error: Class has not been declared despite header inclusion, and the code compiling fine elsewhere

You seem to be saying that the code you are showing doesn't actually produce the compiler error that you are having a problem with. So we can only guess. Here are some possibilities:

You could have forgot to include problemclass.h from the file where you are using ProblemClass.

You could have misspelled the name of ProblemClass either in its own header file or in the place where you are using it. This can be hard to spot if it is a capitalization error such as writing Problemclass or problemClass instead of ProblemClass.

You could have copy-pasted your inclusion guard #defines from one header file to another and then forgot to change the defined names. Then only the first of those two included header files would take effect.

You could have placed ProblemClass in a namespace A, in which case you must refer to ProblemClass as A::ProblemClass if you are referring to it from outside the namespace A.

You may be using templates and not expecting two-phase lookup to work the way it does.

You could have misspelled the file name in your include. The compiler would not report an error on that if you also have an old version of that file under the misspelled name.

You could have made ProblemClass a macro that only gets defined after you include problemclass.h, in which case what you see as ProblemClass gets replaced by something else by the macro preprocessor.

You could have defined ProblemClass in a header file other than problemclass.h and then problemclass.h actually defines something else.

I've had that same error message as a result of a circular dependency in my header files / classes:

foo.hpp:

#ifndef FOO_HPP
#define FOO_HPP

#include <stdio.h>
#include "bar.hpp" // <-- here

class Foo {
public:
    int value = 0;

    void do_foo(Bar myBar) {
        printf("foo + %d\n", myBar.value);
    }
};

#endif //FOO_HPP

bar.hpp:

#ifndef BAR_HPP
#define BAR_HPP

#include <stdio.h>
#include "foo.hpp" // <-- and here

class Bar {
public: 
    int value = 1;      

    void do_bar(Foo myFoo) {
        printf("bar = %d \n", myFoo.value);
    }
};

#endif //BAR_HPP

Compiling with: g++ -std=c++11 foo.hpp -o foo resulted in the following output:

In file included from foo.hpp:5:0:
bar.hpp:11:15: error: ‘Foo’ has not been declared
bar.hpp: In member function ‘void Bar::do_bar(int)’:
bar.hpp:12:32: error: request for member ‘value’ in ‘myFoo’, which is of non-class type ‘int’

Please post the command you are using for compilation. I've seen this issue if you have 2 separate files that include the same header and you are doing a gcc *.cpp. This happens because the #define gets defined for the entire gcc instance and not just for each individual object file being compiled.

Ex.

File1

#ifndef FILE1_HPP
#define FILE1_HPP 1
....
#endif

Then two separate files that reference it.

#include <file1.hpp>

Trying to compile all at the same time will cause one of the cpp files to fail since FILE1_HPP was already defined (causing the header file to be ignored for that cpp file).

gcc -Wall *.cpp

Answer is either remove the #ifndef, or to compile each file into its own object files and then link them into your main application.