This question already has answers here:
error: request for member '..' in '..' which is of non-class type
(9 answers)
Closed 4 years ago.
I was recently giving an online quiz on c++ and a question came that had similar syntax as
class className
{
public:
constructor()
{
print("ABC");
}
}
int main()
{
className ABC();
return 0;
}
I thought it would not compile but rather it compiled and ran without having any effect, I am interested as to what feature this is and in which case do we use this?
Your posted code won't compile. Ignoring that there are 2 reasons it doesn't do anything.
className ABC(); declares a function. To call the default constructor of a class simply omit the brackets: className ABC;
className has a method called constructor but no contructor so doesn't print anything when it is constructed.
Related
This question already has answers here:
Why can't member initializers use parentheses?
(2 answers)
Initialization of member variable via parentheses doesn't work [duplicate]
(1 answer)
can not define and init a class member by Parentheses [duplicate]
(1 answer)
Closed 9 months ago.
I'm trying to neatly create a objects within a class and I've run into what seems to me to be an odd limitation in c++, and I wondered if I'm just missing a syntax trick or whether it's truly impossible; specifically it seems like I cannot explicitly instantiate a class object inside another class if the constructor of the former has parameters (the parameters are constant) generating the odd message: 'Expected parameter declarator'
It seems as though only default constructors are supported in this scenario, or am I missing a bit of magic?
Currently using c++17 (simply because that's the default in this IDE)
class Fred
{
public:
Fred(const int i)
{
}
Fred()
{
}
};
Fred fred1(0); // This compiles
class Charlie
{
Fred fred2(); // This compiles
Fred fred3(0); // This does not compile
};
This question already has answers here:
Default constructor with empty brackets
(9 answers)
Closed 4 years ago.
This code doesn't behave how I expect it to.
#include<iostream>
using namespace std;
class Class
{
Class()
{
cout<<"default constructor called";
}
~Class()
{
cout<<"destrutor called";
}
};
int main()
{
Class object();
}
I expected the output 'default constructor called', but I did not see anything as the output. What is the problem?
Nope. Your line Class object(); Declared a function. What you want to write is Class object;
Try it out.
You may also be interested in the most vexing parse (as others have noted). A great example is in Effective STL Item 6 on page 33. (In 12th printing, September 2009.) Specifically the example at the top of page 35 is what you did, and it explains why the parser handles it as a function declaration.
No call to constructor
Because the constructor never gets called actually.
Class object(); is interpreted as the declaration of a function object taking no argument and returning an object of Class [by value]
Try Class object;
EDIT:
As Mike noticed this is not exactly the same code as what you are feeding to the compiler. Is the constructor/destructor public or is Class a struct?
However google for C++ most vexing parse.
You can use it like this:
Class obj;
//or
Class *obj = new Class(/*constructor arguments*/);
This question already has answers here:
Default constructor with empty brackets
(9 answers)
Closed 4 years ago.
This code doesn't behave how I expect it to.
#include<iostream>
using namespace std;
class Class
{
Class()
{
cout<<"default constructor called";
}
~Class()
{
cout<<"destrutor called";
}
};
int main()
{
Class object();
}
I expected the output 'default constructor called', but I did not see anything as the output. What is the problem?
Nope. Your line Class object(); Declared a function. What you want to write is Class object;
Try it out.
You may also be interested in the most vexing parse (as others have noted). A great example is in Effective STL Item 6 on page 33. (In 12th printing, September 2009.) Specifically the example at the top of page 35 is what you did, and it explains why the parser handles it as a function declaration.
No call to constructor
Because the constructor never gets called actually.
Class object(); is interpreted as the declaration of a function object taking no argument and returning an object of Class [by value]
Try Class object;
EDIT:
As Mike noticed this is not exactly the same code as what you are feeding to the compiler. Is the constructor/destructor public or is Class a struct?
However google for C++ most vexing parse.
You can use it like this:
Class obj;
//or
Class *obj = new Class(/*constructor arguments*/);
This question already has answers here:
How come a non-const reference cannot bind to a temporary object?
(11 answers)
Closed 7 years ago.
Let's say I have a struct :
struct structazauras
{
string bla;
}
And some function (in my case this function is actually a constructor of some class but I don't think this is the issue):
void myfunc(structazauras& mystruct) { }
then some where i call myfunc :
..
myfunc(structazauras());
..
I get an error:
no matching function for call to myfunc(structazauras) candidates are myfunc(structazauras&)
If I change the code to :
structazauras tmp;
myfunc(tmp);
It will work fine.
I feel that he (the compiler) has a problem passing a reference to the anonymous instance if structazauras, but why ? the anonymous instance exist on the stack of the calling function.
That is because you cannot bind a temporary to a non-const reference. Mark your reference as const and it will work.
Also, you are using a standard C++ keyword (struct) in the definition
void myfunc(structazauras& struct) { }
Change the name to something else. Or maybe you meant
void myfunc(struct structazauras&) { }
but the additional struct is superfluous in C++.
This question already has answers here:
Default constructor with empty brackets
(9 answers)
Closed 4 years ago.
This code doesn't behave how I expect it to.
#include<iostream>
using namespace std;
class Class
{
Class()
{
cout<<"default constructor called";
}
~Class()
{
cout<<"destrutor called";
}
};
int main()
{
Class object();
}
I expected the output 'default constructor called', but I did not see anything as the output. What is the problem?
Nope. Your line Class object(); Declared a function. What you want to write is Class object;
Try it out.
You may also be interested in the most vexing parse (as others have noted). A great example is in Effective STL Item 6 on page 33. (In 12th printing, September 2009.) Specifically the example at the top of page 35 is what you did, and it explains why the parser handles it as a function declaration.
No call to constructor
Because the constructor never gets called actually.
Class object(); is interpreted as the declaration of a function object taking no argument and returning an object of Class [by value]
Try Class object;
EDIT:
As Mike noticed this is not exactly the same code as what you are feeding to the compiler. Is the constructor/destructor public or is Class a struct?
However google for C++ most vexing parse.
You can use it like this:
Class obj;
//or
Class *obj = new Class(/*constructor arguments*/);