Alirhgt, i tried to sort this one out myslef but can't. So, i have a task to build a paint program in the console, i have a set of functions dealing with the console. My task being only to connect them logically to do something useful. The problem is that everytime i #include the two files given : the .h and the .cpp file, i get the LNK2005 error that they are already defined. If i only include the header file, the functions don't do anything( i tried using one function but the console just stood there doing nothing). Can anybody tell me what i'm doing wrong? I haven't worked with C++ in a bit, so i might be doing some stupid mistake.
First off, you should never include cpp files.
Second, you might need include guards.
Format headers like this:
#ifndef FILE_H
#define FILE_H
struct foo {
int member;
};
#endif
You can read about why from here: http://en.wikipedia.org/wiki/Include_guard
Related
coming from python and am a bit tripped up on what the proper approach to this is.
I am trying to include this library in my project:
https://github.com/nothings/stb/blob/master/stb_image.h
to do so, i have to #define STB_IMAGE_IMPLEMENTATION exactly once before importing the file (as per that file's doc)
This makes sense, where I am confused is, I have CLASS.h/cpp and in .h I define functions that use typedefs from that file, so I have
#define STB_IMAGE_IMPLEMENTATION
#include <stb_image.h>
in that header file, and can't move these lines to .cpp as headers needs the defs for function def, but as soon as another file includes this header, (#ifndef wont help, i believe), that will be defined twice
I have a structure where TOP creates the CLASS above, but parent also creates OTHER, and OTHER needs to include PARENT, which includes CLASS, which triggers the issue (and prevents me from just moving the #define to PARENT) Note the actual class structure is more complex then this, but this idea seems to be a core issue, and I'm looking for the general best practice.
So, is there some way to ensure these #defines are defined before anything else, and done only once? This seems like a fundamental thing but I can't figure it out - What's the best approach?
This code is a library, and doesn't have a defined entry if that matters
Create a cpp file (or whatever extension you are using for your source files) whose sole purpose is to have
#define STB_IMAGE_IMPLEMENTATION
#include <stb_image.h>
and don't forget to include this cpp file into your project so that it is compiled and the result is linked into your program. In all other places where you need something from this library, just include the stb_image.h header as usual.
These "implementation" macros are a "trick" used by some library authors to make "installing" their library easy. The idea is that when a specific macro (chosen by the library authors) is defined before the header of that library is included, some code with the actual implementation will be added. That is why this must be in a source file instead of a header and only in one source file (otherwise you get multiple definitions for the same functions).
You should have the #define STB_IMAGE_IMPLEMENTATION macro definition in exactly one source file that includes <stb_image.h> (a .cpp file, not in a header).
Since you can also only have one source file that defines main(), it is simple to put the #define in the same file as main() (as long as it also includes <stb_image.h>), but it can be used in any other source file if you prefer. You could even create a source file stb_image_imp.cpp that contains just the two lines shown, and link that into your program (or library) too.
All other source files in the project should only include <stb_image.h> without also defining the macro.
#define is a preprocessor directive and doesn't actually get run everytime the header is accessed so you should
't have any problems
if you are using visual studio you can also do #pragma once to only parse the file once stopping anything from happening twice
//........Project for ABC.dll
//ABC.h
#pragma once
class ABC{
public:
ABC(){}
private:
std::vector<int> m_vector;
};
//ABC.cpp
#include "Stdafx.h"
#include "ABC.h"
//Stdafx.h
#include <vector>
Till today, I've skipped #include <standard-lib.h> in my headers by delegating it to Stdafx.h header.
It's never been a problem when I worked in a single project file.
Now I'm trying to add a new DLL project to gather shared codes in one project.
It compiled well and generated ABC.dll too.
Here's a problem. When another project that uses ABC.dll show compile error saying that std::vector does not exist.
//...........Another Project using ABC.dll
int main(){
ABC abc;
}
Error C2039 'vector': is not a member of 'std'
To get it working, I had to include all the libraries in the consumer's Stdafx.h too.
Maybe I've been misusing the precompiled header.
I want to know whether the way I've been doing with the PCH was wrong or right.
If it's wrong, I would appreciate it if you suggest right ways of using PCH.
Thanks.
Your problems have nothing to do with precompiled headers. A good practice is to include all the stuff directly used by current file. This will prevent changes in includes of one header file from potentially requiring changes in includes in files that are using this header. vector needs to be included in ABC.h because it is directly used there. Otherwise you'll end up with endless struggling to figure out which headers needs to be included when including this particular library header.
Consider 2 files:
func.h:
#ifndef FUNC_H_
#define FUNC_H_
#include "func2.h"
#include "OrderedList.h"
class func{
func2* pointer;
};
#endif /* FUNC_H_ */
and func2.h:
#ifndef FUNC2_H_
#define FUNC2_H_
#include "func.h"
class func2{
func* joe;
};
#endif /* FUNC2_H_ */
I have a project containing this and an empty main.cpp file. It compiles fine, although in my opinion it shouldn't: when func.h is pasted onto func2.h, then the preproccesor recognizes func2_h is already defined and it stops there. Then func doesn't know who func2 is since it comes first.
However, if I include "func.h" in main then it doesn't compile as expected. I'm guessing that compilation ignores things that aren't used? Is this an optimization in eclipse (what I'm using) or something deeper?
Yes, compilation ignores files which aren't included. Your compiler doesn't know anything about your project. All it knows about are the things which are passed to it on the command line, and typically, header files are not one of those things. If you take a look at the compilation command which Eclipse executes, you will most likely see main.cpp there, and no mention of your header files.
Read this question for a more thorough understanding of how C++ compilation works: How does the compilation/linking process work?
I'm working on a big c++ project and change code from other people. During this I got a linker error stating that reference VarA is multiple defined. I found the corresponding variable and it had been defined in a cpp file which had directly been included into the project. I tried to convert the source file in a header file and a source file, which doesn't work.
I then tried to move the variable declaration into new separate h/cpp only containing this variable, which looks then like:
h-file (aaa.h)
#ifndef AAA_H
#define AAA_H
#include "classAdefinition.h"
extern ClassA VarA;
#endif
cpp-file (aaa.cpp)
#include "aaa.h"
ClassA VarA;
If I now include aaa.h in the main file, the linker error adds the new created aaa.obj to the error message (e.g. VarA is also defined there) which is exactly what I expected. But when I remove the definition of VarA in the main file I get a linker error staying that VarA is not defined, which is really confusing.
Does anyone have a clue what might be the cause of this behavior?
I'm using VS2008, and the project is created with cmake. Might this cause the problem? E.g. could there be a configuration issue? We also use templates quite often, can this lead to the problem?
In order to remove the multiple defined error which occurs because of multiple cyclic calls I suggest the first and foremost thing that you should do is to edit your header files to include the following macro (whatever it might be in your case) give each file a unique name
#ifndef CAR_HPP
#define CAR_HPP
class Car
{
public:
....
protected:
....
};
#endif
I haven't found the error, but the problem is now avoided because most of the third-party code has been changed and the error does no longer occure.
I am creating header file for the fist time in dev c++
I have created add.h and add.cpp according to proper format. I don't know where to store them and when I am using header, it is showing many errors
Typically my headers look like this:
#ifndef ADD_H
#define ADD_H
class Add
{
...
};
#endif
and I save them in the same directory as my .cpp files.
In the implementation file:
#include "add.h"
And then in the main cpp file:
#include "add.h"
Doesn't matter where you save them, just put them in the same directory.
You include the header in your .cpp file like this:
#include "add.h"
Try googling for some beginner C++ tutorials.
The problem that it is showing many errors is that you may have written incorrect code. You can start a new question, paste the part of code which you think is cause of the error, with a little description about your code and we'll happily help you out :)