I'm trying to add some utility functions and global variables to my code in such a way that I can be able to use them in every class I want in my project. I would like to use a .hpp file for the definitions end a .cpp file for the implementation.
This is a summary of these two files:
// This is Utilities.hpp
#ifndef utilities_hpp
#define utilities_hpp
namespace utils {
int global_variable1;
int global_variable2;
void utility_function1(...);
void utility_function2(...);
void utility_function3(...);
}
#endif /* utilities_hpp */
and the implementation:
// This is Utilities.cpp
#include "Utilities.hpp"
namespace utils {
void utility_function1(...) {
// Some code
}
void utility_function2(...) {
// Some code
}
void utility_function3(...) {
// Some code
}
}
Other than my main.cpp file I have two other classes. My main.cpp file includes Class1.hpp header that includes Class2.hpp header.
Now I thought that I could put another #include "Utilities.hpp" in Class1.hpp or Class2.hpp without any problems since I've used the guards in that header. The thing is that when I try to do that the linker throws me this error: Apple Mach-O Linker (ld) Error Group - clang: error: linker command failed with exit code 1 (use -v to see invocation) and I can't understand why or what to do to solve it.
I'm using Xcode 8.3 on a macOS Sierra 10.12.4.
I hope I was able to explain my problem, thank you all very much in advance.
You've violated the One Definition Rule. global_variable1 and global_variable2 should be declared extern in your header and defined in exactly one translation unit (probably Utilities.cpp).
You've defined global variables in a header that gets included in multiple translation units, so there's a utils::global_variable1 defined in main.cpp, and one in Utilities.cpp. When it comes to link time, the linker has no way to know which global_variable1 to use, so it throws an error.
To fix it, add the extern keyword to your declarations and add a definition in "Utilities.cpp":
Utilities.hpp:
// This is Utilities.hpp
#ifndef utilities_hpp
#define utilities_hpp
namespace utils {
extern int global_variable1;
//^^^^^^ <-----HERE
extern int global_variable2;
//^^^^^^ <-----HERE
void utility_function1(...);
void utility_function2(...);
void utility_function3(...);
}
#endif /* utilities_hpp */
Utilities.cpp:
// This is Utilities.cpp
#include "Utilities.hpp"
namespace utils {
int global_variable1; //<---- Definitions added
int global_variable2;
void utility_function1(...) {
// Some code
}
void utility_function2(...) {
// Some code
}
void utility_function3(...) {
// Some code
}
}
You're missing the extern keyword on your global variables in your header. As a result, you're defining them, which results in multiple definitions when you include the header in two different source modules.
Once you add the extern in the header file (extern int global_variable1;), you'll need to add the definition in your .cpp file where you also define your functions.
Related
I'm trying to make a namespace and its members available globally however I am running into already defined errors.
Settings.h
#pragma once
#include "boost/property_tree/json_parser.hpp"
#include <string>
using json = boost::property_tree::ptree;
namespace Settings {
extern std::string settingsPath;
extern json settings;
extern void init();
extern void readSettings();
extern void writeSettings();
};
Settings.cpp
#pragma once
#include "Settings.h"
using json = boost::property_tree::ptree;
namespace Settings {
void init() {
}
void readSettings() {
}
void writeSettings() {
}
};
I am forward declaring the Settings namespace and members and using extern. I have no idea what I'm doing wrong. Please could someone point out the error here.
Thanks
Edit: The actual error messages:
Error LNK2005 "class std::basic_string<char,struct std::char_traits<char>,class
std::allocator<char> > Settings::settingsPath" (?settingsPath#Settings##3V?$
basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##A) already defined in
AmalgamService.obj Amalgam F:\Dropbox\CPP\Visual Studio Projects\Amalgam\Amalgam\
main.obj 1
And repeat for all members of the namespace. The Settings.h is #includeed in main.cpp and AmalgamService.cpp
You seem to be including Settings.cpp in another file. Do not include .cpp files. This results in duplicate definitions. This also means that #pragma once in .cpp files is useless.
You need to keep in mind that #include is just a glorified copy&paste tool. When you #include a file, that file is literally being copy&pasted into the spot of the #include statement. So including a .cpp file means you will get multiple definitions of everything defined in that .cpp file.
Furthermore, you don't have to use extern when declaring functions. Functions are extern by default, unless you say otherwise.
Variables are extern by default too, however you need to use extern as a way to declare them without defining them:
extern int var; // declaration
int var; // definition
Functions don't need that, because you can declare them by omitting their body:
void func(); // declaration
void func() { } // definition
You are allowed to declare things multiple times, which is why you can #include header files (like .h, .hpp) in multiple files. But you are not allowed to define things multiple times, which is why you can't #include non-header source files.
I have a function that is the same across all my header files and main.cpp if I define it in main.cpp will they all be able to use it once they are included or will they have a compiler issue?
Still new to this whole header file business. Thanks in advance.
In the header file (myfunction.h), you need to have only declaration of the function:
int foo(int param);
In the main.cpp (or any other cpp file - better choice would be myfunction.cpp - just make sure definition is included in exactly one file!) file, you need to have definition of the function:
int foo(int param)
{
return 1;
}
In all other source (cpp) files where you're using function foo, just include myfunction.h and use function:
#include "myfunction.h"
void someotherfunction()
{
std::cout << foo(1) << std::endl;
}
Compiler only needs to see declaration of the function before it is used. Linker will connect definition of the function with the places you've used the function. If you forget to write definition in main.cpp file, you will not get compiler, but a linker error. It may be worth of mentioning that compiler is compiling each cpp file separately, and linker's job is to combine all compiler object files and to produce final output file. On most setups, linker will be called automatically after compiling, so you may not be familiar with it.
If you include entire function definition in the header file, that definition will be compiled in each translation unit where header file is included, and you will get multiple symbol definition linker error, or something similar - that's why you need to include only declaration of the function inside header file. However, there are exceptions for this - for example, you may declare your function inline - other answers explain this approach.
So, now myfunction.h contains the function declaration:
#ifndef MY_FUNCTION_H
#define MY_FUNCITON_H
// declaration
int myfunction();
#end if
myfunction.cpp contains the function definition:
int myfunction()
{
return 4;
}
Now, in file1.cpp and in file2.cpp you want to use this function, so you're including myfunction.h:
// file1.cpp
#include "myfunction.h"
// somewhere in the file
void foo()
{
std::cout << myfunction();
}
... and in the second file:
// file2.cpp
#include "myfunction.h"
// somewhere in the file
void bar()
{
/// ...
std::cout << myfunction();
}
Header files in C and C++ are a language artifact. They are the consequence of the fact, that C and C++ can be implemented as a single-pass compiler. In contrast, Pascal - for example - has a two-pass compiler, that skips over unknown entities during the first pass, and fills in the missing bits in a second pass. Consequently, in C and C++ every type, object, and method must be declared before it can be used. This is the main responsibility of header files.
Header files are expanded into any file that includes them. In other words: The preprocessor replaces the statement #include "foo.h" with the contents of the file "foo.h". With this being the case you need to be careful to not violate the single definition rule: An entity must not be defined more than once.
To meet both requirements you have two options: Declare and define the function in the header, using the inline keyword, or declaring it in the header only, and defining it in another compilation unit.
The following code illustrates both solutions:
// foo.h
inline void foo() {
// Method is implemented in this header file.
// It is marked 'inline' to prevent linker errors
// concerning multiply defined symbols.
...
}
Delaration in header only, implementation in another compilation unit:
// foo.h
extern void foo();
// foo.cpp (or another compilation unit)
void foo() {
...
}
Regardless of which solution you go with, you can use foo() from any compilation unit. If you want to use it from "main.cpp" the code would look something like this:
// main.cpp
#include "foo.h"
int main() {
foo();
}
So you have a function which is used in all your header files, why don't you make a utility.h which keeps track of these types of functions and inline the functions in the .h ?
Declare the function prototype in a custom header file:
int add(int a, int b);
let say header file name is myfunction.h and include it wherever you need the function.
now you can define a function on another.cpp or main.cpp
int add(int a, int b){
return a+b;
}
include your custom header file like this:
#include "myfunction.h"
remember your main.cpp and other cpp files and the new header file should be in the same path.
If you have two files:
main.cpp
#include "func.h"
int main(){
hello();
std::cout<<" world!\n";
return 0;
}
& func.h
#ifndef FUNC_H
#define FUNC_H
#include <iostream>
void hello(void){
std::cout<<"hello";
}
#endif
iostreams objects and functions e.t.c will work fine from within main.cpp.
This posts answers sum up #ifndef pretty well if you would like to know more.
I'd like to use a certain namespace along with its functions to be used in several forms to have sort of shared functions. However, I am having linker problem.
Here is what I did - I created a new unit and inside of it I wrote in header file:
#ifndef MyHeaderH
#define MyHeaderH
namespace MyHeader
{
enum { SOMETHING1, SOMETHING2 };
void SomeFunction(int Param);
}
#endif
Of course, the actual function is defined in cpp file, the above is just from h file.
And the cpp file is as follows:
#pragma hdrstop
#include "MyHeader.h"
#pragma package(smart_init)
void SomeFunction(int Param)
{
//some code here
}
So in my main form Form1 I include the above in hpp file of Form1
MyHeader::SomeFunction(0);
This all compiles fine but the linker reports unresolved external. So obviously it doesn't see the namespace and function. How do I fix that?
As an additional question - if I use such function set in several forms, it should be all compiled just once and reused right (it won't have several copies of same functions I guess?)
You probably forgot to put the function definition in the namespace.
It's either done like you do in the header file, but with a function body:
namespace MyHeader
{
void SomeFunction(int Param)
{
// ...
}
}
Or using the scope operator :::
void MyHeader::SomeFunction(int Param)
{
// ...
}
The actual function is defined in the cpp file in a way that it does not implement the interface declared in the header file.
Implement
namespace MyHeader
{
void SomeFunction(int Param) { /* Your implementation goes here. */ }
}
in the cpp file.
I'm using Xcode to build a C++ project.
But I don't understand the error message:
"Apple Mach-O linker command failed with exit code 1"
I found that #include is the reason.
I have two .cpp file which include a same .h file. If I remove #include of one, it will be build successfully.
Other header files are fine expect the header file described above. I already used "ifndef".
#ifndef include guards only work at the level of the translation unit (usually a single source file).
If you define the same object twice in two translation units, that won't be fixed by include guards but the linker will complain bitterly when you try to combine the two object files into a single executable.
I suspect your situation is akin to:
hdr.h:
#ifndef HDR_H
#define HDR_H
void rc(void);
int xyzzy;
#endif
prog1.c:
#include "hdr.h"
#include "hdr.h"
int main (void) { rc(); return xyzzy; }
prog2.c:
#include "hdr.h"
void rc(void) { xyzzy = 0; }
In a situation like that, the include guard will prevent the header being included twice in prog1.c but it will still be included in both prog1.c and prog2.c, meaning that each will have a copy of xyzzy.
When you link them together, the linker will not like that.
The solution is to not define things in headers but to merely declare them there, leaving the definition for a C file:
hdr.h:
#ifndef HDR_H
#define HDR_H
int rc(void);
extern int xyzzy; // declare, not define
#endif
prog1.c:
#include "hdr.h"
#include "hdr.h"
int main (void) { rc(); return xyzzy; }
prog2.c:
#include "hdr.h"
int xyzzy; // define
int rc(void) { xyzzy = 0; }
Declarations are things like function prototypes, extern variables, typedefs and so on (simplistically, things that declare something exists without actually creating an "object").
Definition are things that create "objects", like non-extern variables and so on.
You need to track down what "object" is being defined twice (the linker output should have something like doubly-defined symbol 'xyzzy') and then make sure it's not defined in the header.
I need to create structure like this:
// file main.h:
#pragma once
#ifndef _MAIN_H
#define _MAIN_H
#include <iostream>
#include "first.h"
#include "second.h"
#endif
// -----------------
// file first.h:
#pragma once
#ifndef _FIRST_H
#define _FIRST_H
#include "main.h" // because of using SomeFunction() in first.cpp
int SomeVar; // used in first.cpp
#endif
// -----------------
// file second.h:
#pragma once
#ifndef _SECOND_H
#define _SECOND_H
#include "main.h" // because of using SomeVar in second.cpp
int SomeFunction(); // implemented in second.cpp
#endif
// -----------------
By logic, if main.h will be compiled first, then each of this headers will be included only once and all variables/functions will be defined normally.
For example, this configuration compiled normally in IAR C++ Compiler, if set up in options "preinclude file" (not precompiled) to main.h.
But, in Visual Studio 2010 same structure causes linker errors like:
1>second.obj : error LNK2005: "int SomeVar" (?SomeVar##3HA) already defined in first.obj
1>second.obj : error LNK2005: "int SomeFunction" (?SomeFunction##3HA) already defined in first.obj
I think because of including files in alphabetic order. Apparently pragma- and define-guards are ignored by linker.
Errors can be fixed by additional header and external variables definitions, but this is hard and wrong way.
Question is: What should i do?
int SomeVar; // used in first.cpp
Variables should never be defined in headers. They should be declared with extern:
extern int SomeVar; // used in first.cpp
Then you can actually define them in "first.cpp" with int SomeVar;.
Also, "first.h" does not need to include "main.h". Headers should only include files if the definitions in that header absolutely need the contents of those files. The definitions in "first.h" do not need anything in "main.h". Therefore, it should not include it.
If "first.cpp" needs to include something from "second.h", then it is the responsibility of "first.cpp" to include it.
In the header file use extern keyword as:
//first.h
extern int SomeVar; //extern tells the compiler that definition is elsewhere
Then in .cpp file define it and use it:
//first.cpp
int SomeVar; //definition is here
As for SomeFunction, have you defined it header file itself? Re-check it. :-)