicit conversion can be done to resolve ambiguities, if you have a function like
void DoBar (Foo foo)
{
int i = foo.GetFoo();
}
the following is legit:
int main ()
{
DoBar (42); // Implicit conversion
}
And that's exactly where the explicit keyword comes into play: forbids the case above.
To solve your problem, in order to prevent your class from being used as a base class, just mark the constructor with final if you're using C++11 (http://en.wikipedia.org/wiki/C++11#Explicit_overrides_and_final)

explicit keyword is usually used with constructors that have one parameter. It prevents implicit construction of the object from type of the parameter to type of the class.
The example below will compile, and it usually is not what you want:
#include <iostream>
using namespace std;
struct Test
{
Test(int t) {}
};
void test_fun(Test t) {}
int main() {
test_fun(1); //implicit conversion
return 0;
}
With explicit keyword this example won't compile:
#include <iostream>
using namespace std;
struct Test
{
explicit Test(int t) {}
};
void test_fun(Test t) {}
int main() {
test_fun(1); //no implicit conversion, compiler error
return 0;
}

Related

C++ shadowing parent constructor

Is there any harm in shadowing a parent constructor? It doesn't seem like anything is wrong, but my IDE is indicating and not warning me about it. Quick example
#include <iostream>
#include <string>
namespace NS1 {
class A {
public:
A(){std::cout<<"I am A\n";}
virtual ~A(){}
};
}
namespace NS2 {
class A : public NS1::A{
public:
A(): NS1::A() {}
~A(){}
};
}
int main()
{
NS2::A a;
}
It works as expected, but... is it bad to shadow?
There's nothing wrong with this code, except that it could possibly be improved by following the Rule Of Zero:
namespace NS2 {
class A : public NS1::A {
public:
// Has implicitly declared default constructor, copy constructor,
// move constructor, copy assignment, move assignment, and destructor.
};
}
In fact, the constructor NS2::A::A does not "shadow" NS1::A::A at all.
A derived class does not inherit constructors from any base class by default. If you want that, you can say that it should with a declaration like using Base::Base;.
In your example, if NS2::A did not declare its default constructor A();, it would still get one implicitly with exactly the same behavior - but this is because a class with no user-declared constructors gets an implicitly declared/defined default constructor, not because NS1::A has such a constructor. If we add A(std::string); to NS1::A, this doesn't make it valid to create NS2::A("test").
#include <iostream>
#include <string>
namespace NS1 {
class A {
public:
A(){std::cout<<"I am A\n";}
explicit A(std::string s) {
std::cout << "Creating NS1::A from string '" << s << "'\n";
}
virtual ~A(){}
};
}
namespace NS2 {
class A : public NS1::A {
};
}
int main()
{
NS2::A a; // OK
NS2::A b("test"); // Error, no matching constructor!
}
And then, if we add a similar A(std::string); to NS2::A, that's a different function. Its member initializer list will determine whether creating the NS1::A subobject uses the NS1::A::A(); default constructor, the NS1::A::A(std::string); constructor from string, or something else. But we wouldn't say that NS2::A::A(std::string); "hides" NS1::A::A(std::string); because the NS1::A constructor couldn't be used directly to initialize an NS2::A object even without the NS2::A::A(std::string); constructor present.
Your compiler/IDE probably means this:
namespace X {
struct A {
A() = default;
};
}
struct B : X::A {
B () : A() { // <- you can use just A, as opposed to X::A here
A a; // <- also here
}
};
struct A : X::A {
A () : X::A() { // <- you cannot use unqualified A here to refer to X::A,
// because it is shadowed by class's own name
X::A a; // <- neither here
}
};
There is nothing particularly harmful in it, because you can always use the qualified name. So I would say it makes no sense to warn about it, and it's a bug if the compiler does so.

Why does a explicit constructor expecting std::shared_ptr accept a nullptr?

The following code compiles fine with gcc 4.8.1
#include <memory>
class Foo {
public:
explicit Foo(const std::shared_ptr<Foo>& foo) {
}
};
int main() {
Foo foo(nullptr);
}
Why is this possible? Shouldn't the explicit prevent the compiler from calling std::shared_ptr(nullptr) implicitly?
Shouldn't the explicit prevent the compiler from calling std::shared_ptr(nullptr) implicitly?
No, the explicit constructor would stop this from happening:
Foo foo = some_shared_ptr;
It has no effect on the constructors of shared_ptr, so implicit conversion from nullptr to shared_ptr is still allowed.

deleted default constructor headache

My c++ book says this (lippman, c++ primer, fifth ed., p. 508):
The synthesized default constructor is defined as deleted if the class ... has a const member whose type does not explicitly define a default constructor and that member does not have an in-class initializer. (emphesis mine)
Why then does this code produce an error?
class Foo {
Foo() { }
};
class Bar {
private:
const Foo foo;
};
int main() {
Bar f; //error: call to implicitly-deleted default constructor of 'Bar'
return 0;
}
The rule above seems to indicate that it should not be an error, because Foo does explicitly define a default constructor. Any ideas?
To fix your error. You need to make Foo::Foo() public.
class Foo
{
public:
Foo() { }
};
Otherwise I do believe it is private.
Is this what your looking for?
The default constructor is omitted when a a class construction isn't trivial.
That in general means that either there is an explicit constructor that receives parameters (and then you can't assume that it can be constructed without those parameters)
Or if one of the members or base classes need to be initiated in construction (They themselves don't have a trivial constructor)
I think that this should work
class Foo {
public:
Foo() { }
};
class Bar {
public:
Bar() : foo() {}
private:
const Foo foo;
};

c++ class operator self-typecast

How can I create an operator function within a class that serves to typecast other types as an object of that class?
e.g.
class MyClass
{
// ...
// operator ??
// ...
}
int main()
{
MyClass obj;
int Somevar;
obj=(MyClass)Somevar; // class typecast
}
In general, is there an operator that allows this kind of typecast in exact syntax?
Just add a constructor that takes one argument:
class MyClass {
explicit MyClass(int x) { … }
};
called as:
MyClass x = static_cast<MyClass>(10); // or
MyClass y = MyClass(10); // or even
MyClass z(10);
This allows an explicit cast as in your example. (The C-style cast syntax is also supported but I won’t show it here because you should never use C-style casts. They are evil and unnecessary.)
Sometimes (but very rarely), an implicit cast is more appropriate (e.g. to convert from char* to std::string in assignments). In that case, remove the explicit qualifier in front of the constructor:
class MyClass {
MyClass(int x) { … }
};
Now an implicit conversion from int is possible:
MyClass a = 10;
However, this is usually not a good idea because implicit conversions are non-intuitive and error-prone so you should normally mark the constructor as explicit.
Define a constructor taking int argument.
But implicit conversions has some problems, so many that the language has the keyword explicit to prohibit them.
Mainly that's about overload resolution.
So, perhaps think twice before allowing the implicit conversion.
Cheers & hth.,
Provide non-explicit constructor with argument of wanted type:
class MyClass {
public:
MyClass( int x );
...
};
MyClass a = 42;
Note though: this is usually a bad idea.
You need to construct the object implicitly.
class MyClass
{
int x;
public:
MyClass(int X = 0):x(X){} //also serves a default constructor
}
int main()
{
MyClass obj = Somevar; // implicit type construction
}
why not use operator=() ?
class MyClass
{
public:
Myclass& operator=()(int i) {
//do what you want
return *this;
}
}
int main()
{
MyClass obj;
int Somevar;
obj = Somevar; // call operator=(somevar)
}

Can I call a constructor from another constructor (do constructor chaining) in C++?

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.
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) {}
};
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.
Ref: https://isocpp.org/wiki/faq/ctors#init-methods
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
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 upto C++03.
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.
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:
Overload the constructor, using different signatures
Use default values on arguments, to make a "simpler" version available
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.
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 :)