'Class name does not name a type in C++
I just started programming in C++, and I've tried to create 2 classes where one will contain the other.
File A.h:
#ifndef _A_h
#define _A_h
class A{
public:
A(int id);
private:
int _id;
B _b; // HERE I GET A COMPILATION ERROR: B does not name a type
};
#endif
File A.cpp:
#include "A.h"
#include "B.h"
#include <cstdio>
A::A(int id): _id(id), _b(){
printf("hello\n the id is: %d\n", _id);
}
File B.h:
#ifndef _B_h
#define _B_h
class B{
public:
B();
};
#endif
File B.cpp:
#include "B.h"
#include <cstdio>
B::B(){
printf("this is hello from B\n");
}
I first compile the B class and then the A class, but then I get the error message:
A.h:9: error: ‘B’ does not name a type
How do I fix this problem?
Solution 1:[1]
error 'Class' does not name a type
Just in case someone does the same idiotic thing I did ... I was creating a small test program from scratch and I typed Class instead of class (with a small C). I didn't take any notice of the quotes in the error message and spent a little too long not understanding my problem.
My search for a solution brought me here so I guess the same could happen to someone else.
Solution 2:[2]
NOTE: Because people searching with the same keyword will land on this page, I am adding this answer which is not the cause for this compiler error in the above mentioned case.
I was facing this error when I had an enum declared in some file which had one of the elements having the same symbol as my class name.
e.g. if I declare an enum = {A, B, C} in some file which is included in another file where I declare an object of class A.
This was throwing the same compiler error message mentioning that Class A does not name a type. There was no circular dependency in my case.
So, be careful while naming classes and declaring enums (which might be visible, imported and used externally in other files) in C++.
Solution 3:[3]
You must first include B.h from A.h. B b; makes no sense until you have included B.h.
Solution 4:[4]
The problem is that you need to include B.h in your A.h file. The problem is that in the definition of A, the compiler still doesn't know what B is. You should include all the definitions of all the types you are using.
Solution 5:[5]
Include "B.h" in "A.h". That brings in the declaration of 'B' for the compiler while compiling 'A'.
The first bullet holds in the case of OP.
$3.4.1/7 -
"A name used in the definition of a class X outside of a member function body or nested class definition27) shall be declared in one of the following ways:
— before its use in class X or be a member of a base class of X (10.2), or
— if X is a nested class of class Y (9.7), before the definition of X in Y, or shall be a member of a base class of Y (this lookup applies in turn to Y’s enclosing classes, starting with the innermost enclosing class),28) or
— if X is a local class (9.8) or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or
— if X is a member of namespace N, or is a nested class of a class that is a member of N, or is a local class or a nested class within a local class of a function that is a member of N, before the definition of class X in namespace N or in one of N’s enclosing namespaces."
Solution 6:[6]
when you define the class A, in A.h, you explicitely say that the class has a member B.
You MUST include "B.h" in "A.h"
Solution 7:[7]
Aren't you missing the #include "B.h" in A.h?
Solution 8:[8]
The solution to my problem today was slightly different that the other answers here.
In my case, the problem was caused by a missing close bracket (}) at the end of one of the header files in the include chain.
Essentially, what was happening was that A was including B. Because B was missing a } somewhere in the file, the definitions in B were not correctly found in A.
At first I thought I have circular dependency and added the forward declaration B. But then it started complaining about the fact that something in B was an incomplete type. That's how I thought of double checking the files for syntax errors.
Solution 9:[9]
Try to move all includes outside namespace.
//Error
namespace U2 {
#include <Head.h>
#include <LifeDiode.h>
}
//Solution
#include <Head.h>
#include <LifeDiode.h>
namespace U2 {
}
Solution 10:[10]
Not the answer, but for me the thing was that I forgot to add the std:: before the potential type to properly use it.
Sources
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