g ++编译错误基本对象构造

Question

I'm receiving the error:

thing.cpp:5:1: error: ‘SquareThing’ does not name a type
 SquareThing::SquareThing()
 ^
compilation terminated due to -Wfatal-errors.
make: *** [thing.o] Error 1

my thing.h file:

#define THING_H_
#ifndef THING_H_
#include <vector>
#include <iostream>
class SquareThing{

public:
        SquareThing();
        //other functions
private:
    int something;
    //more members

};
#endif

Any my thing.cpp:

#include "thing.h"
#include <vector>
#include <iostream>
using namespace std;
SquareThing::SquareThing()
{
     something = 3;
}
//more functions below

This seems too rudimentary but I really can't seem to find the error. Any help is greatly appreciated.

Answer 1

These two statements are backward:

#define THING_H_
#ifndef THING_H_

You most certainly meant:

#ifndef THING_H_
#define THING_H_

In the backward order, you were ensuring that the body of the header never kicked in, which isn't terribly useful.

Answer 2

You need to switch the order of your macros:

#define THING_H_ // define the macro
#ifndef THING_H_ // if the macro is not defined (well, it is always defined...)

should be

#ifndef THING_H_ // if the macro hasn't been defined ...
#define THING_H_ // ... define it