Please help me with the following problem:
I have the following classes:
class ChemicalElement
{
private:
std::string _name;
void Init(const std::string& name);
public:
ChemicalElement(const std::string& name);
ChemicalElement(const ChemicalElement& ce);
};
class CombinationRule
{
private:
ChemicalElement _ce1;
ChemicalElement _ce2;
void Init(const ChemicalElement& ce1, const ChemicalElement& ce2);
public:
CombinationRule(const ChemicalElement& ce1, const ChemicalElement& ce2);
CombinationRule(const CombinationRule& rule);
};
The implementation is obvious. I intended to initialize the CombinationRule using the Init method to minimize code duplication. Alas, if I do not use "member initialization list" in each constructor the compiler complains with "error C2512: 'ChemicalElement' : no appropriate default constructor available". Is there an elegant way to solve this error instead of using a default constructor or member initialization list?
BTW: if there are any other problems in the classes definition please add it too. Since I'm revisiting C++ I want to be aware of them.
You should implement constructors of CombinationRule as follows so they will use appropriate constructors of ChemicalElement:
CombinationRule::CombinationRule(const ChemicalElement& ce1,
const ChemicalElement& ce2) : _ce1(ce1), _ce2(ce2)
{
...
}
CombinationRule::CombinationRule(const CombinationRule& rule) :
_ce1( rule._ce1 ), _ce2( rule._ce2 )
{
...
}
I think you are required to put a default constructor in any class where you define any other constructors, if you want to use objects of that class in any kind of array or container. The default constructor's implementation can just be an empty/no-op method though.
You shouldn't need to put in a member initialization list (although using a member initialization list can be more efficient in some cases, since that way your member variables are only initialized once, instead of being initialized once via their default constructor, and then written to a second time by your Init() method)
I think you want this
ChemicalElement * ce1;
I say this because I think its trying to run the default constructor on your CombinationRule and in turn needs to get a ChemicalElement for ce1 and ce2 ... but I could be wrong.
Pretty sure Krill's way is the way to specify the constructor of a variable for a specific constructor BUT i said f that and just made it so ce1 doesn't need to be constructed by the compiler :)
In this particular example I would go on with duplication (it's just writing two initializers, nothing to get obsessive about).
But assuming the real story is more complex: use OO facilities to avoid code duplication.
class CombinationRule : public ElementPair ...
or
class Combination { ElementPair twoElements; ...}
Where ElementPair contains two ChemicalElements and a single constructor (with common code), and Combination rule constructors initialize using constructor of ElementPair.
There are other approaches: initializing members with some InvalidChemicalElement instance or using pointers (auto_ptr) with NULL for InvalidChemicalElement.
Related
I'm having trouble with something that seems very easy, so I must be overlooking something.
I need to construct a class that has a field that is also a class (non-POD). The class of the field has a default constructor and a "real" constructor. The thing is that I really can't construct the field in the initializer list, because in reality the constructor has a parameter that is a vector which needs a somewhat complex for loop to fill.
Here is a minimal example that reproduces the problem.
ConstructorsTest.h:
class SomeProperty {
public:
SomeProperty(int param1); //Ordinary constructor.
SomeProperty(); //Default constructor.
int param1;
};
class ConstructorsTest {
ConstructorsTest();
SomeProperty the_property;
};
ConstructorsTest.cpp:
#include "ConstructorsTest.h"
ConstructorsTest::ConstructorsTest() {
the_property(4);
}
SomeProperty::SomeProperty(int param1) : param1(param1) {}
SomeProperty::SomeProperty() : param1(0) {} //Default constructor, doesn't matter.
But this gives a compile error:
ConstructorsTest.cpp: In constructor 'ConstructorsTest::ConstructorsTest()':
ConstructorsTest.cpp:4:19: error: no match for call to '(SomeProperty) (int)'
the_property(4);
^
It gives no suggestions like it usually would of what functions could have been intended instead.
In the above example I would just initialize the_property in the initializer list, but in reality the 4 is actually a complex vector that needs to be generated first, so I really can't. Moving the_property(4) to the initializer list causes the compilation to succeed.
Other similar threads mention that the object must have a default constructor, or that it can't be const. Both requirements seem to have been met, here.
You can't initialize data member inside the constructor's body. (the_property(4); is just trying to invoke the_property as a functor.) You can only assign them like:
ConstructorsTest::ConstructorsTest() {
the_property = ...;
}
but in reality the 4 is actually a complex vector that needs to be generated first
You can add a member function which generate the necessary data, and use it to initialize the data member in member initializer list. e.g.
class ConstructorsTest {
...
static int generateData();
};
int ConstructorsTest::generateData() {
return ...;
}
ConstructorsTest::ConstructorsTest() : the_property(generateData()) {
}
You cannot initialize a variable twice.1 When your constructor has started, all member subobjects will have been constructed. If you do not provide a member initializer in the constructor, or a default member initializer in the class definition, then it will perform default initialization. Regardless of what form it takes, you can't construct it again.
Complex multi-statement initialization is best done via a lambda function:
ConstructorsTest::ConstructorsTest()
: the_property( []{ /* Do Complex Initialization */}() )
{
}
1: Well... you can, but not like that. And you really shouldn't for cases as simple as this.
I often encounter situations where I have to implement custom copy/move constructors. However some time later, it occurs that the class gets extended with a new member and this custom copy/move constructor is not updated, so I'm searching for a way to prevent the code from compiling without updating these methods.
Header code:
class MyClass
{
public:
MyClass(const MyClass &rhs);
// ...
private:
std::string _s;
std::unique_ptr<OtherClass> _owned;
bool _b;
};
Cpp code:
MyClass::MyClass(const MyClass &rhs)
: _s(rhs._s)
, _b(rhs._b)
{
if (rhs._owned)
_owned = rhs._owned->clone();
}
So, if I add some member to MyClass, example: std::size_t _size; than I would like a compilation error for the copy constructor.
My current solution is to add:
static_assert(sizeof(MyClass) == 32, "...");
near this implementation of the copy constructor. All of this works fine, unfortunately this only works when the size of the class increases. So if I instead add bool _b2; all compiles unfortunately.
So instead of checking the size, I would like to check the number of members. Unfortunately I haven't found this yet. Are there any suggestions?
I've already considered:
banning bool in favor of short, though it breaks all sense of intention.
replacing bool by bitset, however different values can't have different names
Put all default copyable members in separate struct, which introduces complexity
Add static const auto with number of members to the class, hoping this gets updated on adding a member
All these ideas however need code/guideline changes, so ideally I would just like to write static_assert(number_of_members<MyClass>::value == 3, "...");, any ideas?
Rule of Zero:
Classes that have custom destructors, copy/move constructors or copy/move assignment operators should deal exclusively with ownership (which follows from the Single Responsibility Principle). Other classes should not have custom destructors, copy/move constructors or copy/move assignment operators.
In this case, if you simply had a:
template <class T>
struct clone_unique_ptr {
std::unique_ptr<T> p;
clone_unique_ptr(const clone_unique_ptr& rhs)
: p(rhs.p ? rhs.p->clone() : nullptr)
{ }
// rest of special members
};
Then you wouldn't have to write anything special:
class MyClass
{
public:
MyClass(const MyClass&) = default;
private:
std::string _s;
clone_unique_ptr<OtherClass> _owned;
bool _b;
};
C++ does not have reflection built in. However, you can try using an external library xCppRefl which should provide reflection (Never used it before and it's quite old so let me know if using the library works for you).
Stepping through the source if you do
std::vector<DataMember> dataMembers = className.getDataMembers();
And then run assert(dataMembers.size() == expectedNumMembers) you should be able to test if the number of members in a class is what you expect.
This is a short question about syntax in c++:
class arrayInit {
public:
bool vars[2];
arrayInit() : vars() {} //1
};
class array {
public:
bool vars[2];
array() {} //2
};
What does 1 and 2 do?
Why they don't zero initialize like this: bool vars[2]={};?
What is the purpose of arrayInit() : and array()? and what is it called so I could search for it?
It's from: https://msujaws.wordpress.com/2010/06/16/initializing-an-array-in-c/
What does 1 and 2 do?
Both 1 and 2 define the default constructor for the respective type
Why they don't zero initialize like this: bool vars[2]={};?
They could if they were using a compiler with c++11 support. Also var() will value initialize the array which is same as vars[2] = {} will explicitly initialize all elements to false
What is the purpose of arrayInit() : and array()? and what is it
called so I could search for it?
They are called the default constructors. C++ compiler will create them for you unless you want to do something special in them. If you were mentioning about what is written beyond the : (colon), that expression is called the initializer list
Read more here
What does 1 and 2 do?
Both allow you to override the default initialization for an array.
InitArray is specifically initializing the vars array with no parameters, I believe it will assume 0 as the default parameter.
Array is not specifically initializing the array, so it is falling back to a default initialization case.
Why they don't zero initialize like this: bool vars[2]={};?
You could do this, this is just another option which encapsulates the bool array in a class to allow you to provide other functionality if you wish.
What is the purpose of arrayInit() : and array()?
If you want default functionality, there is no need to encapsulate the array in its own class. Encapsulation allows you to encapsulate a type to provide different functionality from the default, you could go on to add methods for addition, subtraction, or anything you can think up and have it perform the methods in the way that you specify.
and what is it called so I could search for it?
Good question; Encapsulation, class initialization, array initialization.
http://www.cplusplus.com/doc/tutorial/classes/
Both arrayInit() and array() are default constructors. If the default constructor is missing and other constructors are available, you cannot declare an object from that class type without calling the other constructors e.g. you cannot do this arrayInit arr; without the default constructor.
The part : vars() is called the initialization list. You can read more about them in this link: http://en.cppreference.com/w/cpp/language/initializer_list
I am a beginner in C++ and am trying to learn it by looking at examples.
Here is an example definition for a class that I don't understand the meaning completely:
class MyClass{
public:
std::string name;
void *data;
MyClass(const std::string& t_name, void *t_data) : name(t_name), data(t_data) {}
};
Here is what I understand:
name and *data are variables of the class and, MyClass(...) is the constructor. The meaning of : is the left side class is derived from the right hand side class. However, what is the meaning of this part of the code:
MyClass(const std::string& t_name, void *t_data) : name(t_name), data(t_data) {}
Here are the questions:
what are "t_data" and "t_name"? Are they initial values for "data" and "name"? what is the reason t_ is used here?
what is the meaning of : in the above line?
what is {} at the end of that line?
Thanks for the help.
TJ
what are "t_data" and "t_name"? Are they initial values for "data" and "name"?
They are the arguments passed to the constructor. If an object were created as
MyClass thing("Fred", some_pointer);
then, in the constructor, t_name has the value "Fred" and t_data has the value of some_pointer.
what is the reason t_ is used here?
Some people like to tag the arguments to give them different names to the class members, but there's no need to do that unless you want to.
what is the meaning of : in the above line?
That marks the start of the initialiser list, which initialises the class member variables. The following initialisers, name(t_name), data(t_data) initialise those members with the constructor's arguments.
what is {} at the end of that line?
That's the constructor's body, like a function body. Any code in their will be run after the members have been initialised. In this case, there's nothing else to do, so the body is empty.
It is good C++ practice to use initializer lists to initialize members.
t_name, t_data are the parameter names.
The "t_" prefix is merely used to distinguish it from the similarly named member fields.
The members are being initialized using syntax that resembles a function call, but it is a proper initialization/construction, so you should think of it that way.
The colon signals the beginning of an initializer list
The empty braces {} designate that the body of the constructor (which happens after the members are initialized) is empty.
MyClass(const std::string& t_name, void *t_data) : name(t_name), data(t_data) {}
is constructor of your class, and you should initialize your class with a string and void* parameter, then you initialize your class fields (name and data) with your passed parameters
This is a somewhat compressed example for a beginner, but you're asking good questions.
1) t_data and t_name are the parameters going into the constructor. When you create an instance of this class, you'll do something like:
MyClass * myNewClassInstance = new MyClass("My name", &myData);
Now the constructor has "My name" (in std::string form) as t_name and a pointer to myData as t_data.
2) The colon here does not refer to a derived class. Here it says that an "initializer list" is to follow. Check out Prashant's answer's link, or do a little digging about this. Basically, this is setting the member variable name to t_name (which is the std::string that we passed into the constructor in the above example), and data to t_data (the void pointer). (see additional notes below)
3) The {} is the actual definition of the constructor. It's empty -- there's no code here. So all the constructor will do is initialize the member variables (as defined by the initializer list), and nothing more.
Now let's look at the exact same class in a more beginner-friendly format:
// The declaration of MyClass. This would often show up in a header (.h) file
class MyClass {
public:
// Member variables
std::string name;
void * data;
// Declaration of constructor -- we haven't defined what actually does here, just
// what parameters it takes in.
MyClass(const std::string& t_name, void * t_data);
}
// We declared what the class "looks like" above, but we still have to define what
// the constructor does. This would usually show up in a .cpp file
// This is the constructor definition. We have to specify that it's part of the
// The extra "MyClass::" specifies that it's part of the MyClass namespace that we
// declared above.
MyClass::MyClass(const std::string& t_name, void * t_data)
{
// Set our member variables to what was passed in.
this.name = t_name;
this.data = t_data;
}
The above version does exactly the same thing (with some subtle differences between initializer lists and traditional initialization in the constructor that you probably don't care about yet -- again, see other references regarding this if you're curious).
It's pretty obvious that this takes up a lot more space, and that's exactly why the more compressed format is often used. But when you're just starting out, it makes things a lot more clear doing it this way. It's important to understand the difference between a declaration and a definition when it comes to functions (i.e. the constructor), because the example you posted does both at the same time when often they are separated.
I am a beginner in C++ and am trying to learn it by looking at examples.
For whatever reason, this is harder to do in C++ than it is in Perl, Python, Java, or even C. I don't recommend this course; instead, you might invest in a good book.
Here is what I understand:
Let's start with that. I don't think you understand as much as you claim to.
name and *data are variables of the class
No. name and data are variables of the class. Their types are std::string and void*, respectively.
MyClass(...) is the constructor.
Yes.
The meaning of : is the left side class is derived from the right hand side class.
No, : means different things in different contexts. In the context in which it is used in your sample, it indicates that an initializer list follows.
However, what is the meaning of this part of the code:
what are "t_data" and "t_name"?
They are local variables in the constructor function. Specifically, they are the passed-in parameters to the function.
Are they initial values for "data" and "name"?
That is how they are being used in this sample. Specifically, they are being passed to the data and name initializers in the initializer list.
what is the reason t_ is used here?
This is purely the convention of this particular author. I've never seen that convention before.
what is the meaning of : in the above line?
It introduces an initializer list. This construct is only used in constructor member functions. Each named member is initialized by the values listed. (Note: the values do not need to come from parameters -- they can be any legal expression, so: MyClass(int ii) : i(ii), j(cos(0) / 2), k(&global_variable) {} might be a legitmate constructor. )
what is {} at the end of that line?
The body of the constructor member function. Since all of the work of the constructor is being done in the initializer list, the body of the function is empty.
I am working on a small project in C++ that requires me to create an object of a custom class I wrote in another one of my classes. The class is called FIRFilterModule, It has a simple blank constructor.
Being of a java background, my impulse is to create it like this:
class SensorInput{
public:
FIRFilterModule firFilter;
...More Class Members...
SensorInput():firFilter(FIRFilterModule()){}
...};
However this compiles with the ever so helpful error message of "Error within this context". I'm a little lost why that doesn't work. Increasing my confusion I changed the code to this:
class SensorInput{
public:
FIRFilterModule firFilter;
...More Class Members...
SensorInput(){}
...};
It works.
Can someone help me understand why this is so?
In this particular case, running of the default constructor for a member field, you don't have to do anything. The constructor is run automatically. So you can just write
class SensorInput{
public:
FIRFilterModule firFilter;
SensorInput() { ... }
};
The member initialization list is only needed when you need to call a constructor which has arguments or initialize POD types. For example say the FIRFilterModule had a constructor which took an int. Then you would use the memeber initialization list
SensorInput() : firFilter(42) { ... }
The code you posted is correct.
Maybe you forgot to include the header where FIRFilterModule is declared.
Otherwise, everything should work.