What are the ramifications of assigning a class variable when defining the class versus in the class constructor? Is the variable assigned in the class definition accessible by all class instances?
Example of assignment at instantiation:
class Foo
{
private:
int x;
double y;
public:
Foo()
{
x = 0;
y = 1.;
}
};
Example of assignment in class definition:
class Foo
{
private:
int x = 0;
double y = 1.;
public:
Foo();
};
edit:
As to the class member being accessible by all instances, I think I was looking for the notion of a static declaration, I guess I'm just new to the curly brace languages.
In this code snippet
int x = 0;
double y = 1.;
there is no assignments. There are initializations.
In this code snippet
Foo()
{
x = 0;
y = 1.;
}
there is indeed used the assignment operator.
In general for objects of complex types it can be 1) impossible (either the default constructor or the assignment operator is not available) or 2) requiring many resources because at first default constructors are called that create the objects and after that there are called assignment operators.
It is desirable to define constructors at least like for example
Foo() : x( 0 ), y( 1 )
{
}
using mem-initializer lists.
Is the variable assigned in the class definition accessible by all class instances?
No!
The difference is that you default-initialize the member variables and then assign values to them in the first case. In the second case you value-initialize them - which is preferred. A third option, that also value-initializes them, is to use the member-initializer list:
class Foo
{
private:
int x;
double y;
public:
Foo() : x{0}, y{1.}
{
}
};
My question is about this operator in C++, should we use it as much as possible? I given the following example to show my point:
class Abc
{
public:
int a_;
void fun();
};
void Abc::fun()
{
// option 1
a_ = 3;
// option 2
this->a_ = 3;
}
In the function class member fun(), we can invoke member variables in two ways, one is using this-> and the other is not using it. So my question is: which practice is encouraged? Thanks.
Under ordinary circumstances, you're right that you can use both. In such case, it's just a matter of style and the correct thing to do is follow the style guide of your project. Consistency is more important than personal preference in this regard.
However, there are two situations where using this-> can make a difference. One is when the member function has a parameter with the same name as a member; in such case, the name of the parameter hides the name of the member and you have to use this-> to refer to the member (first pointed out by #Krypton's answer):
void Abc::fun(int a_)
{
a_ = 3; // assigns into the parameter
this->a_ = 3; // assigns into the data member
}
The other situation is when you're working inside a class template and the member is inherited from a base class which depends on template parameters of your class template. In such case, unqualified lookup does not search dependent contexts and so the member would not be found. Using this-> turns the access into a dependent expression, which will be looked up at instantiation time, and thus resolved to the member correctly. Example:
template <class T>
struct Base
{
protected:
T a_;
};
template <class T>
struct Abc : Base<T>
{
void fun() {
a_ = 3; // error, not `a_` in scope
this->a_ = 3; // OK, found at instantiation time
}
};
In this situation, an alternative solution exists: make the name visible explicitly:
template <class T>
struct Abc : Base<T>
{
protected:
using Base<T>::a_;
public:
void fun() {
a_ = 3; // OK, found thanks to `using` declaration
}
};
If the variable and the parameter are named the same, using this is a must.
class Foo
{
public:
int _a;
void func(int _a) {
this->_a = _a;
}
};
this is also required when referring to a member of a base class that depends on a template parameter of the current class.
Otherwise, it is not necessary to use this.
I think it is more a matter of style. The use of additional this-> does not change the produced code.
Though you cannot use operator this-> in the initialization of class members before the constructor body, like
class Abc {
public:
Abc(int i): /*this-> is incorrect*/i(i) {}
private:
int i;
};
I prefer to use this-> to have a clear difference with other no-class members.
Then the code like the following is more readable
void foo(int i) { }
class Abc {
public:
Abc(int j) { this->foo(j); }
private:
void foo (int i) { this->i = i; }
int i;
};
Some people name class data members starting with m_, like m_i, m_j.
Some modern IDEs support semantic syntax highlighting that also helps to distinguish between local variables, class data members, global variables, functions.
I've read about class with no name...
Got that
class
{
int i;
};
is not valied while
class
{
int i;
}A;
is valid.
But how to have a: 1) constructor and :2) destructor and: 3) how to pass as an argument???
If the class is unnamed then it can't have a custom constructor/destructor (although the compiler will generate the usual default ones for you). That means you can't pass an argument to it at construction. However, there's probably nothing stopping you from either (1) adding one or methods to the class to pass data to it after construction, or (2) giving the class a name in the first place.
The second thing is valid, but you cannot access the member i since it will be private. It is valid only because You have created an object A of that class type.
To have a constructor, I think you have to give the class a name. Use this:
class myclass
{
int i;
public:
myclass(int a) //This is the constructor
{
i = a;
}
~myclass() //This is the destructor
{
//Whatever you want to do on destroying the object
}
};
and instantiate that class as follows:
myclass A(10);
or similarly.
Note: This is just my interpretation, it can be done in many other ways.
You are doomed using the class locally and accessing it through the local object A, only.
You can use it in a template, though:
template <typename T>
inline void print(const T& a) {
std::cout << a.x << std::endl;
}
int main()
{
class {
public: int x;
} a;
a.x = 1;
print(a);
}
Whenever I define a variable and give it a value at the same time inside a class, I get an error. What is the reason for this?
As you can see, this doesn't work...
class myClass {
private:
int x = 4; // error
};
But when I keep the variable undefined it does:
class myClass {
private:
int x;
};
Since no one else is using member initialization, I'll introduce you:
class myClass {
private:
int x;
public:
myClass() : x (4){}
};
It's always better to use this over assigning in the body of the constructor, since by the time the body begins, all user-defined members will have already been initialized whether you said so or not. Better to do it once and actually initialize the non-user-defined members, and it is the only method that works for both non-static const members, and reference members.
For example, the following will not work because x isn't being initialized in the body, it's being assigned to:
class myClass {
private:
const int x;
public:
myClass() {x = 4;}
};
Using a member initializer, however, will, because you're initializing it off the bat:
class myClass {
private:
const int x;
public:
myClass() : x (4){}
};
Note also that your int x = 4; syntax is perfectly valid in C++11, where it subs in for any needed initialization, so you'll benefit if you start using it.
Initialize your variables in the constructor.
class myClass {
private:
int x;
public:
myClass()
{
x = 4; // hope that it will work
}
};
Updated Answer:
According to chris, it is better to use member initialization
class myClass {
private:
const int x;
public:
myClass() : x (4){}
};
Instance variable are supposed to be defined using setter methods, IE: setX(input) or inside a constructor.
Do this insted:
class myClass
{
private:
static const int x = 4;
};
If you don't x to be either static or constant, use only int x; instead and initialize x in the constructor of the class.
Your class is like a blue print. It does not have any storage associated with it. When you instantiate an object of your class, that is akin to the building based on that blue print. Your object has storage and that can hold the value you give to the member variables.
Also, as others have pointed out, it is possible to do what you want in C++11. Check out:
http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2008/n2756.htm
In C++03,
only static const integral data members can be initialized within a
class.
Either initialize in the constructor, or make the data member static const:
class myClass {
private:
int x;
public:
myClass() {
x = 4;
}
};
or
class myClass {
private:
static const int x = 4;
};
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 :)