Create and use a header in C++ - c++

I've got three files, add.h, add.cpp and test.cpp
add.h creates a header for a class, and is nothing but skeletons for implementation later.
add.cpp contains the actual code for the class listed in add.h
test.cpp contains the main method and declares an instance of the class in add and uses some of its methods.
However, I'm having trouble compiling it. In add.h, I have #DEFINES to prevent multiple writes of the header, and in add.cpp and test.cpp I have add.h included, but when I attempt to compile using the line
g++ test.cpp -o test
I get an error about undefined references to the class objects and methods in add.h. I've been searching google on how to compile or run this, but so far no help, can StackOverflow help me?
EDIT: Sorry, I should have also included that I did try g++ test.cpp add.cpp -o test and it didn't work either, yielding the same resulting errors.

Compile each file separately, then link:
g++ -Wall -c test.cpp
g++ -Wall -c add.cpp
g++ -o test test.o add.o
Or compile and link all files in one command:
g++ -Wall -o test test.cpp add.cpp

run g++ test.cpp add.cpp -o test
EDIT: copypasted my comment here
You need to understand why your initial approach isn't working. When you reference stuff from add.h header in test.cpp, the compiler looks for definitions, but does not find them, because they are in add.cpp and you did not pass it to the compiler. The compiler can't just guess that it should look for the definitions in the add.cpp file just because you included add.h in test.cpp.

run g++ test.cpp add.cpp -o test
or
g++ -c add.cpp -o add.o
g++ -c test.cpp -o test.o
g++ test.o add.o -o test
the -c flag tells gcc to just compile and not link
the first two steps compile 1 cpp (a compilation unit) in an object file
the last step links those into a single executable
your actual problem comes from the fact that when you compile test.cpp, it refers to some simbols which are undefined.
If you're just compiling (-c flag) that's fine, and the next step is to link with those objects file containing the missing symbols.

You need
g++ test.cpp app.cpp -o myTest
as app.cpp contains code used by test.cpp.

Related

How to compile a project with clases in Clang?

Seen this post:
How do you compile a C++ program with multiple class files from OS X Terminal?
I see that to compile with gcc a project with a class (with .h and .cpp) is easy as:
g++ [list of all source files] -o [executableName]
And it actually works for me, but when i try to use Clang:
clang++-11 main.cpp Person.hpp Person.cpp -o main
I get:
clang: error: cannot specify -o when generating multiple output files
NOTE: i know how to use Makefile's, but i would like to have a quick way to compile in Clang like in gcc.
If you run clang++-11 main.cpp Person.hpp Person.cpp -o main with the header file, clang notices this and compiles the pre-compiled header "Person.pch", the default -o Person.pch is implied. Meanwhile you want getting yet another output file "main". Thus, clang complains it can't generate multiple output files, -o Person.pch and -o main.
Pre-compiled headers should be compiled separately.
clang++-11 Person.hpp
clang++-11 main.cpp Person.cpp -o main
The first step is not required, if skipped, no pre-compiled headers are used.

How to use libraries and headers in C++ with MinGW?

I want to use OpenGL GLEW library. I have the binary downloaded and its folder is in the folder with my .cpp file. My .cpp file uses #include <eglew.h>.
How should I format my command for MinGW to compile my .cpp file? Do I compile with the .lib file like g++ -L./path/to/lib/file.lib test.cpp -o test or do I do something else like link to the header files g++ -I./path/to/headers test.cpp -o test?
To better understand things maybe it's better to split compiling and linking steps.
If you get errors then you will also know in which step the problem occurs.
I'm assuming you have the following folders/files:
/path/to/eglew/include/GL/eglew.h
/path/to/eglew/lib/libglew32.a
Compiling:
g++ -Wall -c -o test.o test.cpp -I/path/to/eglew/include/GL
Linking:
g++ -o test.exe test.o -L/path/to/eglew/lib -lglew32
Though I would expect to see #include <GL/eglew.h> in which case the linker include flag should be -I/path/to/eglew/include.

How do I link multiple files without the use of a makefile?

For example, I'm given carModels.cpp, carModels.h, carType.in, manufacturers.h, manufacturers.o, and lastly my own file tester.cpp. How would I go about linking all of these using g++ in a Linux terminal? Would I have to create any additional ".o" files? I'm supposed to assume that the given files already work. Multiple lines in terminal are fine, I just I want a clear understanding of it. (I'm coming from a C++ IDE that I didn't really care for.)
Compile each source file to its own object file:
g++ -I . -c carModels.cpp -o carModels.o
g++ -I . -c tester.cpp -o tester.o
Now link all object files together:
g++ carModels.o tester.o manufacturers.o -o outputname
Consider adding more options like -O3, -std=c++11, -Wall, etc. as needed.
you can do this in two steps, first compile to *.o files,
gcc -c your.cpp other.cpp .....
then link them
gcc -o you_out_put_name the_object_files.o ...
In a single line, that would be just g++ -o tester *.cpp *.o. GCC will sort everything out. In particular, the *.h files are referenced via #include "" statements in the .cpp files.

C/C++ duplicate symbols error

I'm currently trying to compile an existing project which is a mix of cpp and c.
My makefile is as follows;
execute: mgnsvd.o multifit.o cit.o main.o
g++ -Wall -g multifit.o cit.o main.o mgnsvd.o -lgsl -lgslcblas -o execute
main.o: main.cpp cit.cpp
g++ -Wall -g -c main.cpp
cit.o: cit.cpp mgnsvd.c multifit.h
g++ -Wall -g -c cit.cpp
multifit.o: multifit.c mgnsvd.c multifit.h
g++ -Wall -g -c multifit.c
mgnsvd.o: mgnsvd.c
g++ -Wall -g -c mgnsvd.c
And my main is a rather plain
// global libraries
#include <iostream>
// local files
#include "cit.cpp"
// using directives
using std::endl;
using std::cout;
// main
int main(){
cout << "Hello, world" << endl;
return 0;
}
If is comment out #include "cit.cpp" it compiles fine. However, if i include it the following error happens;
ld: duplicate symbol _first_function_in_cit in main.o and cit.o for architecture x86_64
_first_function is always the first function, and is not defined or even declared/used anywhere else. A grep of the object files confirms the function seems to be incorporated into main.o, but why? I've not worked on a C++/C project before, so maybe I'm making some canonical error? I'm also in OSX, if that makes a difference.
For the record, there are no classes - the .h file contains a few structs and some macros.
cit.cpp is being compiled by itself, and is also included in main.cpp so you're getting two copies of all the code in it.
There is no need for
#include "cit.cpp"
cit.cpp is compiled as a separate unit and later linked.
With the above include you get the code twice with results in duplicate symbols
YOu are compiling cit.cpp to yield cit.o, and you are compiling it again with that #include "cit.cpp" attrocity in your main.cpp. So of course you are getting duplicate symbols.

C++ programs, compiling with g++

I am very aware of compiling C++ programs with g++ in linux environment. But, may be I am missing something, I am getting this strange output/behaviour.
I have source file in test.cpp.
To compile this, I did
(1)
g++ -c test.cpp
g++ -o test test.o
./test
Everything works fine.
But when I did compling and linking in same stage, like this
(2)
g++ test.cpp -o test
./test => Works fine
(3)
g++ -c test.cpp -o test => Doesn't work
In my last case, test is generated but is no more executable; but in my guess it should work fine.
So, what is wrong or do I need to change some settings/configuration ??
I am using g++ 4.3.3
Thanks.
When you say:
g++ -c test.cpp -o test
The -c flag inhibits linking, so no executable is produced - you are renaming the .o file.
Basically, don't do that.
You are forcing compiler to produce an object file and name it like an executable.
Essentially your last line tells: compile this to an object file, but name it test, instead of test.obj.
-c flag means Compile Only
Try
g++ -o test test.cpp
Specifying -o in the g++ command line tells the compiler what name to give the output file. When you tried to do it all in one line, you just told the compiler to compile test.cpp as an object file named test, and no linking was done.
Have a look at the fabulous online manual for GCC for more details.
from the gcc manual:
-c Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
You must link the compiled object files to get the executable file.
More info about compiling and linking and stuff is here.
Read man g++. The switch -c is to compile only but not to link.
g++ -c test.cpp -o test
does what
g++ -c test.cpp
does but the object file will be test istead of the default name test.o. An object file cannot be executed.