My project has this folder structure:
Project/
--Classes/
----Class1.h
----Class1.cpp
--main.cpp
"Class1.h" contains method definitions, "Class1.cpp" is the source code for "Class1.h".
The source code of "Class1.h" is like this:
class Class1 {
public:
void do_something();
};
The source code of "Class1.cpp" is like this:
#include "Class1.h"
void Class1::do_something() {
//
}
The source code of "main.cpp" is like this:
#include "Classes/Class1.h"
int main(int argc,char** args) {
Class1* var = new Class1();
var->do_something();
return 0;
}
However, when compiling "main.cpp", the compiler doesn't know where the implementation of methods in Class1 is, so it shows linking error about undefined reference.
Do I have to add any path into the command line so the compiler knows what source files it has to compile? How to tell the compiler that it must compile "Class1.cpp" also?
You need to feed all files in your project to the compiler, not just "main.cpp". Here you can read about the basics of compiling multiply files together with Gcc.
Another option would be to compile your classes as a dynamic or static library, but you should start with simply compiling them together if you're not quite familiar with libraries.
You need to know about building (compiling and linking) C++ applications. This topic usually don't describe in programming books about C++ and only way to do it - google and programming community sites with articles.
Fast answer is:
g++ -c Classes/Class1.cpp -o Class1.o
g++ -c main.cpp -o main.o
g++ Class1.0 main.0 -o ProjectName
It's a simple set of commands to compiling and linking program. Usually it would be done by build system (make, qmake, cmake, waf, scons, ant etc). Also, IDE can build program without additional configuration, Visual Studio for example.
The correct way to do it is doing the header inclusion in the Class1.cpp file. That way if the Class1.cpp is compiled as a library, you can use the header file to get the declarations.
The other way around, if you will directly use the Class1.cpp, compiling it with your project. You should include Class1.cpp in your main.cpp.
Related
I am a C++ learner and I came across a concept of separating code into multiple files to speed up compiling process on bigger projects.
However what the book doesn't tell me and I have tried to find it in other books and on the web, but with no luck is how does linker (during compilation) knows what files to include.
When I make new file I connect its header to the main file with #include "newfile.h", however in this header I don't write where to find definitions of functions.
So how I imagine the process is that it starts in the main file and there it finds "connections" to other files. The question is how it finds those .cpp files that as far as I see don't need to be named the same as its header file.
Exmple:
Main file:
#include <iostream>
#include "krneki_H.h"
using namespace std;
int main()
{
krneki1();
krneki2();
krneki3();
}
And header file:
void krneki1();
void krneki2();
void krneki3();
And new .cpp file:
#include <iostream>
#include "krneki_H.h"
using namespace std;
void krneki1() {
cout<<"Krneki1"<<endl;}
void krneki2() {
cout<<"Krneki2"<<endl;}
void krneki3() {
cout<<"Krneki3"<<endl;}
Notice that there is no indication to use second cpp file. It just knows to use it. Does it search all .cpp files in the map?
Thank you for answer.
You compile both .cpp files using gcc -c or a similar command line, and then pass both of the .o files produced to the linker. The linker does not magically figure out that you want to compile another .cpp file.
For example
gcc -c main.cpp -o main.o # compile
gcc -c krneki_H.cpp -o krneki_H.o # compile
gcc main.o krneki_H.o -o main # link
Your IDE may take care of these details automatically, in which case it compiles all .cpp files you added to your project, then links all the .o files produced by the compilation step.
No. It doesn't just know to use it. In your example, if you compile with main.cpp and without that new cpp file, you'll get an "undefined reference" error.
What happens is that you're using a fancy IDE that automatically compiles all cpp files. It includes them all by default in your makefile.
I recommend that you try to design a makefile from scratch, and see for yourself how you'll get that "undefined reference" error if you don't include the cpp file that has the implementations of your functions.
This question already has an answer here:
"undefined reference to" errors when linking static C library with C++ code
(1 answer)
Closed 6 years ago.
I realize this question has been asked in a number of ways including this very comprehensive answer but I have looked at many and tried fixing my error to no avail.
I am using .cpp and .c files to create a program. I compiled all files with g++, it seemed to have no more linking errors, though it gave me a bunch of C++ errors related to the C syntax. This was the command I used:
g++ -o program main.cpp /data/*.c -l[...libs]
The main.cpp calls functions in the .c files.
I then understood that one should not try to compile both .c and .cpp files with one compiler, instead to use gcc for one, g++ for the other, and thereafter to simply link the object files.
So I did this (the .c files are part of a library, and already have .o files)
g++ -c main.cpp
g++ -o program main.o /data/*.o -l[..libs]
But then here I will get "undefined reference to" errors for functions called from main.cpp to the precompiled .c files, errors which I didn't get previously.
Could anyone help? Or do I maybe need to provide more information?
EDIT (a more in depth excerpt of code, I've tried to simplify otherwise this will be impossible to read, but let me know if I still need to add stuff, I'll try to remove unnecessary code):
main.cpp :
#include "header.h"
int main(int argc, char** argv) {
string s1 = argv[2];
fn1(s1)
}
header.h
void fn1(string s1)
mycfile.c
#include "header.h"
void fn1(string s1){
fprintf(stdout, " you entered %s", s1);
fflush(stdout);
}
ANSWER:
#Smeehey helped me figure out the solution, it was actually that I was still including the old headers in an .hpp file I was using. But the core solution was indeed to use the external C{}.
This is highly likely to do with C-vs-C++ linkage. In your main.cpp, you probably have something like this:
#include <data/header.h>
where header.h refers to your c library. Replace it as follows:
extern "C" {
#include <data/header.h>
}
This tells your c++ compiler not to use c++-style name mangling when defining the required symbols from the header, allowing the linker to successfully find them in the c-compiled .o files.
You have to compile C files with the gcc command, C++ files with either gcc or g++, and link with the g++ command. This sequence will probably work:
gcc -c data/*.c main.cpp
g++ -o program *.o -l <libs...>
Next step: learn to write proper makefiles.
This is a shot in the dark but the problem might be the way C and
Cpp files are compiled is simillar yet slightly different....
Due to name spaces the function foo would generate the symbol some_prefix#foo
Unlike C whereas goo generates the symbol goo
Try doing the following in your .cpp files
extern "C"
{
#include "yourcfilesheader.h"
}
And please attach your code
I have a main file main.cpp which include myclass.h. I also have myclass.o and myclass.cpp. The visual studio project is only using the files main.cpp and myclass.h.
This compile:
#include "myclass.h"
int main()
{
return 0;
}
This does not:
#include "myclass.h"
int main()
{
myclass M{};
return 0;
}
which gives a link error - probably because the myclass.cpp or myclass.o is not included in the project.
I would prefer to include only myclass.o since compiling this class is very problematic.
See
https://forum.qt.io/topic/8492/solved-cannot-get-quazip-test-to-work/3
if you are curious of which library I am using.
To my question:
Can I compile and use the class with only the header and the object file, i.e without the .cpp file at all?
Thanks!
Visual Studio generates a .obj file as its intermediate format. gcc and gcc are the ones that produce a .o file. You can include this in your project or, since libraries rarely have a single compile unit, you can build a .lib file and include that. Be warned, though, that the compiled units must be compiled using the exact same compiler switches such as run time library and etc. as you are using in you project. It is often simpler to simply compile the file from source.
We can also talk about using a DLL but that seems to be beyond the scope of your question and also introduces the complication of calling conventions.
First of all, I am a beginner programmer so, please give simple answers that are understandable.
I am suing Code::Blocks and I am trying to make a multiple file project in C++. I am using SDL2. My problem is that when I put all my codes and functions altogether, they run and when I seperate them, they don't. This is because the execution file requires object files to be built which it don't get. It don't get them because compiler don't form them. In other words, there are problems with linking. The compiler simply, said that there are not ".o" files. These files are in the project as "Link Files" and not "Compile Files". And when I make them "Compile Files" as well as "Link Files", they produces errors that various variables inside the file where I mentioned them, are not declared. But when I include that file in the file using the variables, it gets deeply nested.
Following are the two ways:
First way which is not working, with seperate files.
In main.cpp:
#include <SDL.h>
#include <other.h>
int var;
int linkVar;
int link1Var;
int link2Var;
#include "link.cpp"
#include "link1.cpp"
#include "link2.cpp"
int main( int argc, char* args[] )
{
linkFunc();
link1Func();
link2Func();
}
In link.cpp/ link1.cpp/ link2.cpp:
void linkFunc()/void link1Func()/void link2Func() //Just a reference
{
//Code associated with var, link1Var, link2Var and link3Var.
}
Second way which is working, with all functions in a single file.
First thing is that I don't want to learn makefile thing. If there is any other way to solve it then be it!
Code::Blocks makes object files for each file that it compiles as separate compilation units. It will compile and link together any files that are part of the project which it identifies as a source file.
First, remove your #includes of the cpp files. Then try the Project > Add files... menu option to add those other cpp files to the project.
First of all, you should not use #include on other cpp files, you can read about it here:
include cpp
Secondly when you are trying to compile a project composed of separate files you should use the g++ command like this: g++ main.cpp link.cpp link2.cpp etc'
you can read a basic explanation about it here: use the g++ command
To do exactly what you asked you should do:
# get rid of including any cpp files
# use forward declerations on any function belonging in other cpp files, before your main function like this:
void linkfunc();
void linkfunc2();
etc
.
.
.
int main()
{
}
# specify the direct path to your .h files like this:
#include <C:/MinGW/SDL2/SDL.h>
# use this command:
g++ -I C:/MinGW/SDL2/ main.cpp link.cpp link2.cpp link3.cpp
The problem I am having is that, when I call a constructor for a class I have created I get the following error.
main.cpp:20: undefined reference to
`StaticObject::StaticObject(Graphics*, sf::String,
sf::Vector2)'
This problem can be 'fixed' adding an include for the .cpp file in main.cpp like so.
...
#include "GameObjects/StaticObject.cpp"
...
Although this solves the problem, this seems like a poor solution and goes against what I have been previously told. Is there any other way to solve this problem? I'm using Netbeans 7.3 with g++ to code/compile this program. Below is the relevant code.
main.cpp
...
#include <SFML/Graphics.hpp>
#include "Graphics/Graphics.hpp"
#include "GameObjects/StaticObject.hpp"
int main(int argc, char** argv) {
//SETUP
Graphics graphics;
background = new StaticObject(&graphics, "Data/Images/BackgroundPlaceholder.png", sf::Vector2f(0,0));
...
main.hpp
...
#include <SFML/Graphics.hpp>
#include "GameObjects/StaticObject.hpp"
...
// Objects
StaticObject *background;
...
StaticObject.hpp
#include <SFML/Graphics.hpp>
#include "../Graphics/Graphics.hpp"
class StaticObject{
public:
StaticObject();
StaticObject(Graphics *_graphics, sf::String texture_filename, sf::Vector2f _position);
StaticObject(const StaticObject& orig);
virtual ~StaticObject();
private:
// The sprite stores the position
sf::Sprite *sprite;
sf::Texture *texture;
};
StaticObject.cpp
#include "StaticObject.hpp"
#include <SFML/Graphics.hpp>
#include "../Graphics/Graphics.hpp"
StaticObject::StaticObject(){
}
StaticObject::StaticObject(Graphics *_graphics, sf::String texture_filename, sf::Vector2f _position) {
sprite = _graphics->AddSprite(texture_filename);
sprite->setPosition(_position);
}
StaticObject::StaticObject(const StaticObject& orig) {
}
StaticObject::~StaticObject() {
}
If I add the following line to main.cpp, the error disappears.
#include "GameObject/StaticObject.cpp"
Can anyone please explain:
Why this fixes the problem?
Why the .cpp was not implicitly included through including the .hpp
file?
Is there a better way of doing this?
The undefined reference error indicates that the definition of a function/method (i.e constructor here) was not found by the linker.
StaticObject::StaticObject(Graphics*, sf::String, sf::Vector2<float>)
And the reason that adding the following line:
#include "GameObject/StaticObject.cpp"
fixes the issue, is it brings in the implementation as part of the main.cpp whereas your actual implementation is in StaticObject.cpp. This is an incorrect way to fix this problem.
I haven't used Netbeans much, but there should be an option to add all the .cpp files into a single project, so that Netbeans takes care of linking all the .o files into a single executable.
If StaticObject.cpp is built into a library of its own (I highly doubt that is the case here), then you might have to specify the path to the location of this library, so that the linker can find the implementation.
This is what ideally happens when you build your program:
Compile: StaticObject.cpp -> StaticObject.o
Compile: main.cpp -> main.o
Link: StaticObject.o, main.o -> main_program
Although there are ways in gcc/g++ to skip all the intermediate .o file generations and directly generate the main_program, if you specify all the source files (and any libraries) in the same command line.
The linker cannot find the definition for StaticObject, which means you have not compiled and linked StaticObject.cpp. Each .cpp file needs to be separately compiled, and the object files the compiler produces for each one needs to be given to the linker.
The reason including StaticObject.cpp in Main.cpp works is that you are telling the preprocessor to insert the contents of StaticObject.cpp into Main.cpp, which you are compiling, so the definitions become part of Main.cpp's compiled output.
including the cpp file causes the compiler to build that code as part of that file (which you should not do), what you need to do is compile the GameObject/StaticObject.cpp file as its own object, and link the 2 objects together.
I dont know much of netbeans, but probably GameObject/StaticObject.cpp is not included as part of the sources to compile.
That's why if you include it manually at main.cpp he found source code and everything is ok.
I had the same issue but mine was because I'm using Eclipse CDT with and Autotools project. So I had to manually add the new .h and .cpp files to the corresponding Makefile.am, and then do a project > reconfigure project, rebuild, an that was it.
I had the same issue in Qt, where I forgot to add the class to the pro file. Apparently, without adding it to the pro file, the compiler doesn't recognize it. So I added it in, and then ran qmake again, and that silved the problem.
Basically, you need to add the file in the sources.