C++ inheritance error : expected class name before '{' token

Question

I have a base class in the file base.h

#ifdef _BASE_H_
#define _BASE_H_

class base {
  public:
    int base_data;
};
#endif

and a derived class

#include "base.h"

class derived: public base {
  public:
    int derived1_data;
    inline void set(int data) { derived1_data = data;}
};

When I try to compile I get this error:

error: expected class-name before ‘{’ token

I am not sure what I am doing wrong here.

You're using a [reserved identifier](http://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier), and the answers' suggestions still do as well. — chris, Dec 19 '13 at 23:13

score 4 · Answer 1 · answered Dec 19 '13 at 23:16

4

#ifdef _BASE_H_ should be #ifndef _BASE_H_.

Or simply use #pragma once :)

answered Dec 19 '13 at 23:16

Eutherpy

4,471
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score 3 · Answer 2 · answered Dec 19 '13 at 23:12

3

Change

#ifdef _BASE_H_

to

#ifndef _BASE_H_

answered Dec 19 '13 at 23:12

Matt

20,108
1
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score 2 · Answer 3 · answered Dec 19 '13 at 23:13

2

Change in the header

#ifdef _BASE_H_

to

#ifndef _BASE_H_

answered Dec 19 '13 at 23:13

Vlad from Moscow

301,070
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score 2 · Accepted Answer · answered Dec 19 '13 at 23:53

The problem is with the first line of your header file #ifdef _BASE_H_. It should be #ifndef _BASE_H_ (notice the extra 'n')

Here is a brief explanation of what is happening:

#ifdef MACRO_NAME
  <line 1>
  <line 2>
  ...
#endif

This code tells the C++ preprocessor to only include the line1, line2, ... in the source code if a macro MACRO_NAME has been defined prior to this point (Note: It's just the definition of the macro which matters, not it's value. If you want to predicate code inclusion on macro value, you may use #if)

The technique you are trying to use here is called Include Guard, and it safeguards against the duplicate inclusion of code (mostly by #include preprocessor directive). The idea is include source code of a header file only if some macro (say "M") has not been defined yet, and then inside the header file code the first thing you do is define that macro "M" (so that next call to #include samefile will not include its code again). For creating a Include guard you need to use the #ifndef preprocessor directive (which is the exact opposite of #ifdef, in fact, the extra n stands for "NOT").

Since you incorrectly used #ifdef, the file base.h was never included (as the only place you define the macro _BASE_H_ is after checking that it is already defined!). Thus the compiler does not know about the class base, leading to the error expected class-name before ‘{’ token

score 1 · Answer 5 · answered Dec 19 '13 at 23:25

Despite from the fact that the name of the macro should not start with an underscore, you have a mistake in the "header guard"

#ifdef _BASE_H_

This will only compile the stuff up to #endif if the macro was already defined, the opposite of what you want to do. That's why you should write

#ifndef _BASE_H_

which essentially ignores the stuff up to #endif if the macro was already defined, which is the case if the file was already be included.

But your compiler can't detect that this is the real problem. Instead, it silently ignores the contents of the header file base.h, which means that the base class will never be defined (it will not find the source code!). So when reaching the line

class derived : public base {

the compiler doesn't know the type base, which means you can't derive from it, hence the error message that a class name was expected.

C++ inheritance error : expected class name before '{' token

5 Answers5